text: /*
   *	linux/mm/filemap.c
   *
   * Copyright (C) 1994-1999  Linus Torvalds
   */
  
  /*
   * This file handles the generic file mmap semantics used by
   * most "normal" filesystems (but you don't /have/ to use this:
   * the NFS filesystem used to do this differently, for example)
   */
  #include <linux/export.h>
  #include <linux/compiler.h>
  #include <linux/dax.h>
  #include <linux/fs.h>
  #include <linux/sched/signal.h>
  #include <linux/uaccess.h>
  #include <linux/capability.h>
  #include <linux/kernel_stat.h>
  #include <linux/gfp.h>
  #include <linux/mm.h>
  #include <linux/swap.h>
  #include <linux/mman.h>
  #include <linux/pagemap.h>
  #include <linux/file.h>
  #include <linux/uio.h>
  #include <linux/hash.h>
  #include <linux/writeback.h>
  #include <linux/backing-dev.h>
  #include <linux/pagevec.h>
  #include <linux/blkdev.h>
  #include <linux/security.h>
  #include <linux/cpuset.h>
  #include <linux/hugetlb.h>
  #include <linux/memcontrol.h>
  #include <linux/cleancache.h>
  #include <linux/shmem_fs.h>
  #include <linux/rmap.h>
  #include "internal.h"
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/filemap.h>
  
  /*
   * FIXME: remove all knowledge of the buffer layer from the core VM
   */
  #include <linux/buffer_head.h> /* for try_to_free_buffers */
  
  #include <asm/mman.h>
  
  /*
   * Shared mappings implemented 30.11.1994. It's not fully working yet,
   * though.
   *
   * Shared mappings now work. 15.8.1995  Bruno.
   *
   * finished 'unifying' the page and buffer cache and SMP-threaded the
   * page-cache, 21.05.1999, Ingo Molnar <mingo@redhat.com>
   *
   * SMP-threaded pagemap-LRU 1999, Andrea Arcangeli <andrea@suse.de>
   */
  
  /*
   * Lock ordering:
   *
   *  ->i_mmap_rwsem		(truncate_pagecache)
   *    ->private_lock		(__free_pte->__set_page_dirty_buffers)
   *      ->swap_lock		(exclusive_swap_page, others)
   *        ->i_pages lock
   *
   *  ->i_mutex
   *    ->i_mmap_rwsem		(truncate->unmap_mapping_range)
   *
   *  ->mmap_sem
   *    ->i_mmap_rwsem
   *      ->page_table_lock or pte_lock	(various, mainly in memory.c)
   *        ->i_pages lock	(arch-dependent flush_dcache_mmap_lock)
   *
   *  ->mmap_sem
   *    ->lock_page		(access_process_vm)
   *
   *  ->i_mutex			(generic_perform_write)
   *    ->mmap_sem		(fault_in_pages_readable->do_page_fault)
   *
   *  bdi->wb.list_lock
   *    sb_lock			(fs/fs-writeback.c)
   *    ->i_pages lock		(__sync_single_inode)
   *
   *  ->i_mmap_rwsem
   *    ->anon_vma.lock		(vma_adjust)
   *
   *  ->anon_vma.lock
   *    ->page_table_lock or pte_lock	(anon_vma_prepare and various)
   *
   *  ->page_table_lock or pte_lock
   *    ->swap_lock		(try_to_unmap_one)
   *    ->private_lock		(try_to_unmap_one)
   *    ->i_pages lock		(try_to_unmap_one)
   *    ->zone_lru_lock(zone)	(follow_page->mark_page_accessed)
   *    ->zone_lru_lock(zone)	(check_pte_range->isolate_lru_page)
   *    ->private_lock		(page_remove_rmap->set_page_dirty)
   *    ->i_pages lock		(page_remove_rmap->set_page_dirty)
   *    bdi.wb->list_lock		(page_remove_rmap->set_page_dirty)
   *    ->inode->i_lock		(page_remove_rmap->set_page_dirty)
   *    ->memcg->move_lock	(page_remove_rmap->lock_page_memcg)
   *    bdi.wb->list_lock		(zap_pte_range->set_page_dirty)
   *    ->inode->i_lock		(zap_pte_range->set_page_dirty)
   *    ->private_lock		(zap_pte_range->__set_page_dirty_buffers)
   *
   * ->i_mmap_rwsem
   *   ->tasklist_lock            (memory_failure, collect_procs_ao)
   */
  
  static int page_cache_tree_insert(struct address_space *mapping,
  				  struct page *page, void **shadowp)
  {
  	struct radix_tree_node *node;
  	void **slot;
  	int error;
  
  	error = __radix_tree_create(&mapping->i_pages, page->index, 0,
  				    &node, &slot);
  	if (error)
  		return error;
  	if (*slot) {
  		void *p;
  
  		p = radix_tree_deref_slot_protected(slot,
  						    &mapping->i_pages.xa_lock);
  		if (!radix_tree_exceptional_entry(p))
  			return -EEXIST;
  
  		mapping->nrexceptional--;
  		if (shadowp)
  			*shadowp = p;
  	}
  	__radix_tree_replace(&mapping->i_pages, node, slot, page,
  			     workingset_lookup_update(mapping));
  	mapping->nrpages++;
  	return 0;
  }
  
  static void page_cache_tree_delete(struct address_space *mapping,
  				   struct page *page, void *shadow)
  {
  	int i, nr;
  
  	/* hugetlb pages are represented by one entry in the radix tree */
  	nr = PageHuge(page) ? 1 : hpage_nr_pages(page);
  
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(PageTail(page), page);
  	VM_BUG_ON_PAGE(nr != 1 && shadow, page);
  
  	for (i = 0; i < nr; i++) {
  		struct radix_tree_node *node;
  		void **slot;
  
  		__radix_tree_lookup(&mapping->i_pages, page->index + i,
  				    &node, &slot);
  
  		VM_BUG_ON_PAGE(!node && nr != 1, page);
  
  		radix_tree_clear_tags(&mapping->i_pages, node, slot);
  		__radix_tree_replace(&mapping->i_pages, node, slot, shadow,
  				workingset_lookup_update(mapping));
  	}
  
  	page->mapping = NULL;
  	/* Leave page->index set: truncation lookup relies upon it */
  
  	if (shadow) {
  		mapping->nrexceptional += nr;
  		/*
  		 * Make sure the nrexceptional update is committed before
  		 * the nrpages update so that final truncate racing
  		 * with reclaim does not see both counters 0 at the
  		 * same time and miss a shadow entry.
  		 */
  		smp_wmb();
  	}
  	mapping->nrpages -= nr;
  }
  
  static void unaccount_page_cache_page(struct address_space *mapping,
  				      struct page *page)
  {
  	int nr;
  
  	/*
  	 * if we're uptodate, flush out into the cleancache, otherwise
  	 * invalidate any existing cleancache entries.  We can't leave
  	 * stale data around in the cleancache once our page is gone
  	 */
  	if (PageUptodate(page) && PageMappedToDisk(page))
  		cleancache_put_page(page);
  	else
  		cleancache_invalidate_page(mapping, page);
  
  	VM_BUG_ON_PAGE(PageTail(page), page);
  	VM_BUG_ON_PAGE(page_mapped(page), page);
  	if (!IS_ENABLED(CONFIG_DEBUG_VM) && unlikely(page_mapped(page))) {
  		int mapcount;
  
  		pr_alert("BUG: Bad page cache in process %s  pfn:%05lx\n",
  			 current->comm, page_to_pfn(page));
  		dump_page(page, "still mapped when deleted");
  		dump_stack();
  		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
  
  		mapcount = page_mapcount(page);
  		if (mapping_exiting(mapping) &&
  		    page_count(page) >= mapcount + 2) {
  			/*
  			 * All vmas have already been torn down, so it's
  			 * a good bet that actually the page is unmapped,
  			 * and we'd prefer not to leak it: if we're wrong,
  			 * some other bad page check should catch it later.
  			 */
  			page_mapcount_reset(page);
  			page_ref_sub(page, mapcount);
  		}
  	}
  
  	/* hugetlb pages do not participate in page cache accounting. */
  	if (PageHuge(page))
  		return;
  
  	nr = hpage_nr_pages(page);
  
  	__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, -nr);
  	if (PageSwapBacked(page)) {
  		__mod_node_page_state(page_pgdat(page), NR_SHMEM, -nr);
  		if (PageTransHuge(page))
  			__dec_node_page_state(page, NR_SHMEM_THPS);
  	} else {
  		VM_BUG_ON_PAGE(PageTransHuge(page), page);
  	}
  
  	/*
  	 * At this point page must be either written or cleaned by
  	 * truncate.  Dirty page here signals a bug and loss of
  	 * unwritten data.
  	 *
  	 * This fixes dirty accounting after removing the page entirely
  	 * but leaves PageDirty set: it has no effect for truncated
  	 * page and anyway will be cleared before returning page into
  	 * buddy allocator.
  	 */
  	if (WARN_ON_ONCE(PageDirty(page)))
  		account_page_cleaned(page, mapping, inode_to_wb(mapping->host));
  }
  
  /*
   * Delete a page from the page cache and free it. Caller has to make
   * sure the page is locked and that nobody else uses it - or that usage
   * is safe.  The caller must hold the i_pages lock.
   */
  void __delete_from_page_cache(struct page *page, void *shadow)
  {
  	struct address_space *mapping = page->mapping;
  
  	trace_mm_filemap_delete_from_page_cache(page);
  
  	unaccount_page_cache_page(mapping, page);
  	page_cache_tree_delete(mapping, page, shadow);
  }
  
  static void page_cache_free_page(struct address_space *mapping,
  				struct page *page)
  {
  	void (*freepage)(struct page *);
  
  	freepage = mapping->a_ops->freepage;
  	if (freepage)
  		freepage(page);
  
  	if (PageTransHuge(page) && !PageHuge(page)) {
  		page_ref_sub(page, HPAGE_PMD_NR);
  		VM_BUG_ON_PAGE(page_count(page) <= 0, page);
  	} else {
  		put_page(page);
  	}
  }
  
  /**
   * delete_from_page_cache - delete page from page cache
   * @page: the page which the kernel is trying to remove from page cache
   *
   * This must be called only on pages that have been verified to be in the page
   * cache and locked.  It will never put the page into the free list, the caller
   * has a reference on the page.
   */
  void delete_from_page_cache(struct page *page)
  {
  	struct address_space *mapping = page_mapping(page);
  	unsigned long flags;
  
  	BUG_ON(!PageLocked(page));
  	xa_lock_irqsave(&mapping->i_pages, flags);
  	__delete_from_page_cache(page, NULL);
  	xa_unlock_irqrestore(&mapping->i_pages, flags);
  
  	page_cache_free_page(mapping, page);
  }
  EXPORT_SYMBOL(delete_from_page_cache);
  
  /*
   * page_cache_tree_delete_batch - delete several pages from page cache
   * @mapping: the mapping to which pages belong
   * @pvec: pagevec with pages to delete
   *
   * The function walks over mapping->i_pages and removes pages passed in @pvec
   * from the mapping. The function expects @pvec to be sorted by page index.
   * It tolerates holes in @pvec (mapping entries at those indices are not
   * modified). The function expects only THP head pages to be present in the
   * @pvec and takes care to delete all corresponding tail pages from the
   * mapping as well.
   *
   * The function expects the i_pages lock to be held.
   */
  static void
  page_cache_tree_delete_batch(struct address_space *mapping,
  			     struct pagevec *pvec)
  {
  	struct radix_tree_iter iter;
  	void **slot;
  	int total_pages = 0;
  	int i = 0, tail_pages = 0;
  	struct page *page;
  	pgoff_t start;
  
  	start = pvec->pages[0]->index;
  	radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
  		if (i >= pagevec_count(pvec) && !tail_pages)
  			break;
  		page = radix_tree_deref_slot_protected(slot,
  						       &mapping->i_pages.xa_lock);
  		if (radix_tree_exceptional_entry(page))
  			continue;
  		if (!tail_pages) {
  			/*
  			 * Some page got inserted in our range? Skip it. We
  			 * have our pages locked so they are protected from
  			 * being removed.
  			 */
  			if (page != pvec->pages[i])
  				continue;
  			WARN_ON_ONCE(!PageLocked(page));
  			if (PageTransHuge(page) && !PageHuge(page))
  				tail_pages = HPAGE_PMD_NR - 1;
  			page->mapping = NULL;
  			/*
  			 * Leave page->index set: truncation lookup relies
  			 * upon it
  			 */
  			i++;
  		} else {
  			tail_pages--;
  		}
  		radix_tree_clear_tags(&mapping->i_pages, iter.node, slot);
  		__radix_tree_replace(&mapping->i_pages, iter.node, slot, NULL,
  				workingset_lookup_update(mapping));
  		total_pages++;
  	}
  	mapping->nrpages -= total_pages;
  }
  
  void delete_from_page_cache_batch(struct address_space *mapping,
  				  struct pagevec *pvec)
  {
  	int i;
  	unsigned long flags;
  
  	if (!pagevec_count(pvec))
  		return;
  
  	xa_lock_irqsave(&mapping->i_pages, flags);
  	for (i = 0; i < pagevec_count(pvec); i++) {
  		trace_mm_filemap_delete_from_page_cache(pvec->pages[i]);
  
  		unaccount_page_cache_page(mapping, pvec->pages[i]);
  	}
  	page_cache_tree_delete_batch(mapping, pvec);
  	xa_unlock_irqrestore(&mapping->i_pages, flags);
  
  	for (i = 0; i < pagevec_count(pvec); i++)
  		page_cache_free_page(mapping, pvec->pages[i]);
  }
  
  int filemap_check_errors(struct address_space *mapping)
  {
  	int ret = 0;
  	/* Check for outstanding write errors */
  	if (test_bit(AS_ENOSPC, &mapping->flags) &&
  	    test_and_clear_bit(AS_ENOSPC, &mapping->flags))
  		ret = -ENOSPC;
  	if (test_bit(AS_EIO, &mapping->flags) &&
  	    test_and_clear_bit(AS_EIO, &mapping->flags))
  		ret = -EIO;
  	return ret;
  }
  EXPORT_SYMBOL(filemap_check_errors);
  
  static int filemap_check_and_keep_errors(struct address_space *mapping)
  {
  	/* Check for outstanding write errors */
  	if (test_bit(AS_EIO, &mapping->flags))
  		return -EIO;
  	if (test_bit(AS_ENOSPC, &mapping->flags))
  		return -ENOSPC;
  	return 0;
  }
  
  /**
   * __filemap_fdatawrite_range - start writeback on mapping dirty pages in range
   * @mapping:	address space structure to write
   * @start:	offset in bytes where the range starts
   * @end:	offset in bytes where the range ends (inclusive)
   * @sync_mode:	enable synchronous operation
   *
   * Start writeback against all of a mapping's dirty pages that lie
   * within the byte offsets <start, end> inclusive.
   *
   * If sync_mode is WB_SYNC_ALL then this is a "data integrity" operation, as
   * opposed to a regular memory cleansing writeback.  The difference between
   * these two operations is that if a dirty page/buffer is encountered, it must
   * be waited upon, and not just skipped over.
   */
  int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
  				loff_t end, int sync_mode)
  {
  	int ret;
  	struct writeback_control wbc = {
  		.sync_mode = sync_mode,
  		.nr_to_write = LONG_MAX,
  		.range_start = start,
  		.range_end = end,
  	};
  
  	if (!mapping_cap_writeback_dirty(mapping))
  		return 0;
  
  	wbc_attach_fdatawrite_inode(&wbc, mapping->host);
  	ret = do_writepages(mapping, &wbc);
  	wbc_detach_inode(&wbc);
  	return ret;
  }
  
  static inline int __filemap_fdatawrite(struct address_space *mapping,
  	int sync_mode)
  {
  	return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);
  }
  
  int filemap_fdatawrite(struct address_space *mapping)
  {
  	return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
  }
  EXPORT_SYMBOL(filemap_fdatawrite);
  
  int filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
  				loff_t end)
  {
  	return __filemap_fdatawrite_range(mapping, start, end, WB_SYNC_ALL);
  }
  EXPORT_SYMBOL(filemap_fdatawrite_range);
  
  /**
   * filemap_flush - mostly a non-blocking flush
   * @mapping:	target address_space
   *
   * This is a mostly non-blocking flush.  Not suitable for data-integrity
   * purposes - I/O may not be started against all dirty pages.
   */
  int filemap_flush(struct address_space *mapping)
  {
  	return __filemap_fdatawrite(mapping, WB_SYNC_NONE);
  }
  EXPORT_SYMBOL(filemap_flush);
  
  /**
   * filemap_range_has_page - check if a page exists in range.
   * @mapping:           address space within which to check
   * @start_byte:        offset in bytes where the range starts
   * @end_byte:          offset in bytes where the range ends (inclusive)
   *
   * Find at least one page in the range supplied, usually used to check if
   * direct writing in this range will trigger a writeback.
   */
  bool filemap_range_has_page(struct address_space *mapping,
  			   loff_t start_byte, loff_t end_byte)
  {
  	pgoff_t index = start_byte >> PAGE_SHIFT;
  	pgoff_t end = end_byte >> PAGE_SHIFT;
  	struct page *page;
  
  	if (end_byte < start_byte)
  		return false;
  
  	if (mapping->nrpages == 0)
  		return false;
  
  	if (!find_get_pages_range(mapping, &index, end, 1, &page))
  		return false;
  	put_page(page);
  	return true;
  }
  EXPORT_SYMBOL(filemap_range_has_page);
  
  static void __filemap_fdatawait_range(struct address_space *mapping,
  				     loff_t start_byte, loff_t end_byte)
  {
  	pgoff_t index = start_byte >> PAGE_SHIFT;
  	pgoff_t end = end_byte >> PAGE_SHIFT;
  	struct pagevec pvec;
  	int nr_pages;
  
  	if (end_byte < start_byte)
  		return;
  
  	pagevec_init(&pvec);
  	while (index <= end) {
  		unsigned i;
  
  		nr_pages = pagevec_lookup_range_tag(&pvec, mapping, &index,
  				end, PAGECACHE_TAG_WRITEBACK);
  		if (!nr_pages)
  			break;
  
  		for (i = 0; i < nr_pages; i++) {
  			struct page *page = pvec.pages[i];
  
  			wait_on_page_writeback(page);
  			ClearPageError(page);
  		}
  		pagevec_release(&pvec);
  		cond_resched();
  	}
  }
  
  /**
   * filemap_fdatawait_range - wait for writeback to complete
   * @mapping:		address space structure to wait for
   * @start_byte:		offset in bytes where the range starts
   * @end_byte:		offset in bytes where the range ends (inclusive)
   *
   * Walk the list of under-writeback pages of the given address space
   * in the given range and wait for all of them.  Check error status of
   * the address space and return it.
   *
   * Since the error status of the address space is cleared by this function,
   * callers are responsible for checking the return value and handling and/or
   * reporting the error.
   */
  int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
  			    loff_t end_byte)
  {
  	__filemap_fdatawait_range(mapping, start_byte, end_byte);
  	return filemap_check_errors(mapping);
  }
  EXPORT_SYMBOL(filemap_fdatawait_range);
  
  /**
   * file_fdatawait_range - wait for writeback to complete
   * @file:		file pointing to address space structure to wait for
   * @start_byte:		offset in bytes where the range starts
   * @end_byte:		offset in bytes where the range ends (inclusive)
   *
   * Walk the list of under-writeback pages of the address space that file
   * refers to, in the given range and wait for all of them.  Check error
   * status of the address space vs. the file->f_wb_err cursor and return it.
   *
   * Since the error status of the file is advanced by this function,
   * callers are responsible for checking the return value and handling and/or
   * reporting the error.
   */
  int file_fdatawait_range(struct file *file, loff_t start_byte, loff_t end_byte)
  {
  	struct address_space *mapping = file->f_mapping;
  
  	__filemap_fdatawait_range(mapping, start_byte, end_byte);
  	return file_check_and_advance_wb_err(file);
  }
  EXPORT_SYMBOL(file_fdatawait_range);
  
  /**
   * filemap_fdatawait_keep_errors - wait for writeback without clearing errors
   * @mapping: address space structure to wait for
   *
   * Walk the list of under-writeback pages of the given address space
   * and wait for all of them.  Unlike filemap_fdatawait(), this function
   * does not clear error status of the address space.
   *
   * Use this function if callers don't handle errors themselves.  Expected
   * call sites are system-wide / filesystem-wide data flushers: e.g. sync(2),
   * fsfreeze(8)
   */
  int filemap_fdatawait_keep_errors(struct address_space *mapping)
  {
  	__filemap_fdatawait_range(mapping, 0, LLONG_MAX);
  	return filemap_check_and_keep_errors(mapping);
  }
  EXPORT_SYMBOL(filemap_fdatawait_keep_errors);
  
  static bool mapping_needs_writeback(struct address_space *mapping)
  {
  	return (!dax_mapping(mapping) && mapping->nrpages) ||
  	    (dax_mapping(mapping) && mapping->nrexceptional);
  }
  
  int filemap_write_and_wait(struct address_space *mapping)
  {
  	int err = 0;
  
  	if (mapping_needs_writeback(mapping)) {
  		err = filemap_fdatawrite(mapping);
  		/*
  		 * Even if the above returned error, the pages may be
  		 * written partially (e.g. -ENOSPC), so we wait for it.
  		 * But the -EIO is special case, it may indicate the worst
  		 * thing (e.g. bug) happened, so we avoid waiting for it.
  		 */
  		if (err != -EIO) {
  			int err2 = filemap_fdatawait(mapping);
  			if (!err)
  				err = err2;
  		} else {
  			/* Clear any previously stored errors */
  			filemap_check_errors(mapping);
  		}
  	} else {
  		err = filemap_check_errors(mapping);
  	}
  	return err;
  }
  EXPORT_SYMBOL(filemap_write_and_wait);
  
  /**
   * filemap_write_and_wait_range - write out & wait on a file range
   * @mapping:	the address_space for the pages
   * @lstart:	offset in bytes where the range starts
   * @lend:	offset in bytes where the range ends (inclusive)
   *
   * Write out and wait upon file offsets lstart->lend, inclusive.
   *
   * Note that @lend is inclusive (describes the last byte to be written) so
   * that this function can be used to write to the very end-of-file (end = -1).
   */
  int filemap_write_and_wait_range(struct address_space *mapping,
  				 loff_t lstart, loff_t lend)
  {
  	int err = 0;
  
  	if (mapping_needs_writeback(mapping)) {
  		err = __filemap_fdatawrite_range(mapping, lstart, lend,
  						 WB_SYNC_ALL);
  		/* See comment of filemap_write_and_wait() */
  		if (err != -EIO) {
  			int err2 = filemap_fdatawait_range(mapping,
  						lstart, lend);
  			if (!err)
  				err = err2;
  		} else {
  			/* Clear any previously stored errors */
  			filemap_check_errors(mapping);
  		}
  	} else {
  		err = filemap_check_errors(mapping);
  	}
  	return err;
  }
  EXPORT_SYMBOL(filemap_write_and_wait_range);
  
  void __filemap_set_wb_err(struct address_space *mapping, int err)
  {
  	errseq_t eseq = errseq_set(&mapping->wb_err, err);
  
  	trace_filemap_set_wb_err(mapping, eseq);
  }
  EXPORT_SYMBOL(__filemap_set_wb_err);
  
  /**
   * file_check_and_advance_wb_err - report wb error (if any) that was previously
   * 				   and advance wb_err to current one
   * @file: struct file on which the error is being reported
   *
   * When userland calls fsync (or something like nfsd does the equivalent), we
   * want to report any writeback errors that occurred since the last fsync (or
   * since the file was opened if there haven't been any).
   *
   * Grab the wb_err from the mapping. If it matches what we have in the file,
   * then just quickly return 0. The file is all caught up.
   *
   * If it doesn't match, then take the mapping value, set the "seen" flag in
   * it and try to swap it into place. If it works, or another task beat us
   * to it with the new value, then update the f_wb_err and return the error
   * portion. The error at this point must be reported via proper channels
   * (a'la fsync, or NFS COMMIT operation, etc.).
   *
   * While we handle mapping->wb_err with atomic operations, the f_wb_err
   * value is protected by the f_lock since we must ensure that it reflects
   * the latest value swapped in for this file descriptor.
   */
  int file_check_and_advance_wb_err(struct file *file)
  {
  	int err = 0;
  	errseq_t old = READ_ONCE(file->f_wb_err);
  	struct address_space *mapping = file->f_mapping;
  
  	/* Locklessly handle the common case where nothing has changed */
  	if (errseq_check(&mapping->wb_err, old)) {
  		/* Something changed, must use slow path */
  		spin_lock(&file->f_lock);
  		old = file->f_wb_err;
  		err = errseq_check_and_advance(&mapping->wb_err,
  						&file->f_wb_err);
  		trace_file_check_and_advance_wb_err(file, old);
  		spin_unlock(&file->f_lock);
  	}
  
  	/*
  	 * We're mostly using this function as a drop in replacement for
  	 * filemap_check_errors. Clear AS_EIO/AS_ENOSPC to emulate the effect
  	 * that the legacy code would have had on these flags.
  	 */
  	clear_bit(AS_EIO, &mapping->flags);
  	clear_bit(AS_ENOSPC, &mapping->flags);
  	return err;
  }
  EXPORT_SYMBOL(file_check_and_advance_wb_err);
  
  /**
   * file_write_and_wait_range - write out & wait on a file range
   * @file:	file pointing to address_space with pages
   * @lstart:	offset in bytes where the range starts
   * @lend:	offset in bytes where the range ends (inclusive)
   *
   * Write out and wait upon file offsets lstart->lend, inclusive.
   *
   * Note that @lend is inclusive (describes the last byte to be written) so
   * that this function can be used to write to the very end-of-file (end = -1).
   *
   * After writing out and waiting on the data, we check and advance the
   * f_wb_err cursor to the latest value, and return any errors detected there.
   */
  int file_write_and_wait_range(struct file *file, loff_t lstart, loff_t lend)
  {
  	int err = 0, err2;
  	struct address_space *mapping = file->f_mapping;
  
  	if (mapping_needs_writeback(mapping)) {
  		err = __filemap_fdatawrite_range(mapping, lstart, lend,
  						 WB_SYNC_ALL);
  		/* See comment of filemap_write_and_wait() */
  		if (err != -EIO)
  			__filemap_fdatawait_range(mapping, lstart, lend);
  	}
  	err2 = file_check_and_advance_wb_err(file);
  	if (!err)
  		err = err2;
  	return err;
  }
  EXPORT_SYMBOL(file_write_and_wait_range);
  
  /**
   * replace_page_cache_page - replace a pagecache page with a new one
   * @old:	page to be replaced
   * @new:	page to replace with
   * @gfp_mask:	allocation mode
   *
   * This function replaces a page in the pagecache with a new one.  On
   * success it acquires the pagecache reference for the new page and
   * drops it for the old page.  Both the old and new pages must be
   * locked.  This function does not add the new page to the LRU, the
   * caller must do that.
   *
   * The remove + add is atomic.  The only way this function can fail is
   * memory allocation failure.
   */
  int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
  {
  	int error;
  
  	VM_BUG_ON_PAGE(!PageLocked(old), old);
  	VM_BUG_ON_PAGE(!PageLocked(new), new);
  	VM_BUG_ON_PAGE(new->mapping, new);
  
  	error = radix_tree_preload(gfp_mask & GFP_RECLAIM_MASK);
  	if (!error) {
  		struct address_space *mapping = old->mapping;
  		void (*freepage)(struct page *);
  		unsigned long flags;
  
  		pgoff_t offset = old->index;
  		freepage = mapping->a_ops->freepage;
  
  		get_page(new);
  		new->mapping = mapping;
  		new->index = offset;
  
  		xa_lock_irqsave(&mapping->i_pages, flags);
  		__delete_from_page_cache(old, NULL);
  		error = page_cache_tree_insert(mapping, new, NULL);
  		BUG_ON(error);
  
  		/*
  		 * hugetlb pages do not participate in page cache accounting.
  		 */
  		if (!PageHuge(new))
  			__inc_node_page_state(new, NR_FILE_PAGES);
  		if (PageSwapBacked(new))
  			__inc_node_page_state(new, NR_SHMEM);
  		xa_unlock_irqrestore(&mapping->i_pages, flags);
  		mem_cgroup_migrate(old, new);
  		radix_tree_preload_end();
  		if (freepage)
  			freepage(old);
  		put_page(old);
  	}
  
  	return error;
  }
  EXPORT_SYMBOL_GPL(replace_page_cache_page);
  
  static int __add_to_page_cache_locked(struct page *page,
  				      struct address_space *mapping,
  				      pgoff_t offset, gfp_t gfp_mask,
  				      void **shadowp)
  {
  	int huge = PageHuge(page);
  	struct mem_cgroup *memcg;
  	int error;
  
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(PageSwapBacked(page), page);
  
  	if (!huge) {
  		error = mem_cgroup_try_charge(page, current->mm,
  					      gfp_mask, &memcg, false);
  		if (error)
  			return error;
  	}
  
  	error = radix_tree_maybe_preload(gfp_mask & GFP_RECLAIM_MASK);
  	if (error) {
  		if (!huge)
  			mem_cgroup_cancel_charge(page, memcg, false);
  		return error;
  	}
  
  	get_page(page);
  	page->mapping = mapping;
  	page->index = offset;
  
  	xa_lock_irq(&mapping->i_pages);
  	error = page_cache_tree_insert(mapping, page, shadowp);
  	radix_tree_preload_end();
  	if (unlikely(error))
  		goto err_insert;
  
  	/* hugetlb pages do not participate in page cache accounting. */
  	if (!huge)
  		__inc_node_page_state(page, NR_FILE_PAGES);
  	xa_unlock_irq(&mapping->i_pages);
  	if (!huge)
  		mem_cgroup_commit_charge(page, memcg, false, false);
  	trace_mm_filemap_add_to_page_cache(page);
  	return 0;
  err_insert:
  	page->mapping = NULL;
  	/* Leave page->index set: truncation relies upon it */
  	xa_unlock_irq(&mapping->i_pages);
  	if (!huge)
  		mem_cgroup_cancel_charge(page, memcg, false);
  	put_page(page);
  	return error;
  }
  
  /**
   * add_to_page_cache_locked - add a locked page to the pagecache
   * @page:	page to add
   * @mapping:	the page's address_space
   * @offset:	page index
   * @gfp_mask:	page allocation mode
   *
   * This function is used to add a page to the pagecache. It must be locked.
   * This function does not add the page to the LRU.  The caller must do that.
   */
  int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
  		pgoff_t offset, gfp_t gfp_mask)
  {
  	return __add_to_page_cache_locked(page, mapping, offset,
  					  gfp_mask, NULL);
  }
  EXPORT_SYMBOL(add_to_page_cache_locked);
  
  int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
  				pgoff_t offset, gfp_t gfp_mask)
  {
  	void *shadow = NULL;
  	int ret;
  
  	__SetPageLocked(page);
  	ret = __add_to_page_cache_locked(page, mapping, offset,
  					 gfp_mask, &shadow);
  	if (unlikely(ret))
  		__ClearPageLocked(page);
  	else {
  		/*
  		 * The page might have been evicted from cache only
  		 * recently, in which case it should be activated like
  		 * any other repeatedly accessed page.
  		 * The exception is pages getting rewritten; evicting other
  		 * data from the working set, only to cache data that will
  		 * get overwritten with something else, is a waste of memory.
  		 */
  		if (!(gfp_mask & __GFP_WRITE) &&
  		    shadow && workingset_refault(shadow)) {
  			SetPageActive(page);
  			workingset_activation(page);
  		} else
  			ClearPageActive(page);
  		lru_cache_add(page);
  	}
  	return ret;
  }
  EXPORT_SYMBOL_GPL(add_to_page_cache_lru);
  
  #ifdef CONFIG_NUMA
  struct page *__page_cache_alloc(gfp_t gfp)
  {
  	int n;
  	struct page *page;
  
  	if (cpuset_do_page_mem_spread()) {
  		unsigned int cpuset_mems_cookie;
  		do {
  			cpuset_mems_cookie = read_mems_allowed_begin();
  			n = cpuset_mem_spread_node();
  			page = __alloc_pages_node(n, gfp, 0);
  		} while (!page && read_mems_allowed_retry(cpuset_mems_cookie));
  
  		return page;
  	}
  	return alloc_pages(gfp, 0);
  }
  EXPORT_SYMBOL(__page_cache_alloc);
  #endif
  
  /*
   * In order to wait for pages to become available there must be
   * waitqueues associated with pages. By using a hash table of
   * waitqueues where the bucket discipline is to maintain all
   * waiters on the same queue and wake all when any of the pages
   * become available, and for the woken contexts to check to be
   * sure the appropriate page became available, this saves space
   * at a cost of "thundering herd" phenomena during rare hash
   * collisions.
   */
  #define PAGE_WAIT_TABLE_BITS 8
  #define PAGE_WAIT_TABLE_SIZE (1 << PAGE_WAIT_TABLE_BITS)
  static wait_queue_head_t page_wait_table[PAGE_WAIT_TABLE_SIZE] __cacheline_aligned;
  
  static wait_queue_head_t *page_waitqueue(struct page *page)
  {
  	return &page_wait_table[hash_ptr(page, PAGE_WAIT_TABLE_BITS)];
  }
  
  void __init pagecache_init(void)
  {
  	int i;
  
  	for (i = 0; i < PAGE_WAIT_TABLE_SIZE; i++)
  		init_waitqueue_head(&page_wait_table[i]);
  
  	page_writeback_init();
  }
  
  /* This has the same layout as wait_bit_key - see fs/cachefiles/rdwr.c */
  struct wait_page_key {
  	struct page *page;
  	int bit_nr;
  	int page_match;
  };
  
  struct wait_page_queue {
  	struct page *page;
  	int bit_nr;
  	wait_queue_entry_t wait;
  };
  
  static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync, void *arg)
  {
  	struct wait_page_key *key = arg;
  	struct wait_page_queue *wait_page
  		= container_of(wait, struct wait_page_queue, wait);
  
  	if (wait_page->page != key->page)
  	       return 0;
  	key->page_match = 1;
  
  	if (wait_page->bit_nr != key->bit_nr)
  		return 0;
  
  	/* Stop walking if it's locked */
  	if (test_bit(key->bit_nr, &key->page->flags))
  		return -1;
  
  	return autoremove_wake_function(wait, mode, sync, key);
  }
  
  static void wake_up_page_bit(struct page *page, int bit_nr)
  {
  	wait_queue_head_t *q = page_waitqueue(page);
  	struct wait_page_key key;
  	unsigned long flags;
  	wait_queue_entry_t bookmark;
  
  	key.page = page;
  	key.bit_nr = bit_nr;
  	key.page_match = 0;
  
  	bookmark.flags = 0;
  	bookmark.private = NULL;
  	bookmark.func = NULL;
  	INIT_LIST_HEAD(&bookmark.entry);
  
  	spin_lock_irqsave(&q->lock, flags);
  	__wake_up_locked_key_bookmark(q, TASK_NORMAL, &key, &bookmark);
  
  	while (bookmark.flags & WQ_FLAG_BOOKMARK) {
  		/*
  		 * Take a breather from holding the lock,
  		 * allow pages that finish wake up asynchronously
  		 * to acquire the lock and remove themselves
  		 * from wait queue
  		 */
  		spin_unlock_irqrestore(&q->lock, flags);
  		cpu_relax();
  		spin_lock_irqsave(&q->lock, flags);
  		__wake_up_locked_key_bookmark(q, TASK_NORMAL, &key, &bookmark);
  	}
  
  	/*
  	 * It is possible for other pages to have collided on the waitqueue
  	 * hash, so in that case check for a page match. That prevents a long-
  	 * term waiter
  	 *
  	 * It is still possible to miss a case here, when we woke page waiters
  	 * and removed them from the waitqueue, but there are still other
  	 * page waiters.
  	 */
  	if (!waitqueue_active(q) || !key.page_match) {
  		ClearPageWaiters(page);
  		/*
  		 * It's possible to miss clearing Waiters here, when we woke
  		 * our page waiters, but the hashed waitqueue has waiters for
  		 * other pages on it.
  		 *
  		 * That's okay, it's a rare case. The next waker will clear it.
  		 */
  	}
  	spin_unlock_irqrestore(&q->lock, flags);
  }
  
  static void wake_up_page(struct page *page, int bit)
  {
  	if (!PageWaiters(page))
  		return;
  	wake_up_page_bit(page, bit);
  }
  
  static inline int wait_on_page_bit_common(wait_queue_head_t *q,
  		struct page *page, int bit_nr, int state, bool lock)
  {
  	struct wait_page_queue wait_page;
  	wait_queue_entry_t *wait = &wait_page.wait;
  	int ret = 0;
  
  	init_wait(wait);
  	wait->flags = lock ? WQ_FLAG_EXCLUSIVE : 0;
  	wait->func = wake_page_function;
  	wait_page.page = page;
  	wait_page.bit_nr = bit_nr;
  
  	for (;;) {
  		spin_lock_irq(&q->lock);
  
  		if (likely(list_empty(&wait->entry))) {
  			__add_wait_queue_entry_tail(q, wait);
  			SetPageWaiters(page);
  		}
  
  		set_current_state(state);
  
  		spin_unlock_irq(&q->lock);
  
  		if (likely(test_bit(bit_nr, &page->flags))) {
  			io_schedule();
  		}
  
  		if (lock) {
  			if (!test_and_set_bit_lock(bit_nr, &page->flags))
  				break;
  		} else {
  			if (!test_bit(bit_nr, &page->flags))
  				break;
  		}
  
  		if (unlikely(signal_pending_state(state, current))) {
  			ret = -EINTR;
  			break;
  		}
  	}
  
  	finish_wait(q, wait);
  
  	/*
  	 * A signal could leave PageWaiters set. Clearing it here if
  	 * !waitqueue_active would be possible (by open-coding finish_wait),
  	 * but still fail to catch it in the case of wait hash collision. We
  	 * already can fail to clear wait hash collision cases, so don't
  	 * bother with signals either.
  	 */
  
  	return ret;
  }
  
  void wait_on_page_bit(struct page *page, int bit_nr)
  {
  	wait_queue_head_t *q = page_waitqueue(page);
  	wait_on_page_bit_common(q, page, bit_nr, TASK_UNINTERRUPTIBLE, false);
  }
  EXPORT_SYMBOL(wait_on_page_bit);
  
  int wait_on_page_bit_killable(struct page *page, int bit_nr)
  {
  	wait_queue_head_t *q = page_waitqueue(page);
  	return wait_on_page_bit_common(q, page, bit_nr, TASK_KILLABLE, false);
  }
  EXPORT_SYMBOL(wait_on_page_bit_killable);
  
  /**
   * add_page_wait_queue - Add an arbitrary waiter to a page's wait queue
   * @page: Page defining the wait queue of interest
   * @waiter: Waiter to add to the queue
   *
   * Add an arbitrary @waiter to the wait queue for the nominated @page.
   */
  void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter)
  {
  	wait_queue_head_t *q = page_waitqueue(page);
  	unsigned long flags;
  
  	spin_lock_irqsave(&q->lock, flags);
  	__add_wait_queue_entry_tail(q, waiter);
  	SetPageWaiters(page);
  	spin_unlock_irqrestore(&q->lock, flags);
  }
  EXPORT_SYMBOL_GPL(add_page_wait_queue);
  
  #ifndef clear_bit_unlock_is_negative_byte
  
  /*
   * PG_waiters is the high bit in the same byte as PG_lock.
   *
   * On x86 (and on many other architectures), we can clear PG_lock and
   * test the sign bit at the same time. But if the architecture does
   * not support that special operation, we just do this all by hand
   * instead.
   *
   * The read of PG_waiters has to be after (or concurrently with) PG_locked
   * being cleared, but a memory barrier should be unneccssary since it is
   * in the same byte as PG_locked.
   */
  static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem)
  {
  	clear_bit_unlock(nr, mem);
  	/* smp_mb__after_atomic(); */
  	return test_bit(PG_waiters, mem);
  }
  
  #endif
  
  /**
   * unlock_page - unlock a locked page
   * @page: the page
   *
   * Unlocks the page and wakes up sleepers in ___wait_on_page_locked().
   * Also wakes sleepers in wait_on_page_writeback() because the wakeup
   * mechanism between PageLocked pages and PageWriteback pages is shared.
   * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
   *
   * Note that this depends on PG_waiters being the sign bit in the byte
   * that contains PG_locked - thus the BUILD_BUG_ON(). That allows us to
   * clear the PG_locked bit and test PG_waiters at the same time fairly
   * portably (architectures that do LL/SC can test any bit, while x86 can
   * test the sign bit).
   */
  void unlock_page(struct page *page)
  {
  	BUILD_BUG_ON(PG_waiters != 7);
  	page = compound_head(page);
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	if (clear_bit_unlock_is_negative_byte(PG_locked, &page->flags))
  		wake_up_page_bit(page, PG_locked);
  }
  EXPORT_SYMBOL(unlock_page);
  
  /**
   * end_page_writeback - end writeback against a page
   * @page: the page
   */
  void end_page_writeback(struct page *page)
  {
  	/*
  	 * TestClearPageReclaim could be used here but it is an atomic
  	 * operation and overkill in this particular case. Failing to
  	 * shuffle a page marked for immediate reclaim is too mild to
  	 * justify taking an atomic operation penalty at the end of
  	 * ever page writeback.
  	 */
  	if (PageReclaim(page)) {
  		ClearPageReclaim(page);
  		rotate_reclaimable_page(page);
  	}
  
  	if (!test_clear_page_writeback(page))
  		BUG();
  
  	smp_mb__after_atomic();
  	wake_up_page(page, PG_writeback);
  }
  EXPORT_SYMBOL(end_page_writeback);
  
  /*
   * After completing I/O on a page, call this routine to update the page
   * flags appropriately
   */
  void page_endio(struct page *page, bool is_write, int err)
  {
  	if (!is_write) {
  		if (!err) {
  			SetPageUptodate(page);
  		} else {
  			ClearPageUptodate(page);
  			SetPageError(page);
  		}
  		unlock_page(page);
  	} else {
  		if (err) {
  			struct address_space *mapping;
  
  			SetPageError(page);
  			mapping = page_mapping(page);
  			if (mapping)
  				mapping_set_error(mapping, err);
  		}
  		end_page_writeback(page);
  	}
  }
  EXPORT_SYMBOL_GPL(page_endio);
  
  /**
   * __lock_page - get a lock on the page, assuming we need to sleep to get it
   * @__page: the page to lock
   */
  void __lock_page(struct page *__page)
  {
  	struct page *page = compound_head(__page);
  	wait_queue_head_t *q = page_waitqueue(page);
  	wait_on_page_bit_common(q, page, PG_locked, TASK_UNINTERRUPTIBLE, true);
  }
  EXPORT_SYMBOL(__lock_page);
  
  int __lock_page_killable(struct page *__page)
  {
  	struct page *page = compound_head(__page);
  	wait_queue_head_t *q = page_waitqueue(page);
  	return wait_on_page_bit_common(q, page, PG_locked, TASK_KILLABLE, true);
  }
  EXPORT_SYMBOL_GPL(__lock_page_killable);
  
  /*
   * Return values:
   * 1 - page is locked; mmap_sem is still held.
   * 0 - page is not locked.
   *     mmap_sem has been released (up_read()), unless flags had both
   *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
   *     which case mmap_sem is still held.
   *
   * If neither ALLOW_RETRY nor KILLABLE are set, will always return 1
   * with the page locked and the mmap_sem unperturbed.
   */
  int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
  			 unsigned int flags)
  {
  	if (flags & FAULT_FLAG_ALLOW_RETRY) {
  		/*
  		 * CAUTION! In this case, mmap_sem is not released
  		 * even though return 0.
  		 */
  		if (flags & FAULT_FLAG_RETRY_NOWAIT)
  			return 0;
  
  		up_read(&mm->mmap_sem);
  		if (flags & FAULT_FLAG_KILLABLE)
  			wait_on_page_locked_killable(page);
  		else
  			wait_on_page_locked(page);
  		return 0;
  	} else {
  		if (flags & FAULT_FLAG_KILLABLE) {
  			int ret;
  
  			ret = __lock_page_killable(page);
  			if (ret) {
  				up_read(&mm->mmap_sem);
  				return 0;
  			}
  		} else
  			__lock_page(page);
  		return 1;
  	}
  }
  
  /**
   * page_cache_next_hole - find the next hole (not-present entry)
   * @mapping: mapping
   * @index: index
   * @max_scan: maximum range to search
   *
   * Search the set [index, min(index+max_scan-1, MAX_INDEX)] for the
   * lowest indexed hole.
   *
   * Returns: the index of the hole if found, otherwise returns an index
   * outside of the set specified (in which case 'return - index >=
   * max_scan' will be true). In rare cases of index wrap-around, 0 will
   * be returned.
   *
   * page_cache_next_hole may be called under rcu_read_lock. However,
   * like radix_tree_gang_lookup, this will not atomically search a
   * snapshot of the tree at a single point in time. For example, if a
   * hole is created at index 5, then subsequently a hole is created at
   * index 10, page_cache_next_hole covering both indexes may return 10
   * if called under rcu_read_lock.
   */
  pgoff_t page_cache_next_hole(struct address_space *mapping,
  			     pgoff_t index, unsigned long max_scan)
  {
  	unsigned long i;
  
  	for (i = 0; i < max_scan; i++) {
  		struct page *page;
  
  		page = radix_tree_lookup(&mapping->i_pages, index);
  		if (!page || radix_tree_exceptional_entry(page))
  			break;
  		index++;
  		if (index == 0)
  			break;
  	}
  
  	return index;
  }
  EXPORT_SYMBOL(page_cache_next_hole);
  
  /**
   * page_cache_prev_hole - find the prev hole (not-present entry)
   * @mapping: mapping
   * @index: index
   * @max_scan: maximum range to search
   *
   * Search backwards in the range [max(index-max_scan+1, 0), index] for
   * the first hole.
   *
   * Returns: the index of the hole if found, otherwise returns an index
   * outside of the set specified (in which case 'index - return >=
   * max_scan' will be true). In rare cases of wrap-around, ULONG_MAX
   * will be returned.
   *
   * page_cache_prev_hole may be called under rcu_read_lock. However,
   * like radix_tree_gang_lookup, this will not atomically search a
   * snapshot of the tree at a single point in time. For example, if a
   * hole is created at index 10, then subsequently a hole is created at
   * index 5, page_cache_prev_hole covering both indexes may return 5 if
   * called under rcu_read_lock.
   */
  pgoff_t page_cache_prev_hole(struct address_space *mapping,
  			     pgoff_t index, unsigned long max_scan)
  {
  	unsigned long i;
  
  	for (i = 0; i < max_scan; i++) {
  		struct page *page;
  
  		page = radix_tree_lookup(&mapping->i_pages, index);
  		if (!page || radix_tree_exceptional_entry(page))
  			break;
  		index--;
  		if (index == ULONG_MAX)
  			break;
  	}
  
  	return index;
  }
  EXPORT_SYMBOL(page_cache_prev_hole);
  
  /**
   * find_get_entry - find and get a page cache entry
   * @mapping: the address_space to search
   * @offset: the page cache index
   *
   * Looks up the page cache slot at @mapping & @offset.  If there is a
   * page cache page, it is returned with an increased refcount.
   *
   * If the slot holds a shadow entry of a previously evicted page, or a
   * swap entry from shmem/tmpfs, it is returned.
   *
   * Otherwise, %NULL is returned.
   */
  struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
  {
  	void **pagep;
  	struct page *head, *page;
  
  	rcu_read_lock();
  repeat:
  	page = NULL;
  	pagep = radix_tree_lookup_slot(&mapping->i_pages, offset);
  	if (pagep) {
  		page = radix_tree_deref_slot(pagep);
  		if (unlikely(!page))
  			goto out;
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page))
  				goto repeat;
  			/*
  			 * A shadow entry of a recently evicted page,
  			 * or a swap entry from shmem/tmpfs.  Return
  			 * it without attempting to raise page count.
  			 */
  			goto out;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/*
  		 * Has the page moved?
  		 * This is part of the lockless pagecache protocol. See
  		 * include/linux/pagemap.h for details.
  		 */
  		if (unlikely(page != *pagep)) {
  			put_page(head);
  			goto repeat;
  		}
  	}
  out:
  	rcu_read_unlock();
  
  	return page;
  }
  EXPORT_SYMBOL(find_get_entry);
  
  /**
   * find_lock_entry - locate, pin and lock a page cache entry
   * @mapping: the address_space to search
   * @offset: the page cache index
   *
   * Looks up the page cache slot at @mapping & @offset.  If there is a
   * page cache page, it is returned locked and with an increased
   * refcount.
   *
   * If the slot holds a shadow entry of a previously evicted page, or a
   * swap entry from shmem/tmpfs, it is returned.
   *
   * Otherwise, %NULL is returned.
   *
   * find_lock_entry() may sleep.
   */
  struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)
  {
  	struct page *page;
  
  repeat:
  	page = find_get_entry(mapping, offset);
  	if (page && !radix_tree_exception(page)) {
  		lock_page(page);
  		/* Has the page been truncated? */
  		if (unlikely(page_mapping(page) != mapping)) {
  			unlock_page(page);
  			put_page(page);
  			goto repeat;
  		}
  		VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
  	}
  	return page;
  }
  EXPORT_SYMBOL(find_lock_entry);
  
  /**
   * pagecache_get_page - find and get a page reference
   * @mapping: the address_space to search
   * @offset: the page index
   * @fgp_flags: PCG flags
   * @gfp_mask: gfp mask to use for the page cache data page allocation
   *
   * Looks up the page cache slot at @mapping & @offset.
   *
   * PCG flags modify how the page is returned.
   *
   * @fgp_flags can be:
   *
   * - FGP_ACCESSED: the page will be marked accessed
   * - FGP_LOCK: Page is return locked
   * - FGP_CREAT: If page is not present then a new page is allocated using
   *   @gfp_mask and added to the page cache and the VM's LRU
   *   list. The page is returned locked and with an increased
   *   refcount. Otherwise, NULL is returned.
   *
   * If FGP_LOCK or FGP_CREAT are specified then the function may sleep even
   * if the GFP flags specified for FGP_CREAT are atomic.
   *
   * If there is a page cache page, it is returned with an increased refcount.
   */
  struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
  	int fgp_flags, gfp_t gfp_mask)
  {
  	struct page *page;
  
  repeat:
  	page = find_get_entry(mapping, offset);
  	if (radix_tree_exceptional_entry(page))
  		page = NULL;
  	if (!page)
  		goto no_page;
  
  	if (fgp_flags & FGP_LOCK) {
  		if (fgp_flags & FGP_NOWAIT) {
  			if (!trylock_page(page)) {
  				put_page(page);
  				return NULL;
  			}
  		} else {
  			lock_page(page);
  		}
  
  		/* Has the page been truncated? */
  		if (unlikely(page->mapping != mapping)) {
  			unlock_page(page);
  			put_page(page);
  			goto repeat;
  		}
  		VM_BUG_ON_PAGE(page->index != offset, page);
  	}
  
  	if (page && (fgp_flags & FGP_ACCESSED))
  		mark_page_accessed(page);
  
  no_page:
  	if (!page && (fgp_flags & FGP_CREAT)) {
  		int err;
  		if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
  			gfp_mask |= __GFP_WRITE;
  		if (fgp_flags & FGP_NOFS)
  			gfp_mask &= ~__GFP_FS;
  
  		page = __page_cache_alloc(gfp_mask);
  		if (!page)
  			return NULL;
  
  		if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK)))
  			fgp_flags |= FGP_LOCK;
  
  		/* Init accessed so avoid atomic mark_page_accessed later */
  		if (fgp_flags & FGP_ACCESSED)
  			__SetPageReferenced(page);
  
  		err = add_to_page_cache_lru(page, mapping, offset, gfp_mask);
  		if (unlikely(err)) {
  			put_page(page);
  			page = NULL;
  			if (err == -EEXIST)
  				goto repeat;
  		}
  	}
  
  	return page;
  }
  EXPORT_SYMBOL(pagecache_get_page);
  
  /**
   * find_get_entries - gang pagecache lookup
   * @mapping:	The address_space to search
   * @start:	The starting page cache index
   * @nr_entries:	The maximum number of entries
   * @entries:	Where the resulting entries are placed
   * @indices:	The cache indices corresponding to the entries in @entries
   *
   * find_get_entries() will search for and return a group of up to
   * @nr_entries entries in the mapping.  The entries are placed at
   * @entries.  find_get_entries() takes a reference against any actual
   * pages it returns.
   *
   * The search returns a group of mapping-contiguous page cache entries
   * with ascending indexes.  There may be holes in the indices due to
   * not-present pages.
   *
   * Any shadow entries of evicted pages, or swap entries from
   * shmem/tmpfs, are included in the returned array.
   *
   * find_get_entries() returns the number of pages and shadow entries
   * which were found.
   */
  unsigned find_get_entries(struct address_space *mapping,
  			  pgoff_t start, unsigned int nr_entries,
  			  struct page **entries, pgoff_t *indices)
  {
  	void **slot;
  	unsigned int ret = 0;
  	struct radix_tree_iter iter;
  
  	if (!nr_entries)
  		return 0;
  
  	rcu_read_lock();
  	radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
  		struct page *head, *page;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				slot = radix_tree_iter_retry(&iter);
  				continue;
  			}
  			/*
  			 * A shadow entry of a recently evicted page, a swap
  			 * entry from shmem/tmpfs or a DAX entry.  Return it
  			 * without attempting to raise page count.
  			 */
  			goto export;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			put_page(head);
  			goto repeat;
  		}
  export:
  		indices[ret] = iter.index;
  		entries[ret] = page;
  		if (++ret == nr_entries)
  			break;
  	}
  	rcu_read_unlock();
  	return ret;
  }
  
  /**
   * find_get_pages_range - gang pagecache lookup
   * @mapping:	The address_space to search
   * @start:	The starting page index
   * @end:	The final page index (inclusive)
   * @nr_pages:	The maximum number of pages
   * @pages:	Where the resulting pages are placed
   *
   * find_get_pages_range() will search for and return a group of up to @nr_pages
   * pages in the mapping starting at index @start and up to index @end
   * (inclusive).  The pages are placed at @pages.  find_get_pages_range() takes
   * a reference against the returned pages.
   *
   * The search returns a group of mapping-contiguous pages with ascending
   * indexes.  There may be holes in the indices due to not-present pages.
   * We also update @start to index the next page for the traversal.
   *
   * find_get_pages_range() returns the number of pages which were found. If this
   * number is smaller than @nr_pages, the end of specified range has been
   * reached.
   */
  unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
  			      pgoff_t end, unsigned int nr_pages,
  			      struct page **pages)
  {
  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned ret = 0;
  
  	if (unlikely(!nr_pages))
  		return 0;
  
  	rcu_read_lock();
  	radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, *start) {
  		struct page *head, *page;
  
  		if (iter.index > end)
  			break;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				slot = radix_tree_iter_retry(&iter);
  				continue;
  			}
  			/*
  			 * A shadow entry of a recently evicted page,
  			 * or a swap entry from shmem/tmpfs.  Skip
  			 * over it.
  			 */
  			continue;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			put_page(head);
  			goto repeat;
  		}
  
  		pages[ret] = page;
  		if (++ret == nr_pages) {
  			*start = pages[ret - 1]->index + 1;
  			goto out;
  		}
  	}
  
  	/*
  	 * We come here when there is no page beyond @end. We take care to not
  	 * overflow the index @start as it confuses some of the callers. This
  	 * breaks the iteration when there is page at index -1 but that is
  	 * already broken anyway.
  	 */
  	if (end == (pgoff_t)-1)
  		*start = (pgoff_t)-1;
  	else
  		*start = end + 1;
  out:
  	rcu_read_unlock();
  
  	return ret;
  }
  
  /**
   * find_get_pages_contig - gang contiguous pagecache lookup
   * @mapping:	The address_space to search
   * @index:	The starting page index
   * @nr_pages:	The maximum number of pages
   * @pages:	Where the resulting pages are placed
   *
   * find_get_pages_contig() works exactly like find_get_pages(), except
   * that the returned number of pages are guaranteed to be contiguous.
   *
   * find_get_pages_contig() returns the number of pages which were found.
   */
  unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
  			       unsigned int nr_pages, struct page **pages)
  {
  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned int ret = 0;
  
  	if (unlikely(!nr_pages))
  		return 0;
  
  	rcu_read_lock();
  	radix_tree_for_each_contig(slot, &mapping->i_pages, &iter, index) {
  		struct page *head, *page;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		/* The hole, there no reason to continue */
  		if (unlikely(!page))
  			break;
  
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				slot = radix_tree_iter_retry(&iter);
  				continue;
  			}
  			/*
  			 * A shadow entry of a recently evicted page,
  			 * or a swap entry from shmem/tmpfs.  Stop
  			 * looking for contiguous pages.
  			 */
  			break;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/*
  		 * must check mapping and index after taking the ref.
  		 * otherwise we can get both false positives and false
  		 * negatives, which is just confusing to the caller.
  		 */
  		if (page->mapping == NULL || page_to_pgoff(page) != iter.index) {
  			put_page(page);
  			break;
  		}
  
  		pages[ret] = page;
  		if (++ret == nr_pages)
  			break;
  	}
  	rcu_read_unlock();
  	return ret;
  }
  EXPORT_SYMBOL(find_get_pages_contig);
  
  /**
   * find_get_pages_range_tag - find and return pages in given range matching @tag
   * @mapping:	the address_space to search
   * @index:	the starting page index
   * @end:	The final page index (inclusive)
   * @tag:	the tag index
   * @nr_pages:	the maximum number of pages
   * @pages:	where the resulting pages are placed
   *
   * Like find_get_pages, except we only return pages which are tagged with
   * @tag.   We update @index to index the next page for the traversal.
   */
  unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
  			pgoff_t end, int tag, unsigned int nr_pages,
  			struct page **pages)
  {
  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned ret = 0;
  
  	if (unlikely(!nr_pages))
  		return 0;
  
  	rcu_read_lock();
  	radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter, *index, tag) {
  		struct page *head, *page;
  
  		if (iter.index > end)
  			break;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				slot = radix_tree_iter_retry(&iter);
  				continue;
  			}
  			/*
  			 * A shadow entry of a recently evicted page.
  			 *
  			 * Those entries should never be tagged, but
  			 * this tree walk is lockless and the tags are
  			 * looked up in bulk, one radix tree node at a
  			 * time, so there is a sizable window for page
  			 * reclaim to evict a page we saw tagged.
  			 *
  			 * Skip over it.
  			 */
  			continue;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			put_page(head);
  			goto repeat;
  		}
  
  		pages[ret] = page;
  		if (++ret == nr_pages) {
  			*index = pages[ret - 1]->index + 1;
  			goto out;
  		}
  	}
  
  	/*
  	 * We come here when we got at @end. We take care to not overflow the
  	 * index @index as it confuses some of the callers. This breaks the
  	 * iteration when there is page at index -1 but that is already broken
  	 * anyway.
  	 */
  	if (end == (pgoff_t)-1)
  		*index = (pgoff_t)-1;
  	else
  		*index = end + 1;
  out:
  	rcu_read_unlock();
  
  	return ret;
  }
  EXPORT_SYMBOL(find_get_pages_range_tag);
  
  /**
   * find_get_entries_tag - find and return entries that match @tag
   * @mapping:	the address_space to search
   * @start:	the starting page cache index
   * @tag:	the tag index
   * @nr_entries:	the maximum number of entries
   * @entries:	where the resulting entries are placed
   * @indices:	the cache indices corresponding to the entries in @entries
   *
   * Like find_get_entries, except we only return entries which are tagged with
   * @tag.
   */
  unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
  			int tag, unsigned int nr_entries,
  			struct page **entries, pgoff_t *indices)
  {
  	void **slot;
  	unsigned int ret = 0;
  	struct radix_tree_iter iter;
  
  	if (!nr_entries)
  		return 0;
  
  	rcu_read_lock();
  	radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter, start, tag) {
  		struct page *head, *page;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				slot = radix_tree_iter_retry(&iter);
  				continue;
  			}
  
  			/*
  			 * A shadow entry of a recently evicted page, a swap
  			 * entry from shmem/tmpfs or a DAX entry.  Return it
  			 * without attempting to raise page count.
  			 */
  			goto export;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			put_page(head);
  			goto repeat;
  		}
  export:
  		indices[ret] = iter.index;
  		entries[ret] = page;
  		if (++ret == nr_entries)
  			break;
  	}
  	rcu_read_unlock();
  	return ret;
  }
  EXPORT_SYMBOL(find_get_entries_tag);
  
  /*
   * CD/DVDs are error prone. When a medium error occurs, the driver may fail
   * a _large_ part of the i/o request. Imagine the worst scenario:
   *
   *      ---R__________________________________________B__________
   *         ^ reading here                             ^ bad block(assume 4k)
   *
   * read(R) => miss => readahead(R...B) => media error => frustrating retries
   * => failing the whole request => read(R) => read(R+1) =>
   * readahead(R+1...B+1) => bang => read(R+2) => read(R+3) =>
   * readahead(R+3...B+2) => bang => read(R+3) => read(R+4) =>
   * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
   *
   * It is going insane. Fix it by quickly scaling down the readahead size.
   */
  static void shrink_readahead_size_eio(struct file *filp,
  					struct file_ra_state *ra)
  {
  	ra->ra_pages /= 4;
  }
  
  /**
   * generic_file_buffered_read - generic file read routine
   * @iocb:	the iocb to read
   * @iter:	data destination
   * @written:	already copied
   *
   * This is a generic file read routine, and uses the
   * mapping->a_ops->readpage() function for the actual low-level stuff.
   *
   * This is really ugly. But the goto's actually try to clarify some
   * of the logic when it comes to error handling etc.
   */
  static ssize_t generic_file_buffered_read(struct kiocb *iocb,
  		struct iov_iter *iter, ssize_t written)
  {
  	struct file *filp = iocb->ki_filp;
  	struct address_space *mapping = filp->f_mapping;
  	struct inode *inode = mapping->host;
  	struct file_ra_state *ra = &filp->f_ra;
  	loff_t *ppos = &iocb->ki_pos;
  	pgoff_t index;
  	pgoff_t last_index;
  	pgoff_t prev_index;
  	unsigned long offset;      /* offset into pagecache page */
  	unsigned int prev_offset;
  	int error = 0;
  
  	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
  		return 0;
  	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
  
  	index = *ppos >> PAGE_SHIFT;
  	prev_index = ra->prev_pos >> PAGE_SHIFT;
  	prev_offset = ra->prev_pos & (PAGE_SIZE-1);
  	last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
  	offset = *ppos & ~PAGE_MASK;
  
  	for (;;) {
  		struct page *page;
  		pgoff_t end_index;
  		loff_t isize;
  		unsigned long nr, ret;
  
  		cond_resched();
  find_page:
  		if (fatal_signal_pending(current)) {
  			error = -EINTR;
  			goto out;
  		}
  
  		page = find_get_page(mapping, index);
  		if (!page) {
  			if (iocb->ki_flags & IOCB_NOWAIT)
  				goto would_block;
  			page_cache_sync_readahead(mapping,
  					ra, filp,
  					index, last_index - index);
  			page = find_get_page(mapping, index);
  			if (unlikely(page == NULL))
  				goto no_cached_page;
  		}
  		if (PageReadahead(page)) {
  			page_cache_async_readahead(mapping,
  					ra, filp, page,
  					index, last_index - index);
  		}
  		if (!PageUptodate(page)) {
  			if (iocb->ki_flags & IOCB_NOWAIT) {
  				put_page(page);
  				goto would_block;
  			}
  
  			/*
  			 * See comment in do_read_cache_page on why
  			 * wait_on_page_locked is used to avoid unnecessarily
  			 * serialisations and why it's safe.
  			 */
  			error = wait_on_page_locked_killable(page);
  			if (unlikely(error))
  				goto readpage_error;
  			if (PageUptodate(page))
  				goto page_ok;
  
  			if (inode->i_blkbits == PAGE_SHIFT ||
  					!mapping->a_ops->is_partially_uptodate)
  				goto page_not_up_to_date;
  			/* pipes can't handle partially uptodate pages */
  			if (unlikely(iter->type & ITER_PIPE))
  				goto page_not_up_to_date;
  			if (!trylock_page(page))
  				goto page_not_up_to_date;
  			/* Did it get truncated before we got the lock? */
  			if (!page->mapping)
  				goto page_not_up_to_date_locked;
  			if (!mapping->a_ops->is_partially_uptodate(page,
  							offset, iter->count))
  				goto page_not_up_to_date_locked;
  			unlock_page(page);
  		}
  page_ok:
  		/*
  		 * i_size must be checked after we know the page is Uptodate.
  		 *
  		 * Checking i_size after the check allows us to calculate
  		 * the correct value for "nr", which means the zero-filled
  		 * part of the page is not copied back to userspace (unless
  		 * another truncate extends the file - this is desired though).
  		 */
  
  		isize = i_size_read(inode);
  		end_index = (isize - 1) >> PAGE_SHIFT;
  		if (unlikely(!isize || index > end_index)) {
  			put_page(page);
  			goto out;
  		}
  
  		/* nr is the maximum number of bytes to copy from this page */
  		nr = PAGE_SIZE;
  		if (index == end_index) {
  			nr = ((isize - 1) & ~PAGE_MASK) + 1;
  			if (nr <= offset) {
  				put_page(page);
  				goto out;
  			}
  		}
  		nr = nr - offset;
  
  		/* If users can be writing to this page using arbitrary
  		 * virtual addresses, take care about potential aliasing
  		 * before reading the page on the kernel side.
  		 */
  		if (mapping_writably_mapped(mapping))
  			flush_dcache_page(page);
  
  		/*
  		 * When a sequential read accesses a page several times,
  		 * only mark it as accessed the first time.
  		 */
  		if (prev_index != index || offset != prev_offset)
  			mark_page_accessed(page);
  		prev_index = index;
  
  		/*
  		 * Ok, we have the page, and it's up-to-date, so
  		 * now we can copy it to user space...
  		 */
  
  		ret = copy_page_to_iter(page, offset, nr, iter);
  		offset += ret;
  		index += offset >> PAGE_SHIFT;
  		offset &= ~PAGE_MASK;
  		prev_offset = offset;
  
  		put_page(page);
  		written += ret;
  		if (!iov_iter_count(iter))
  			goto out;
  		if (ret < nr) {
  			error = -EFAULT;
  			goto out;
  		}
  		continue;
  
  page_not_up_to_date:
  		/* Get exclusive access to the page ... */
  		error = lock_page_killable(page);
  		if (unlikely(error))
  			goto readpage_error;
  
  page_not_up_to_date_locked:
  		/* Did it get truncated before we got the lock? */
  		if (!page->mapping) {
  			unlock_page(page);
  			put_page(page);
  			continue;
  		}
  
  		/* Did somebody else fill it already? */
  		if (PageUptodate(page)) {
  			unlock_page(page);
  			goto page_ok;
  		}
  
  readpage:
  		/*
  		 * A previous I/O error may have been due to temporary
  		 * failures, eg. multipath errors.
  		 * PG_error will be set again if readpage fails.
  		 */
  		ClearPageError(page);
  		/* Start the actual read. The read will unlock the page. */
  		error = mapping->a_ops->readpage(filp, page);
  
  		if (unlikely(error)) {
  			if (error == AOP_TRUNCATED_PAGE) {
  				put_page(page);
  				error = 0;
  				goto find_page;
  			}
  			goto readpage_error;
  		}
  
  		if (!PageUptodate(page)) {
  			error = lock_page_killable(page);
  			if (unlikely(error))
  				goto readpage_error;
  			if (!PageUptodate(page)) {
  				if (page->mapping == NULL) {
  					/*
  					 * invalidate_mapping_pages got it
  					 */
  					unlock_page(page);
  					put_page(page);
  					goto find_page;
  				}
  				unlock_page(page);
  				shrink_readahead_size_eio(filp, ra);
  				error = -EIO;
  				goto readpage_error;
  			}
  			unlock_page(page);
  		}
  
  		goto page_ok;
  
  readpage_error:
  		/* UHHUH! A synchronous read error occurred. Report it */
  		put_page(page);
  		goto out;
  
  no_cached_page:
  		/*
  		 * Ok, it wasn't cached, so we need to create a new
  		 * page..
  		 */
  		page = page_cache_alloc(mapping);
  		if (!page) {
  			error = -ENOMEM;
  			goto out;
  		}
  		error = add_to_page_cache_lru(page, mapping, index,
  				mapping_gfp_constraint(mapping, GFP_KERNEL));
  		if (error) {
  			put_page(page);
  			if (error == -EEXIST) {
  				error = 0;
  				goto find_page;
  			}
  			goto out;
  		}
  		goto readpage;
  	}
  
  would_block:
  	error = -EAGAIN;
  out:
  	ra->prev_pos = prev_index;
  	ra->prev_pos <<= PAGE_SHIFT;
  	ra->prev_pos |= prev_offset;
  
  	*ppos = ((loff_t)index << PAGE_SHIFT) + offset;
  	file_accessed(filp);
  	return written ? written : error;
  }
  
  /**
   * generic_file_read_iter - generic filesystem read routine
   * @iocb:	kernel I/O control block
   * @iter:	destination for the data read
   *
   * This is the "read_iter()" routine for all filesystems
   * that can use the page cache directly.
   */
  ssize_t
  generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
  {
  	size_t count = iov_iter_count(iter);
  	ssize_t retval = 0;
  
  	if (!count)
  		goto out; /* skip atime */
  
  	if (iocb->ki_flags & IOCB_DIRECT) {
  		struct file *file = iocb->ki_filp;
  		struct address_space *mapping = file->f_mapping;
  		struct inode *inode = mapping->host;
  		loff_t size;
  
  		size = i_size_read(inode);
  		if (iocb->ki_flags & IOCB_NOWAIT) {
  			if (filemap_range_has_page(mapping, iocb->ki_pos,
  						   iocb->ki_pos + count - 1))
  				return -EAGAIN;
  		} else {
  			retval = filemap_write_and_wait_range(mapping,
  						iocb->ki_pos,
  					        iocb->ki_pos + count - 1);
  			if (retval < 0)
  				goto out;
  		}
  
  		file_accessed(file);
  
  		retval = mapping->a_ops->direct_IO(iocb, iter);
  		if (retval >= 0) {
  			iocb->ki_pos += retval;
  			count -= retval;
  		}
  		iov_iter_revert(iter, count - iov_iter_count(iter));
  
  		/*
  		 * Btrfs can have a short DIO read if we encounter
  		 * compressed extents, so if there was an error, or if
  		 * we've already read everything we wanted to, or if
  		 * there was a short read because we hit EOF, go ahead
  		 * and return.  Otherwise fallthrough to buffered io for
  		 * the rest of the read.  Buffered reads will not work for
  		 * DAX files, so don't bother trying.
  		 */
  		if (retval < 0 || !count || iocb->ki_pos >= size ||
  		    IS_DAX(inode))
  			goto out;
  	}
  
  	retval = generic_file_buffered_read(iocb, iter, retval);
  out:
  	return retval;
  }
  EXPORT_SYMBOL(generic_file_read_iter);
  
  #ifdef CONFIG_MMU
  /**
   * page_cache_read - adds requested page to the page cache if not already there
   * @file:	file to read
   * @offset:	page index
   * @gfp_mask:	memory allocation flags
   *
   * This adds the requested page to the page cache if it isn't already there,
   * and schedules an I/O to read in its contents from disk.
   */
  static int page_cache_read(struct file *file, pgoff_t offset, gfp_t gfp_mask)
  {
  	struct address_space *mapping = file->f_mapping;
  	struct page *page;
  	int ret;
  
  	do {
  		page = __page_cache_alloc(gfp_mask);
  		if (!page)
  			return -ENOMEM;
  
  		ret = add_to_page_cache_lru(page, mapping, offset, gfp_mask);
  		if (ret == 0)
  			ret = mapping->a_ops->readpage(file, page);
  		else if (ret == -EEXIST)
  			ret = 0; /* losing race to add is OK */
  
  		put_page(page);
  
  	} while (ret == AOP_TRUNCATED_PAGE);
  
  	return ret;
  }
  
  #define MMAP_LOTSAMISS  (100)
  
  /*
   * Synchronous readahead happens when we don't even find
   * a page in the page cache at all.
   */
  static void do_sync_mmap_readahead(struct vm_area_struct *vma,
  				   struct file_ra_state *ra,
  				   struct file *file,
  				   pgoff_t offset)
  {
  	struct address_space *mapping = file->f_mapping;
  
  	/* If we don't want any read-ahead, don't bother */
  	if (vma->vm_flags & VM_RAND_READ)
  		return;
  	if (!ra->ra_pages)
  		return;
  
  	if (vma->vm_flags & VM_SEQ_READ) {
  		page_cache_sync_readahead(mapping, ra, file, offset,
  					  ra->ra_pages);
  		return;
  	}
  
  	/* Avoid banging the cache line if not needed */
  	if (ra->mmap_miss < MMAP_LOTSAMISS * 10)
  		ra->mmap_miss++;
  
  	/*
  	 * Do we miss much more than hit in this file? If so,
  	 * stop bothering with read-ahead. It will only hurt.
  	 */
  	if (ra->mmap_miss > MMAP_LOTSAMISS)
  		return;
  
  	/*
  	 * mmap read-around
  	 */
  	ra->start = max_t(long, 0, offset - ra->ra_pages / 2);
  	ra->size = ra->ra_pages;
  	ra->async_size = ra->ra_pages / 4;
  	ra_submit(ra, mapping, file);
  }
  
  /*
   * Asynchronous readahead happens when we find the page and PG_readahead,
   * so we want to possibly extend the readahead further..
   */
  static void do_async_mmap_readahead(struct vm_area_struct *vma,
  				    struct file_ra_state *ra,
  				    struct file *file,
  				    struct page *page,
  				    pgoff_t offset)
  {
  	struct address_space *mapping = file->f_mapping;
  
  	/* If we don't want any read-ahead, don't bother */
  	if (vma->vm_flags & VM_RAND_READ)
  		return;
  	if (ra->mmap_miss > 0)
  		ra->mmap_miss--;
  	if (PageReadahead(page))
  		page_cache_async_readahead(mapping, ra, file,
  					   page, offset, ra->ra_pages);
  }
  
  /**
   * filemap_fault - read in file data for page fault handling
   * @vmf:	struct vm_fault containing details of the fault
   *
   * filemap_fault() is invoked via the vma operations vector for a
   * mapped memory region to read in file data during a page fault.
   *
   * The goto's are kind of ugly, but this streamlines the normal case of having
   * it in the page cache, and handles the special cases reasonably without
   * having a lot of duplicated code.
   *
   * vma->vm_mm->mmap_sem must be held on entry.
   *
   * If our return value has VM_FAULT_RETRY set, it's because
   * lock_page_or_retry() returned 0.
   * The mmap_sem has usually been released in this case.
   * See __lock_page_or_retry() for the exception.
   *
   * If our return value does not have VM_FAULT_RETRY set, the mmap_sem
   * has not been released.
   *
   * We never return with VM_FAULT_RETRY and a bit from VM_FAULT_ERROR set.
   */
  int filemap_fault(struct vm_fault *vmf)
  {
  	int error;
  	struct file *file = vmf->vma->vm_file;
  	struct address_space *mapping = file->f_mapping;
  	struct file_ra_state *ra = &file->f_ra;
  	struct inode *inode = mapping->host;
  	pgoff_t offset = vmf->pgoff;
  	pgoff_t max_off;
  	struct page *page;
  	int ret = 0;
  
  	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
  	if (unlikely(offset >= max_off))
  		return VM_FAULT_SIGBUS;
  
  	/*
  	 * Do we have something in the page cache already?
  	 */
  	page = find_get_page(mapping, offset);
  	if (likely(page) && !(vmf->flags & FAULT_FLAG_TRIED)) {
  		/*
  		 * We found the page, so try async readahead before
  		 * waiting for the lock.
  		 */
  		do_async_mmap_readahead(vmf->vma, ra, file, page, offset);
  	} else if (!page) {
  		/* No page in the page cache at all */
  		do_sync_mmap_readahead(vmf->vma, ra, file, offset);
  		count_vm_event(PGMAJFAULT);
  		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
  		ret = VM_FAULT_MAJOR;
  retry_find:
  		page = find_get_page(mapping, offset);
  		if (!page)
  			goto no_cached_page;
  	}
  
  	if (!lock_page_or_retry(page, vmf->vma->vm_mm, vmf->flags)) {
  		put_page(page);
  		return ret | VM_FAULT_RETRY;
  	}
  
  	/* Did it get truncated? */
  	if (unlikely(page->mapping != mapping)) {
  		unlock_page(page);
  		put_page(page);
  		goto retry_find;
  	}
  	VM_BUG_ON_PAGE(page->index != offset, page);
  
  	/*
  	 * We have a locked page in the page cache, now we need to check
  	 * that it's up-to-date. If not, it is going to be due to an error.
  	 */
  	if (unlikely(!PageUptodate(page)))
  		goto page_not_uptodate;
  
  	/*
  	 * Found the page and have a reference on it.
  	 * We must recheck i_size under page lock.
  	 */
  	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
  	if (unlikely(offset >= max_off)) {
  		unlock_page(page);
  		put_page(page);
  		return VM_FAULT_SIGBUS;
  	}
  
  	vmf->page = page;
  	return ret | VM_FAULT_LOCKED;
  
  no_cached_page:
  	/*
  	 * We're only likely to ever get here if MADV_RANDOM is in
  	 * effect.
  	 */
  	error = page_cache_read(file, offset, vmf->gfp_mask);
  
  	/*
  	 * The page we want has now been added to the page cache.
  	 * In the unlikely event that someone removed it in the
  	 * meantime, we'll just come back here and read it again.
  	 */
  	if (error >= 0)
  		goto retry_find;
  
  	/*
  	 * An error return from page_cache_read can result if the
  	 * system is low on memory, or a problem occurs while trying
  	 * to schedule I/O.
  	 */
  	if (error == -ENOMEM)
  		return VM_FAULT_OOM;
  	return VM_FAULT_SIGBUS;
  
  page_not_uptodate:
  	/*
  	 * Umm, take care of errors if the page isn't up-to-date.
  	 * Try to re-read it _once_. We do this synchronously,
  	 * because there really aren't any performance issues here
  	 * and we need to check for errors.
  	 */
  	ClearPageError(page);
  	error = mapping->a_ops->readpage(file, page);
  	if (!error) {
  		wait_on_page_locked(page);
  		if (!PageUptodate(page))
  			error = -EIO;
  	}
  	put_page(page);
  
  	if (!error || error == AOP_TRUNCATED_PAGE)
  		goto retry_find;
  
  	/* Things didn't work out. Return zero to tell the mm layer so. */
  	shrink_readahead_size_eio(file, ra);
  	return VM_FAULT_SIGBUS;
  }
  EXPORT_SYMBOL(filemap_fault);
  
  void filemap_map_pages(struct vm_fault *vmf,
  		pgoff_t start_pgoff, pgoff_t end_pgoff)
  {
  	struct radix_tree_iter iter;
  	void **slot;
  	struct file *file = vmf->vma->vm_file;
  	struct address_space *mapping = file->f_mapping;
  	pgoff_t last_pgoff = start_pgoff;
  	unsigned long max_idx;
  	struct page *head, *page;
  
  	rcu_read_lock();
  	radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start_pgoff) {
  		if (iter.index > end_pgoff)
  			break;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			goto next;
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				slot = radix_tree_iter_retry(&iter);
  				continue;
  			}
  			goto next;
  		}
  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			put_page(head);
  			goto repeat;
  		}
  
  		if (!PageUptodate(page) ||
  				PageReadahead(page) ||
  				PageHWPoison(page))
  			goto skip;
  		if (!trylock_page(page))
  			goto skip;
  
  		if (page->mapping != mapping || !PageUptodate(page))
  			goto unlock;
  
  		max_idx = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
  		if (page->index >= max_idx)
  			goto unlock;
  
  		if (file->f_ra.mmap_miss > 0)
  			file->f_ra.mmap_miss--;
  
  		vmf->address += (iter.index - last_pgoff) << PAGE_SHIFT;
  		if (vmf->pte)
  			vmf->pte += iter.index - last_pgoff;
  		last_pgoff = iter.index;
  		if (alloc_set_pte(vmf, NULL, page))
  			goto unlock;
  		unlock_page(page);
  		goto next;
  unlock:
  		unlock_page(page);
  skip:
  		put_page(page);
  next:
  		/* Huge page is mapped? No need to proceed. */
  		if (pmd_trans_huge(*vmf->pmd))
  			break;
  		if (iter.index == end_pgoff)
  			break;
  	}
  	rcu_read_unlock();
  }
  EXPORT_SYMBOL(filemap_map_pages);
  
  int filemap_page_mkwrite(struct vm_fault *vmf)
  {
  	struct page *page = vmf->page;
  	struct inode *inode = file_inode(vmf->vma->vm_file);
  	int ret = VM_FAULT_LOCKED;
  
  	sb_start_pagefault(inode->i_sb);
  	file_update_time(vmf->vma->vm_file);
  	lock_page(page);
  	if (page->mapping != inode->i_mapping) {
  		unlock_page(page);
  		ret = VM_FAULT_NOPAGE;
  		goto out;
  	}
  	/*
  	 * We mark the page dirty already here so that when freeze is in
  	 * progress, we are guaranteed that writeback during freezing will
  	 * see the dirty page and writeprotect it again.
  	 */
  	set_page_dirty(page);
  	wait_for_stable_page(page);
  out:
  	sb_end_pagefault(inode->i_sb);
  	return ret;
  }
  
  const struct vm_operations_struct generic_file_vm_ops = {
  	.fault		= filemap_fault,
  	.map_pages	= filemap_map_pages,
  	.page_mkwrite	= filemap_page_mkwrite,
  };
  
  /* This is used for a general mmap of a disk file */
  
  int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
  {
  	struct address_space *mapping = file->f_mapping;
  
  	if (!mapping->a_ops->readpage)
  		return -ENOEXEC;
  	file_accessed(file);
  	vma->vm_ops = &generic_file_vm_ops;
  	return 0;
  }
  
  /*
   * This is for filesystems which do not implement ->writepage.
   */
  int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
  {
  	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
  		return -EINVAL;
  	return generic_file_mmap(file, vma);
  }
  #else
  int filemap_page_mkwrite(struct vm_fault *vmf)
  {
  	return -ENOSYS;
  }
  int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
  {
  	return -ENOSYS;
  }
  int generic_file_readonly_mmap(struct file * file, struct vm_area_struct * vma)
  {
  	return -ENOSYS;
  }
  #endif /* CONFIG_MMU */
  
  EXPORT_SYMBOL(filemap_page_mkwrite);
  EXPORT_SYMBOL(generic_file_mmap);
  EXPORT_SYMBOL(generic_file_readonly_mmap);
  
  static struct page *wait_on_page_read(struct page *page)
  {
  	if (!IS_ERR(page)) {
  		wait_on_page_locked(page);
  		if (!PageUptodate(page)) {
  			put_page(page);
  			page = ERR_PTR(-EIO);
  		}
  	}
  	return page;
  }
  
  static struct page *do_read_cache_page(struct address_space *mapping,
  				pgoff_t index,
  				int (*filler)(void *, struct page *),
  				void *data,
  				gfp_t gfp)
  {
  	struct page *page;
  	int err;
  repeat:
  	page = find_get_page(mapping, index);
  	if (!page) {
  		page = __page_cache_alloc(gfp);
  		if (!page)
  			return ERR_PTR(-ENOMEM);
  		err = add_to_page_cache_lru(page, mapping, index, gfp);
  		if (unlikely(err)) {
  			put_page(page);
  			if (err == -EEXIST)
  				goto repeat;
  			/* Presumably ENOMEM for radix tree node */
  			return ERR_PTR(err);
  		}
  
  filler:
  		err = filler(data, page);
  		if (err < 0) {
  			put_page(page);
  			return ERR_PTR(err);
  		}
  
  		page = wait_on_page_read(page);
  		if (IS_ERR(page))
  			return page;
  		goto out;
  	}
  	if (PageUptodate(page))
  		goto out;
  
  	/*
  	 * Page is not up to date and may be locked due one of the following
  	 * case a: Page is being filled and the page lock is held
  	 * case b: Read/write error clearing the page uptodate status
  	 * case c: Truncation in progress (page locked)
  	 * case d: Reclaim in progress
  	 *
  	 * Case a, the page will be up to date when the page is unlocked.
  	 *    There is no need to serialise on the page lock here as the page
  	 *    is pinned so the lock gives no additional protection. Even if the
  	 *    the page is truncated, the data is still valid if PageUptodate as
  	 *    it's a race vs truncate race.
  	 * Case b, the page will not be up to date
  	 * Case c, the page may be truncated but in itself, the data may still
  	 *    be valid after IO completes as it's a read vs truncate race. The
  	 *    operation must restart if the page is not uptodate on unlock but
  	 *    otherwise serialising on page lock to stabilise the mapping gives
  	 *    no additional guarantees to the caller as the page lock is
  	 *    released before return.
  	 * Case d, similar to truncation. If reclaim holds the page lock, it
  	 *    will be a race with remove_mapping that determines if the mapping
  	 *    is valid on unlock but otherwise the data is valid and there is
  	 *    no need to serialise with page lock.
  	 *
  	 * As the page lock gives no additional guarantee, we optimistically
  	 * wait on the page to be unlocked and check if it's up to date and
  	 * use the page if it is. Otherwise, the page lock is required to
  	 * distinguish between the different cases. The motivation is that we
  	 * avoid spurious serialisations and wakeups when multiple processes
  	 * wait on the same page for IO to complete.
  	 */
  	wait_on_page_locked(page);
  	if (PageUptodate(page))
  		goto out;
  
  	/* Distinguish between all the cases under the safety of the lock */
  	lock_page(page);
  
  	/* Case c or d, restart the operation */
  	if (!page->mapping) {
  		unlock_page(page);
  		put_page(page);
  		goto repeat;
  	}
  
  	/* Someone else locked and filled the page in a very small window */
  	if (PageUptodate(page)) {
  		unlock_page(page);
  		goto out;
  	}
  	goto filler;
  
  out:
  	mark_page_accessed(page);
  	return page;
  }
  
  /**
   * read_cache_page - read into page cache, fill it if needed
   * @mapping:	the page's address_space
   * @index:	the page index
   * @filler:	function to perform the read
   * @data:	first arg to filler(data, page) function, often left as NULL
   *
   * Read into the page cache. If a page already exists, and PageUptodate() is
   * not set, try to fill the page and wait for it to become unlocked.
   *
   * If the page does not get brought uptodate, return -EIO.
   */
  struct page *read_cache_page(struct address_space *mapping,
  				pgoff_t index,
  				int (*filler)(void *, struct page *),
  				void *data)
  {
  	return do_read_cache_page(mapping, index, filler, data, mapping_gfp_mask(mapping));
  }
  EXPORT_SYMBOL(read_cache_page);
  
  /**
   * read_cache_page_gfp - read into page cache, using specified page allocation flags.
   * @mapping:	the page's address_space
   * @index:	the page index
   * @gfp:	the page allocator flags to use if allocating
   *
   * This is the same as "read_mapping_page(mapping, index, NULL)", but with
   * any new page allocations done using the specified allocation flags.
   *
   * If the page does not get brought uptodate, return -EIO.
   */
  struct page *read_cache_page_gfp(struct address_space *mapping,
  				pgoff_t index,
  				gfp_t gfp)
  {
  	filler_t *filler = (filler_t *)mapping->a_ops->readpage;
  
  	return do_read_cache_page(mapping, index, filler, NULL, gfp);
  }
  EXPORT_SYMBOL(read_cache_page_gfp);
  
  /*
   * Performs necessary checks before doing a write
   *
   * Can adjust writing position or amount of bytes to write.
   * Returns appropriate error code that caller should return or
   * zero in case that write should be allowed.
   */
  inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
  {
  	struct file *file = iocb->ki_filp;
  	struct inode *inode = file->f_mapping->host;
  	unsigned long limit = rlimit(RLIMIT_FSIZE);
  	loff_t pos;
  
  	if (!iov_iter_count(from))
  		return 0;
  
  	/* FIXME: this is for backwards compatibility with 2.4 */
  	if (iocb->ki_flags & IOCB_APPEND)
  		iocb->ki_pos = i_size_read(inode);
  
  	pos = iocb->ki_pos;
  
  	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
  		return -EINVAL;
  
  	if (limit != RLIM_INFINITY) {
  		if (iocb->ki_pos >= limit) {
  			send_sig(SIGXFSZ, current, 0);
  			return -EFBIG;
  		}
  		iov_iter_truncate(from, limit - (unsigned long)pos);
  	}
  
  	/*
  	 * LFS rule
  	 */
  	if (unlikely(pos + iov_iter_count(from) > MAX_NON_LFS &&
  				!(file->f_flags & O_LARGEFILE))) {
  		if (pos >= MAX_NON_LFS)
  			return -EFBIG;
  		iov_iter_truncate(from, MAX_NON_LFS - (unsigned long)pos);
  	}
  
  	/*
  	 * Are we about to exceed the fs block limit ?
  	 *
  	 * If we have written data it becomes a short write.  If we have
  	 * exceeded without writing data we send a signal and return EFBIG.
  	 * Linus frestrict idea will clean these up nicely..
  	 */
  	if (unlikely(pos >= inode->i_sb->s_maxbytes))
  		return -EFBIG;
  
  	iov_iter_truncate(from, inode->i_sb->s_maxbytes - pos);
  	return iov_iter_count(from);
  }
  EXPORT_SYMBOL(generic_write_checks);
  
  int pagecache_write_begin(struct file *file, struct address_space *mapping,
  				loff_t pos, unsigned len, unsigned flags,
  				struct page **pagep, void **fsdata)
  {
  	const struct address_space_operations *aops = mapping->a_ops;
  
  	return aops->write_begin(file, mapping, pos, len, flags,
  							pagep, fsdata);
  }
  EXPORT_SYMBOL(pagecache_write_begin);
  
  int pagecache_write_end(struct file *file, struct address_space *mapping,
  				loff_t pos, unsigned len, unsigned copied,
  				struct page *page, void *fsdata)
  {
  	const struct address_space_operations *aops = mapping->a_ops;
  
  	return aops->write_end(file, mapping, pos, len, copied, page, fsdata);
  }
  EXPORT_SYMBOL(pagecache_write_end);
  
  ssize_t
  generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
  {
  	struct file	*file = iocb->ki_filp;
  	struct address_space *mapping = file->f_mapping;
  	struct inode	*inode = mapping->host;
  	loff_t		pos = iocb->ki_pos;
  	ssize_t		written;
  	size_t		write_len;
  	pgoff_t		end;
  
  	write_len = iov_iter_count(from);
  	end = (pos + write_len - 1) >> PAGE_SHIFT;
  
  	if (iocb->ki_flags & IOCB_NOWAIT) {
  		/* If there are pages to writeback, return */
  		if (filemap_range_has_page(inode->i_mapping, pos,
  					   pos + iov_iter_count(from)))
  			return -EAGAIN;
  	} else {
  		written = filemap_write_and_wait_range(mapping, pos,
  							pos + write_len - 1);
  		if (written)
  			goto out;
  	}
  
  	/*
  	 * After a write we want buffered reads to be sure to go to disk to get
  	 * the new data.  We invalidate clean cached page from the region we're
  	 * about to write.  We do this *before* the write so that we can return
  	 * without clobbering -EIOCBQUEUED from ->direct_IO().
  	 */
  	written = invalidate_inode_pages2_range(mapping,
  					pos >> PAGE_SHIFT, end);
  	/*
  	 * If a page can not be invalidated, return 0 to fall back
  	 * to buffered write.
  	 */
  	if (written) {
  		if (written == -EBUSY)
  			return 0;
  		goto out;
  	}
  
  	written = mapping->a_ops->direct_IO(iocb, from);
  
  	/*
  	 * Finally, try again to invalidate clean pages which might have been
  	 * cached by non-direct readahead, or faulted in by get_user_pages()
  	 * if the source of the write was an mmap'ed region of the file
  	 * we're writing.  Either one is a pretty crazy thing to do,
  	 * so we don't support it 100%.  If this invalidation
  	 * fails, tough, the write still worked...
  	 *
  	 * Most of the time we do not need this since dio_complete() will do
  	 * the invalidation for us. However there are some file systems that
  	 * do not end up with dio_complete() being called, so let's not break
  	 * them by removing it completely
  	 */
  	if (mapping->nrpages)
  		invalidate_inode_pages2_range(mapping,
  					pos >> PAGE_SHIFT, end);
  
  	if (written > 0) {
  		pos += written;
  		write_len -= written;
  		if (pos > i_size_read(inode) && !S_ISBLK(inode->i_mode)) {
  			i_size_write(inode, pos);
  			mark_inode_dirty(inode);
  		}
  		iocb->ki_pos = pos;
  	}
  	iov_iter_revert(from, write_len - iov_iter_count(from));
  out:
  	return written;
  }
  EXPORT_SYMBOL(generic_file_direct_write);
  
  /*
   * Find or create a page at the given pagecache position. Return the locked
   * page. This function is specifically for buffered writes.
   */
  struct page *grab_cache_page_write_begin(struct address_space *mapping,
  					pgoff_t index, unsigned flags)
  {
  	struct page *page;
  	int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT;
  
  	if (flags & AOP_FLAG_NOFS)
  		fgp_flags |= FGP_NOFS;
  
  	page = pagecache_get_page(mapping, index, fgp_flags,
  			mapping_gfp_mask(mapping));
  	if (page)
  		wait_for_stable_page(page);
  
  	return page;
  }
  EXPORT_SYMBOL(grab_cache_page_write_begin);
  
  ssize_t generic_perform_write(struct file *file,
  				struct iov_iter *i, loff_t pos)
  {
  	struct address_space *mapping = file->f_mapping;
  	const struct address_space_operations *a_ops = mapping->a_ops;
  	long status = 0;
  	ssize_t written = 0;
  	unsigned int flags = 0;
  
  	do {
  		struct page *page;
  		unsigned long offset;	/* Offset into pagecache page */
  		unsigned long bytes;	/* Bytes to write to page */
  		size_t copied;		/* Bytes copied from user */
  		void *fsdata;
  
  		offset = (pos & (PAGE_SIZE - 1));
  		bytes = min_t(unsigned long, PAGE_SIZE - offset,
  						iov_iter_count(i));
  
  again:
  		/*
  		 * Bring in the user page that we will copy from _first_.
  		 * Otherwise there's a nasty deadlock on copying from the
  		 * same page as we're writing to, without it being marked
  		 * up-to-date.
  		 *
  		 * Not only is this an optimisation, but it is also required
  		 * to check that the address is actually valid, when atomic
  		 * usercopies are used, below.
  		 */
  		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
  			status = -EFAULT;
  			break;
  		}
  
  		if (fatal_signal_pending(current)) {
  			status = -EINTR;
  			break;
  		}
  
  		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
  						&page, &fsdata);
  		if (unlikely(status < 0))
  			break;
  
  		if (mapping_writably_mapped(mapping))
  			flush_dcache_page(page);
  
  		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
  		flush_dcache_page(page);
  
  		status = a_ops->write_end(file, mapping, pos, bytes, copied,
  						page, fsdata);
  		if (unlikely(status < 0))
  			break;
  		copied = status;
  
  		cond_resched();
  
  		iov_iter_advance(i, copied);
  		if (unlikely(copied == 0)) {
  			/*
  			 * If we were unable to copy any data at all, we must
  			 * fall back to a single segment length write.
  			 *
  			 * If we didn't fallback here, we could livelock
  			 * because not all segments in the iov can be copied at
  			 * once without a pagefault.
  			 */
  			bytes = min_t(unsigned long, PAGE_SIZE - offset,
  						iov_iter_single_seg_count(i));
  			goto again;
  		}
  		pos += copied;
  		written += copied;
  
  		balance_dirty_pages_ratelimited(mapping);
  	} while (iov_iter_count(i));
  
  	return written ? written : status;
  }
  EXPORT_SYMBOL(generic_perform_write);
  
  /**
   * __generic_file_write_iter - write data to a file
   * @iocb:	IO state structure (file, offset, etc.)
   * @from:	iov_iter with data to write
   *
   * This function does all the work needed for actually writing data to a
   * file. It does all basic checks, removes SUID from the file, updates
   * modification times and calls proper subroutines depending on whether we
   * do direct IO or a standard buffered write.
   *
   * It expects i_mutex to be grabbed unless we work on a block device or similar
   * object which does not need locking at all.
   *
   * This function does *not* take care of syncing data in case of O_SYNC write.
   * A caller has to handle it. This is mainly due to the fact that we want to
   * avoid syncing under i_mutex.
   */
  ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
  {
  	struct file *file = iocb->ki_filp;
  	struct address_space * mapping = file->f_mapping;
  	struct inode 	*inode = mapping->host;
  	ssize_t		written = 0;
  	ssize_t		err;
  	ssize_t		status;
  
  	/* We can write back this queue in page reclaim */
  	current->backing_dev_info = inode_to_bdi(inode);
  	err = file_remove_privs(file);
  	if (err)
  		goto out;
  
  	err = file_update_time(file);
  	if (err)
  		goto out;
  
  	if (iocb->ki_flags & IOCB_DIRECT) {
  		loff_t pos, endbyte;
  
  		written = generic_file_direct_write(iocb, from);
  		/*
  		 * If the write stopped short of completing, fall back to
  		 * buffered writes.  Some filesystems do this for writes to
  		 * holes, for example.  For DAX files, a buffered write will
  		 * not succeed (even if it did, DAX does not handle dirty
  		 * page-cache pages correctly).
  		 */
  		if (written < 0 || !iov_iter_count(from) || IS_DAX(inode))
  			goto out;
  
  		status = generic_perform_write(file, from, pos = iocb->ki_pos);
  		/*
  		 * If generic_perform_write() returned a synchronous error
  		 * then we want to return the number of bytes which were
  		 * direct-written, or the error code if that was zero.  Note
  		 * that this differs from normal direct-io semantics, which
  		 * will return -EFOO even if some bytes were written.
  		 */
  		if (unlikely(status < 0)) {
  			err = status;
  			goto out;
  		}
  		/*
  		 * We need to ensure that the page cache pages are written to
  		 * disk and invalidated to preserve the expected O_DIRECT
  		 * semantics.
  		 */
  		endbyte = pos + status - 1;
  		err = filemap_write_and_wait_range(mapping, pos, endbyte);
  		if (err == 0) {
  			iocb->ki_pos = endbyte + 1;
  			written += status;
  			invalidate_mapping_pages(mapping,
  						 pos >> PAGE_SHIFT,
  						 endbyte >> PAGE_SHIFT);
  		} else {
  			/*
  			 * We don't know how much we wrote, so just return
  			 * the number of bytes which were direct-written
  			 */
  		}
  	} else {
  		written = generic_perform_write(file, from, iocb->ki_pos);
  		if (likely(written > 0))
  			iocb->ki_pos += written;
  	}
  out:
  	current->backing_dev_info = NULL;
  	return written ? written : err;
  }
  EXPORT_SYMBOL(__generic_file_write_iter);
  
  /**
   * generic_file_write_iter - write data to a file
   * @iocb:	IO state structure
   * @from:	iov_iter with data to write
   *
   * This is a wrapper around __generic_file_write_iter() to be used by most
   * filesystems. It takes care of syncing the file in case of O_SYNC file
   * and acquires i_mutex as needed.
   */
  ssize_t generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
  {
  	struct file *file = iocb->ki_filp;
  	struct inode *inode = file->f_mapping->host;
  	ssize_t ret;
  
  	inode_lock(inode);
  	ret = generic_write_checks(iocb, from);
  	if (ret > 0)
  		ret = __generic_file_write_iter(iocb, from);
  	inode_unlock(inode);
  
  	if (ret > 0)
  		ret = generic_write_sync(iocb, ret);
  	return ret;
  }
  EXPORT_SYMBOL(generic_file_write_iter);
  
  /**
   * try_to_release_page() - release old fs-specific metadata on a page
   *
   * @page: the page which the kernel is trying to free
   * @gfp_mask: memory allocation flags (and I/O mode)
   *
   * The address_space is to try to release any data against the page
   * (presumably at page->private).  If the release was successful, return '1'.
   * Otherwise return zero.
   *
   * This may also be called if PG_fscache is set on a page, indicating that the
   * page is known to the local caching routines.
   *
   * The @gfp_mask argument specifies whether I/O may be performed to release
   * this page (__GFP_IO), and whether the call may block (__GFP_RECLAIM & __GFP_FS).
   *
   */
  int try_to_release_page(struct page *page, gfp_t gfp_mask)
  {
  	struct address_space * const mapping = page->mapping;
  
  	BUG_ON(!PageLocked(page));
  	if (PageWriteback(page))
  		return 0;
  
  	if (mapping && mapping->a_ops->releasepage)
  		return mapping->a_ops->releasepage(page, gfp_mask);
  	return try_to_free_buffers(page);
  }
  
  EXPORT_SYMBOL(try_to_release_page);
  