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/fs.h>
  #include <linux/uaccess.h>
  #include <linux/aio.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/hardirq.h> /* for BUG_ON(!in_atomic()) only */
  #include <linux/memcontrol.h>
  #include <linux/cleancache.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_mutex		(truncate_pagecache)
   *    ->private_lock		(__free_pte->__set_page_dirty_buffers)
   *      ->swap_lock		(exclusive_swap_page, others)
   *        ->mapping->tree_lock
   *
   *  ->i_mutex
   *    ->i_mmap_mutex		(truncate->unmap_mapping_range)
   *
   *  ->mmap_sem
   *    ->i_mmap_mutex
   *      ->page_table_lock or pte_lock	(various, mainly in memory.c)
   *        ->mapping->tree_lock	(arch-dependent flush_dcache_mmap_lock)
   *
   *  ->mmap_sem
   *    ->lock_page		(access_process_vm)
   *
   *  ->i_mutex			(generic_file_buffered_write)
   *    ->mmap_sem		(fault_in_pages_readable->do_page_fault)
   *
   *  bdi->wb.list_lock
   *    sb_lock			(fs/fs-writeback.c)
   *    ->mapping->tree_lock	(__sync_single_inode)
   *
   *  ->i_mmap_mutex
   *    ->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)
   *    ->tree_lock		(try_to_unmap_one)
   *    ->zone.lru_lock		(follow_page->mark_page_accessed)
   *    ->zone.lru_lock		(check_pte_range->isolate_lru_page)
   *    ->private_lock		(page_remove_rmap->set_page_dirty)
   *    ->tree_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)
   *    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_mutex
   *   ->tasklist_lock            (memory_failure, collect_procs_ao)
   */
  
  /*
   * 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 mapping's tree_lock.
   */
  void __delete_from_page_cache(struct page *page)
  {
  	struct address_space *mapping = page->mapping;
  
  	trace_mm_filemap_delete_from_page_cache(page);
  	/*
  	 * 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);
  
  	radix_tree_delete(&mapping->page_tree, page->index);
  	page->mapping = NULL;
  	/* Leave page->index set: truncation lookup relies upon it */
  	mapping->nrpages--;
  	__dec_zone_page_state(page, NR_FILE_PAGES);
  	if (PageSwapBacked(page))
  		__dec_zone_page_state(page, NR_SHMEM);
  	BUG_ON(page_mapped(page));
  
  	/*
  	 * Some filesystems seem to re-dirty the page even after
  	 * the VM has canceled the dirty bit (eg ext3 journaling).
  	 *
  	 * Fix it up by doing a final dirty accounting check after
  	 * having removed the page entirely.
  	 */
  	if (PageDirty(page) && mapping_cap_account_dirty(mapping)) {
  		dec_zone_page_state(page, NR_FILE_DIRTY);
  		dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
  	}
  }
  
  /**
   * 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;
  	void (*freepage)(struct page *);
  
  	BUG_ON(!PageLocked(page));
  
  	freepage = mapping->a_ops->freepage;
  	spin_lock_irq(&mapping->tree_lock);
  	__delete_from_page_cache(page);
  	spin_unlock_irq(&mapping->tree_lock);
  	mem_cgroup_uncharge_cache_page(page);
  
  	if (freepage)
  		freepage(page);
  	page_cache_release(page);
  }
  EXPORT_SYMBOL(delete_from_page_cache);
  
  static int sleep_on_page(void *word)
  {
  	io_schedule();
  	return 0;
  }
  
  static int sleep_on_page_killable(void *word)
  {
  	sleep_on_page(word);
  	return fatal_signal_pending(current) ? -EINTR : 0;
  }
  
  static int filemap_check_errors(struct address_space *mapping)
  {
  	int ret = 0;
  	/* Check for outstanding write errors */
  	if (test_and_clear_bit(AS_ENOSPC, &mapping->flags))
  		ret = -ENOSPC;
  	if (test_and_clear_bit(AS_EIO, &mapping->flags))
  		ret = -EIO;
  	return ret;
  }
  
  /**
   * __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;
  
  	ret = do_writepages(mapping, &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_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.
   */
  int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
  			    loff_t end_byte)
  {
  	pgoff_t index = start_byte >> PAGE_CACHE_SHIFT;
  	pgoff_t end = end_byte >> PAGE_CACHE_SHIFT;
  	struct pagevec pvec;
  	int nr_pages;
  	int ret2, ret = 0;
  
  	if (end_byte < start_byte)
  		goto out;
  
  	pagevec_init(&pvec, 0);
  	while ((index <= end) &&
  			(nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
  			PAGECACHE_TAG_WRITEBACK,
  			min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1)) != 0) {
  		unsigned i;
  
  		for (i = 0; i < nr_pages; i++) {
  			struct page *page = pvec.pages[i];
  
  			/* until radix tree lookup accepts end_index */
  			if (page->index > end)
  				continue;
  
  			wait_on_page_writeback(page);
  			if (TestClearPageError(page))
  				ret = -EIO;
  		}
  		pagevec_release(&pvec);
  		cond_resched();
  	}
  out:
  	ret2 = filemap_check_errors(mapping);
  	if (!ret)
  		ret = ret2;
  
  	return ret;
  }
  EXPORT_SYMBOL(filemap_fdatawait_range);
  
  /**
   * filemap_fdatawait - wait for all under-writeback pages to complete
   * @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.
   */
  int filemap_fdatawait(struct address_space *mapping)
  {
  	loff_t i_size = i_size_read(mapping->host);
  
  	if (i_size == 0)
  		return 0;
  
  	return filemap_fdatawait_range(mapping, 0, i_size - 1);
  }
  EXPORT_SYMBOL(filemap_fdatawait);
  
  int filemap_write_and_wait(struct address_space *mapping)
  {
  	int err = 0;
  
  	if (mapping->nrpages) {
  		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 {
  		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->nrpages) {
  		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 {
  		err = filemap_check_errors(mapping);
  	}
  	return err;
  }
  EXPORT_SYMBOL(filemap_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(!PageLocked(old));
  	VM_BUG_ON(!PageLocked(new));
  	VM_BUG_ON(new->mapping);
  
  	error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
  	if (!error) {
  		struct address_space *mapping = old->mapping;
  		void (*freepage)(struct page *);
  
  		pgoff_t offset = old->index;
  		freepage = mapping->a_ops->freepage;
  
  		page_cache_get(new);
  		new->mapping = mapping;
  		new->index = offset;
  
  		spin_lock_irq(&mapping->tree_lock);
  		__delete_from_page_cache(old);
  		error = radix_tree_insert(&mapping->page_tree, offset, new);
  		BUG_ON(error);
  		mapping->nrpages++;
  		__inc_zone_page_state(new, NR_FILE_PAGES);
  		if (PageSwapBacked(new))
  			__inc_zone_page_state(new, NR_SHMEM);
  		spin_unlock_irq(&mapping->tree_lock);
  		/* mem_cgroup codes must not be called under tree_lock */
  		mem_cgroup_replace_page_cache(old, new);
  		radix_tree_preload_end();
  		if (freepage)
  			freepage(old);
  		page_cache_release(old);
  	}
  
  	return error;
  }
  EXPORT_SYMBOL_GPL(replace_page_cache_page);
  
  /**
   * 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)
  {
  	int error;
  
  	VM_BUG_ON(!PageLocked(page));
  	VM_BUG_ON(PageSwapBacked(page));
  
  	error = mem_cgroup_cache_charge(page, current->mm,
  					gfp_mask & GFP_RECLAIM_MASK);
  	if (error)
  		return error;
  
  	error = radix_tree_maybe_preload(gfp_mask & ~__GFP_HIGHMEM);
  	if (error) {
  		mem_cgroup_uncharge_cache_page(page);
  		return error;
  	}
  
  	page_cache_get(page);
  	page->mapping = mapping;
  	page->index = offset;
  
  	spin_lock_irq(&mapping->tree_lock);
  	error = radix_tree_insert(&mapping->page_tree, offset, page);
  	radix_tree_preload_end();
  	if (unlikely(error))
  		goto err_insert;
  	mapping->nrpages++;
  	__inc_zone_page_state(page, NR_FILE_PAGES);
  	spin_unlock_irq(&mapping->tree_lock);
  	trace_mm_filemap_add_to_page_cache(page);
  	return 0;
  err_insert:
  	page->mapping = NULL;
  	/* Leave page->index set: truncation relies upon it */
  	spin_unlock_irq(&mapping->tree_lock);
  	mem_cgroup_uncharge_cache_page(page);
  	page_cache_release(page);
  	return error;
  }
  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)
  {
  	int ret;
  
  	ret = add_to_page_cache(page, mapping, offset, gfp_mask);
  	if (ret == 0)
  		lru_cache_add_file(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 = get_mems_allowed();
  			n = cpuset_mem_spread_node();
  			page = alloc_pages_exact_node(n, gfp, 0);
  		} while (!put_mems_allowed(cpuset_mems_cookie) && !page);
  
  		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.
   */
  static wait_queue_head_t *page_waitqueue(struct page *page)
  {
  	const struct zone *zone = page_zone(page);
  
  	return &zone->wait_table[hash_ptr(page, zone->wait_table_bits)];
  }
  
  static inline void wake_up_page(struct page *page, int bit)
  {
  	__wake_up_bit(page_waitqueue(page), &page->flags, bit);
  }
  
  void wait_on_page_bit(struct page *page, int bit_nr)
  {
  	DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);
  
  	if (test_bit(bit_nr, &page->flags))
  		__wait_on_bit(page_waitqueue(page), &wait, sleep_on_page,
  							TASK_UNINTERRUPTIBLE);
  }
  EXPORT_SYMBOL(wait_on_page_bit);
  
  int wait_on_page_bit_killable(struct page *page, int bit_nr)
  {
  	DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);
  
  	if (!test_bit(bit_nr, &page->flags))
  		return 0;
  
  	return __wait_on_bit(page_waitqueue(page), &wait,
  			     sleep_on_page_killable, TASK_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_t *waiter)
  {
  	wait_queue_head_t *q = page_waitqueue(page);
  	unsigned long flags;
  
  	spin_lock_irqsave(&q->lock, flags);
  	__add_wait_queue(q, waiter);
  	spin_unlock_irqrestore(&q->lock, flags);
  }
  EXPORT_SYMBOL_GPL(add_page_wait_queue);
  
  /**
   * 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
   * mechananism between PageLocked pages and PageWriteback pages is shared.
   * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
   *
   * The mb is necessary to enforce ordering between the clear_bit and the read
   * of the waitqueue (to avoid SMP races with a parallel wait_on_page_locked()).
   */
  void unlock_page(struct page *page)
  {
  	VM_BUG_ON(!PageLocked(page));
  	clear_bit_unlock(PG_locked, &page->flags);
  	smp_mb__after_clear_bit();
  	wake_up_page(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)
  {
  	if (TestClearPageReclaim(page))
  		rotate_reclaimable_page(page);
  
  	if (!test_clear_page_writeback(page))
  		BUG();
  
  	smp_mb__after_clear_bit();
  	wake_up_page(page, PG_writeback);
  }
  EXPORT_SYMBOL(end_page_writeback);
  
  /**
   * __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)
  {
  	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
  
  	__wait_on_bit_lock(page_waitqueue(page), &wait, sleep_on_page,
  							TASK_UNINTERRUPTIBLE);
  }
  EXPORT_SYMBOL(__lock_page);
  
  int __lock_page_killable(struct page *page)
  {
  	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
  
  	return __wait_on_bit_lock(page_waitqueue(page), &wait,
  					sleep_on_page_killable, TASK_KILLABLE);
  }
  EXPORT_SYMBOL_GPL(__lock_page_killable);
  
  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;
  	}
  }
  
  /**
   * find_get_page - find and get a page reference
   * @mapping: the address_space to search
   * @offset: the page index
   *
   * Is there a pagecache struct page at the given (mapping, offset) tuple?
   * If yes, increment its refcount and return it; if no, return NULL.
   */
  struct page *find_get_page(struct address_space *mapping, pgoff_t offset)
  {
  	void **pagep;
  	struct page *page;
  
  	rcu_read_lock();
  repeat:
  	page = NULL;
  	pagep = radix_tree_lookup_slot(&mapping->page_tree, 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;
  			/*
  			 * Otherwise, shmem/tmpfs must be storing a swap entry
  			 * here as an exceptional entry: so return it without
  			 * attempting to raise page count.
  			 */
  			goto out;
  		}
  		if (!page_cache_get_speculative(page))
  			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)) {
  			page_cache_release(page);
  			goto repeat;
  		}
  	}
  out:
  	rcu_read_unlock();
  
  	return page;
  }
  EXPORT_SYMBOL(find_get_page);
  
  /**
   * find_lock_page - locate, pin and lock a pagecache page
   * @mapping: the address_space to search
   * @offset: the page index
   *
   * Locates the desired pagecache page, locks it, increments its reference
   * count and returns its address.
   *
   * Returns zero if the page was not present. find_lock_page() may sleep.
   */
  struct page *find_lock_page(struct address_space *mapping, pgoff_t offset)
  {
  	struct page *page;
  
  repeat:
  	page = find_get_page(mapping, offset);
  	if (page && !radix_tree_exception(page)) {
  		lock_page(page);
  		/* Has the page been truncated? */
  		if (unlikely(page->mapping != mapping)) {
  			unlock_page(page);
  			page_cache_release(page);
  			goto repeat;
  		}
  		VM_BUG_ON(page->index != offset);
  	}
  	return page;
  }
  EXPORT_SYMBOL(find_lock_page);
  
  /**
   * find_or_create_page - locate or add a pagecache page
   * @mapping: the page's address_space
   * @index: the page's index into the mapping
   * @gfp_mask: page allocation mode
   *
   * Locates a page in the pagecache.  If the page is not present, a new page
   * is allocated using @gfp_mask and is added to the pagecache and to the VM's
   * LRU list.  The returned page is locked and has its reference count
   * incremented.
   *
   * find_or_create_page() may sleep, even if @gfp_flags specifies an atomic
   * allocation!
   *
   * find_or_create_page() returns the desired page's address, or zero on
   * memory exhaustion.
   */
  struct page *find_or_create_page(struct address_space *mapping,
  		pgoff_t index, gfp_t gfp_mask)
  {
  	struct page *page;
  	int err;
  repeat:
  	page = find_lock_page(mapping, index);
  	if (!page) {
  		page = __page_cache_alloc(gfp_mask);
  		if (!page)
  			return NULL;
  		/*
  		 * We want a regular kernel memory (not highmem or DMA etc)
  		 * allocation for the radix tree nodes, but we need to honour
  		 * the context-specific requirements the caller has asked for.
  		 * GFP_RECLAIM_MASK collects those requirements.
  		 */
  		err = add_to_page_cache_lru(page, mapping, index,
  			(gfp_mask & GFP_RECLAIM_MASK));
  		if (unlikely(err)) {
  			page_cache_release(page);
  			page = NULL;
  			if (err == -EEXIST)
  				goto repeat;
  		}
  	}
  	return page;
  }
  EXPORT_SYMBOL(find_or_create_page);
  
  /**
   * find_get_pages - gang pagecache lookup
   * @mapping:	The address_space to search
   * @start:	The starting page index
   * @nr_pages:	The maximum number of pages
   * @pages:	Where the resulting pages are placed
   *
   * find_get_pages() will search for and return a group of up to
   * @nr_pages pages in the mapping.  The pages are placed at @pages.
   * find_get_pages() 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.
   *
   * find_get_pages() returns the number of pages which were found.
   */
  unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
  			    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();
  restart:
  	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
  		struct page *page;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				/*
  				 * Transient condition which can only trigger
  				 * when entry at index 0 moves out of or back
  				 * to root: none yet gotten, safe to restart.
  				 */
  				WARN_ON(iter.index);
  				goto restart;
  			}
  			/*
  			 * Otherwise, shmem/tmpfs must be storing a swap entry
  			 * here as an exceptional entry: so skip over it -
  			 * we only reach this from invalidate_mapping_pages().
  			 */
  			continue;
  		}
  
  		if (!page_cache_get_speculative(page))
  			goto repeat;
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			page_cache_release(page);
  			goto repeat;
  		}
  
  		pages[ret] = page;
  		if (++ret == nr_pages)
  			break;
  	}
  
  	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();
  restart:
  	radix_tree_for_each_contig(slot, &mapping->page_tree, &iter, index) {
  		struct page *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)) {
  				/*
  				 * Transient condition which can only trigger
  				 * when entry at index 0 moves out of or back
  				 * to root: none yet gotten, safe to restart.
  				 */
  				goto restart;
  			}
  			/*
  			 * Otherwise, shmem/tmpfs must be storing a swap entry
  			 * here as an exceptional entry: so stop looking for
  			 * contiguous pages.
  			 */
  			break;
  		}
  
  		if (!page_cache_get_speculative(page))
  			goto repeat;
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			page_cache_release(page);
  			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->index != iter.index) {
  			page_cache_release(page);
  			break;
  		}
  
  		pages[ret] = page;
  		if (++ret == nr_pages)
  			break;
  	}
  	rcu_read_unlock();
  	return ret;
  }
  EXPORT_SYMBOL(find_get_pages_contig);
  
  /**
   * find_get_pages_tag - find and return pages that match @tag
   * @mapping:	the address_space to search
   * @index:	the starting page index
   * @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_tag(struct address_space *mapping, pgoff_t *index,
  			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();
  restart:
  	radix_tree_for_each_tagged(slot, &mapping->page_tree,
  				   &iter, *index, tag) {
  		struct page *page;
  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {
  				/*
  				 * Transient condition which can only trigger
  				 * when entry at index 0 moves out of or back
  				 * to root: none yet gotten, safe to restart.
  				 */
  				goto restart;
  			}
  			/*
  			 * This function is never used on a shmem/tmpfs
  			 * mapping, so a swap entry won't be found here.
  			 */
  			BUG();
  		}
  
  		if (!page_cache_get_speculative(page))
  			goto repeat;
  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {
  			page_cache_release(page);
  			goto repeat;
  		}
  
  		pages[ret] = page;
  		if (++ret == nr_pages)
  			break;
  	}
  
  	rcu_read_unlock();
  
  	if (ret)
  		*index = pages[ret - 1]->index + 1;
  
  	return ret;
  }
  EXPORT_SYMBOL(find_get_pages_tag);
  
  /**
   * grab_cache_page_nowait - returns locked page at given index in given cache
   * @mapping: target address_space
   * @index: the page index
   *
   * Same as grab_cache_page(), but do not wait if the page is unavailable.
   * This is intended for speculative data generators, where the data can
   * be regenerated if the page couldn't be grabbed.  This routine should
   * be safe to call while holding the lock for another page.
   *
   * Clear __GFP_FS when allocating the page to avoid recursion into the fs
   * and deadlock against the caller's locked page.
   */
  struct page *
  grab_cache_page_nowait(struct address_space *mapping, pgoff_t index)
  {
  	struct page *page = find_get_page(mapping, index);
  
  	if (page) {
  		if (trylock_page(page))
  			return page;
  		page_cache_release(page);
  		return NULL;
  	}
  	page = __page_cache_alloc(mapping_gfp_mask(mapping) & ~__GFP_FS);
  	if (page && add_to_page_cache_lru(page, mapping, index, GFP_NOFS)) {
  		page_cache_release(page);
  		page = NULL;
  	}
  	return page;
  }
  EXPORT_SYMBOL(grab_cache_page_nowait);
  
  /*
   * 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;
  }
  
  /**
   * do_generic_file_read - generic file read routine
   * @filp:	the file to read
   * @ppos:	current file position
   * @desc:	read_descriptor
   *
   * 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 void do_generic_file_read(struct file *filp, loff_t *ppos,
  		read_descriptor_t *desc)
  {
  	struct address_space *mapping = filp->f_mapping;
  	struct inode *inode = mapping->host;
  	struct file_ra_state *ra = &filp->f_ra;
  	pgoff_t index;
  	pgoff_t last_index;
  	pgoff_t prev_index;
  	unsigned long offset;      /* offset into pagecache page */
  	unsigned int prev_offset;
  	int error;
  
  	index = *ppos >> PAGE_CACHE_SHIFT;
  	prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT;
  	prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1);
  	last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
  	offset = *ppos & ~PAGE_CACHE_MASK;
  
  	for (;;) {
  		struct page *page;
  		pgoff_t end_index;
  		loff_t isize;
  		unsigned long nr, ret;
  
  		cond_resched();
  find_page:
  		page = find_get_page(mapping, index);
  		if (!page) {
  			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 (inode->i_blkbits == PAGE_CACHE_SHIFT ||
  					!mapping->a_ops->is_partially_uptodate)
  				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,
  								desc, offset))
  				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_CACHE_SHIFT;
  		if (unlikely(!isize || index > end_index)) {
  			page_cache_release(page);
  			goto out;
  		}
  
  		/* nr is the maximum number of bytes to copy from this page */
  		nr = PAGE_CACHE_SIZE;
  		if (index == end_index) {
  			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
  			if (nr <= offset) {
  				page_cache_release(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...
  		 *
  		 * The file_read_actor routine returns how many bytes were
  		 * actually used..
  		 * NOTE! This may not be the same as how much of a user buffer
  		 * we filled up (we may be padding etc), so we can only update
  		 * "pos" here (the actor routine has to update the user buffer
  		 * pointers and the remaining count).
  		 */
  		ret = file_read_actor(desc, page, offset, nr);
  		offset += ret;
  		index += offset >> PAGE_CACHE_SHIFT;
  		offset &= ~PAGE_CACHE_MASK;
  		prev_offset = offset;
  
  		page_cache_release(page);
  		if (ret == nr && desc->count)
  			continue;
  		goto out;
  
  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);
  			page_cache_release(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) {
  				page_cache_release(page);
  				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);
  					page_cache_release(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 */
  		desc->error = error;
  		page_cache_release(page);
  		goto out;
  
  no_cached_page:
  		/*
  		 * Ok, it wasn't cached, so we need to create a new
  		 * page..
  		 */
  		page = page_cache_alloc_cold(mapping);
  		if (!page) {
  			desc->error = -ENOMEM;
  			goto out;
  		}
  		error = add_to_page_cache_lru(page, mapping,
  						index, GFP_KERNEL);
  		if (error) {
  			page_cache_release(page);
  			if (error == -EEXIST)
  				goto find_page;
  			desc->error = error;
  			goto out;
  		}
  		goto readpage;
  	}
  
  out:
  	ra->prev_pos = prev_index;
  	ra->prev_pos <<= PAGE_CACHE_SHIFT;
  	ra->prev_pos |= prev_offset;
  
  	*ppos = ((loff_t)index << PAGE_CACHE_SHIFT) + offset;
  	file_accessed(filp);
  }
  
  int file_read_actor(read_descriptor_t *desc, struct page *page,
  			unsigned long offset, unsigned long size)
  {
  	char *kaddr;
  	unsigned long left, count = desc->count;
  
  	if (size > count)
  		size = count;
  
  	/*
  	 * Faults on the destination of a read are common, so do it before
  	 * taking the kmap.
  	 */
  	if (!fault_in_pages_writeable(desc->arg.buf, size)) {
  		kaddr = kmap_atomic(page);
  		left = __copy_to_user_inatomic(desc->arg.buf,
  						kaddr + offset, size);
  		kunmap_atomic(kaddr);
  		if (left == 0)
  			goto success;
  	}
  
  	/* Do it the slow way */
  	kaddr = kmap(page);
  	left = __copy_to_user(desc->arg.buf, kaddr + offset, size);
  	kunmap(page);
  
  	if (left) {
  		size -= left;
  		desc->error = -EFAULT;
  	}
  success:
  	desc->count = count - size;
  	desc->written += size;
  	desc->arg.buf += size;
  	return size;
  }
  
  /*
   * Performs necessary checks before doing a write
   * @iov:	io vector request
   * @nr_segs:	number of segments in the iovec
   * @count:	number of bytes to write
   * @access_flags: type of access: %VERIFY_READ or %VERIFY_WRITE
   *
   * Adjust number of segments and amount of bytes to write (nr_segs should be
   * properly initialized first). Returns appropriate error code that caller
   * should return or zero in case that write should be allowed.
   */
  int generic_segment_checks(const struct iovec *iov,
  			unsigned long *nr_segs, size_t *count, int access_flags)
  {
  	unsigned long   seg;
  	size_t cnt = 0;
  	for (seg = 0; seg < *nr_segs; seg++) {
  		const struct iovec *iv = &iov[seg];
  
  		/*
  		 * If any segment has a negative length, or the cumulative
  		 * length ever wraps negative then return -EINVAL.
  		 */
  		cnt += iv->iov_len;
  		if (unlikely((ssize_t)(cnt|iv->iov_len) < 0))
  			return -EINVAL;
  		if (access_ok(access_flags, iv->iov_base, iv->iov_len))
  			continue;
  		if (seg == 0)
  			return -EFAULT;
  		*nr_segs = seg;
  		cnt -= iv->iov_len;	/* This segment is no good */
  		break;
  	}
  	*count = cnt;
  	return 0;
  }
  EXPORT_SYMBOL(generic_segment_checks);
  
  /**
   * generic_file_aio_read - generic filesystem read routine
   * @iocb:	kernel I/O control block
   * @iov:	io vector request
   * @nr_segs:	number of segments in the iovec
   * @pos:	current file position
   *
   * This is the "read()" routine for all filesystems
   * that can use the page cache directly.
   */
  ssize_t
  generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
  		unsigned long nr_segs, loff_t pos)
  {
  	struct file *filp = iocb->ki_filp;
  	ssize_t retval;
  	unsigned long seg = 0;
  	size_t count;
  	loff_t *ppos = &iocb->ki_pos;
  
  	count = 0;
  	retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
  	if (retval)
  		return retval;
  
  	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
  	if (filp->f_flags & O_DIRECT) {
  		loff_t size;
  		struct address_space *mapping;
  		struct inode *inode;
  
  		mapping = filp->f_mapping;
  		inode = mapping->host;
  		if (!count)
  			goto out; /* skip atime */
  		size = i_size_read(inode);
  		if (pos < size) {
  			retval = filemap_write_and_wait_range(mapping, pos,
  					pos + iov_length(iov, nr_segs) - 1);
  			if (!retval) {
  				retval = mapping->a_ops->direct_IO(READ, iocb,
  							iov, pos, nr_segs);
  			}
  			if (retval > 0) {
  				*ppos = pos + retval;
  				count -= retval;
  			}
  
  			/*
  			 * 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.
  			 */
  			if (retval < 0 || !count || *ppos >= size) {
  				file_accessed(filp);
  				goto out;
  			}
  		}
  	}
  
  	count = retval;
  	for (seg = 0; seg < nr_segs; seg++) {
  		read_descriptor_t desc;
  		loff_t offset = 0;
  
  		/*
  		 * If we did a short DIO read we need to skip the section of the
  		 * iov that we've already read data into.
  		 */
  		if (count) {
  			if (count > iov[seg].iov_len) {
  				count -= iov[seg].iov_len;
  				continue;
  			}
  			offset = count;
  			count = 0;
  		}
  
  		desc.written = 0;
  		desc.arg.buf = iov[seg].iov_base + offset;
  		desc.count = iov[seg].iov_len - offset;
  		if (desc.count == 0)
  			continue;
  		desc.error = 0;
  		do_generic_file_read(filp, ppos, &desc);
  		retval += desc.written;
  		if (desc.error) {
  			retval = retval ?: desc.error;
  			break;
  		}
  		if (desc.count > 0)
  			break;
  	}
  out:
  	return retval;
  }
  EXPORT_SYMBOL(generic_file_aio_read);
  
  #ifdef CONFIG_MMU
  /**
   * page_cache_read - adds requested page to the page cache if not already there
   * @file:	file to read
   * @offset:	page index
   *
   * 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)
  {
  	struct address_space *mapping = file->f_mapping;
  	struct page *page; 
  	int ret;
  
  	do {
  		page = page_cache_alloc_cold(mapping);
  		if (!page)
  			return -ENOMEM;
  
  		ret = add_to_page_cache_lru(page, mapping, offset, GFP_KERNEL);
  		if (ret == 0)
  			ret = mapping->a_ops->readpage(file, page);
  		else if (ret == -EEXIST)
  			ret = 0; /* losing race to add is OK */
  
  		page_cache_release(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)
  {
  	unsigned long ra_pages;
  	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_pages = max_sane_readahead(ra->ra_pages);
  	ra->start = max_t(long, 0, offset - ra_pages / 2);
  	ra->size = ra_pages;
  	ra->async_size = 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
   * @vma:	vma in which the fault was taken
   * @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.
   */
  int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
  {
  	int error;
  	struct file *file = 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;
  	struct page *page;
  	pgoff_t size;
  	int ret = 0;
  
  	size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
  	if (offset >= size)
  		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(vma, ra, file, page, offset);
  	} else if (!page) {
  		/* No page in the page cache at all */
  		do_sync_mmap_readahead(vma, ra, file, offset);
  		count_vm_event(PGMAJFAULT);
  		mem_cgroup_count_vm_event(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, vma->vm_mm, vmf->flags)) {
  		page_cache_release(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->index != offset);
  
  	/*
  	 * 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.
  	 */
  	size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
  	if (unlikely(offset >= size)) {
  		unlock_page(page);
  		page_cache_release(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);
  
  	/*
  	 * 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;
  	}
  	page_cache_release(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);
  
  int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
  {
  	struct page *page = vmf->page;
  	struct inode *inode = file_inode(vma->vm_file);
  	int ret = VM_FAULT_LOCKED;
  
  	sb_start_pagefault(inode->i_sb);
  	file_update_time(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;
  }
  EXPORT_SYMBOL(filemap_page_mkwrite);
  
  const struct vm_operations_struct generic_file_vm_ops = {
  	.fault		= filemap_fault,
  	.page_mkwrite	= filemap_page_mkwrite,
  	.remap_pages	= generic_file_remap_pages,
  };
  
  /* 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 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(generic_file_mmap);
  EXPORT_SYMBOL(generic_file_readonly_mmap);
  
  static struct page *__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 | __GFP_COLD);
  		if (!page)
  			return ERR_PTR(-ENOMEM);
  		err = add_to_page_cache_lru(page, mapping, index, gfp);
  		if (unlikely(err)) {
  			page_cache_release(page);
  			if (err == -EEXIST)
  				goto repeat;
  			/* Presumably ENOMEM for radix tree node */
  			return ERR_PTR(err);
  		}
  		err = filler(data, page);
  		if (err < 0) {
  			page_cache_release(page);
  			page = ERR_PTR(err);
  		}
  	}
  	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;
  
  retry:
  	page = __read_cache_page(mapping, index, filler, data, gfp);
  	if (IS_ERR(page))
  		return page;
  	if (PageUptodate(page))
  		goto out;
  
  	lock_page(page);
  	if (!page->mapping) {
  		unlock_page(page);
  		page_cache_release(page);
  		goto retry;
  	}
  	if (PageUptodate(page)) {
  		unlock_page(page);
  		goto out;
  	}
  	err = filler(data, page);
  	if (err < 0) {
  		page_cache_release(page);
  		return ERR_PTR(err);
  	}
  out:
  	mark_page_accessed(page);
  	return page;
  }
  
  /**
   * read_cache_page_async - 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
   *
   * Same as read_cache_page, but don't wait for page to become unlocked
   * after submitting it to the filler.
   *
   * Read into the page cache. If a page already exists, and PageUptodate() is
   * not set, try to fill the page but don't wait for it to become unlocked.
   *
   * If the page does not get brought uptodate, return -EIO.
   */
  struct page *read_cache_page_async(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_async);
  
  static struct page *wait_on_page_read(struct page *page)
  {
  	if (!IS_ERR(page)) {
  		wait_on_page_locked(page);
  		if (!PageUptodate(page)) {
  			page_cache_release(page);
  			page = ERR_PTR(-EIO);
  		}
  	}
  	return 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 wait_on_page_read(do_read_cache_page(mapping, index, filler, NULL, gfp));
  }
  EXPORT_SYMBOL(read_cache_page_gfp);
  
  /**
   * 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 then 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 wait_on_page_read(read_cache_page_async(mapping, index, filler, data));
  }
  EXPORT_SYMBOL(read_cache_page);
  
  static size_t __iovec_copy_from_user_inatomic(char *vaddr,
  			const struct iovec *iov, size_t base, size_t bytes)
  {
  	size_t copied = 0, left = 0;
  
  	while (bytes) {
  		char __user *buf = iov->iov_base + base;
  		int copy = min(bytes, iov->iov_len - base);
  
  		base = 0;
  		left = __copy_from_user_inatomic(vaddr, buf, copy);
  		copied += copy;
  		bytes -= copy;
  		vaddr += copy;
  		iov++;
  
  		if (unlikely(left))
  			break;
  	}
  	return copied - left;
  }
  
  /*
   * Copy as much as we can into the page and return the number of bytes which
   * were successfully copied.  If a fault is encountered then return the number of
   * bytes which were copied.
   */
  size_t iov_iter_copy_from_user_atomic(struct page *page,
  		struct iov_iter *i, unsigned long offset, size_t bytes)
  {
  	char *kaddr;
  	size_t copied;
  
  	BUG_ON(!in_atomic());
  	kaddr = kmap_atomic(page);
  	if (likely(i->nr_segs == 1)) {
  		int left;
  		char __user *buf = i->iov->iov_base + i->iov_offset;
  		left = __copy_from_user_inatomic(kaddr + offset, buf, bytes);
  		copied = bytes - left;
  	} else {
  		copied = __iovec_copy_from_user_inatomic(kaddr + offset,
  						i->iov, i->iov_offset, bytes);
  	}
  	kunmap_atomic(kaddr);
  
  	return copied;
  }
  EXPORT_SYMBOL(iov_iter_copy_from_user_atomic);
  
  /*
   * This has the same sideeffects and return value as
   * iov_iter_copy_from_user_atomic().
   * The difference is that it attempts to resolve faults.
   * Page must not be locked.
   */
  size_t iov_iter_copy_from_user(struct page *page,
  		struct iov_iter *i, unsigned long offset, size_t bytes)
  {
  	char *kaddr;
  	size_t copied;
  
  	kaddr = kmap(page);
  	if (likely(i->nr_segs == 1)) {
  		int left;
  		char __user *buf = i->iov->iov_base + i->iov_offset;
  		left = __copy_from_user(kaddr + offset, buf, bytes);
  		copied = bytes - left;
  	} else {
  		copied = __iovec_copy_from_user_inatomic(kaddr + offset,
  						i->iov, i->iov_offset, bytes);
  	}
  	kunmap(page);
  	return copied;
  }
  EXPORT_SYMBOL(iov_iter_copy_from_user);
  
  void iov_iter_advance(struct iov_iter *i, size_t bytes)
  {
  	BUG_ON(i->count < bytes);
  
  	if (likely(i->nr_segs == 1)) {
  		i->iov_offset += bytes;
  		i->count -= bytes;
  	} else {
  		const struct iovec *iov = i->iov;
  		size_t base = i->iov_offset;
  		unsigned long nr_segs = i->nr_segs;
  
  		/*
  		 * The !iov->iov_len check ensures we skip over unlikely
  		 * zero-length segments (without overruning the iovec).
  		 */
  		while (bytes || unlikely(i->count && !iov->iov_len)) {
  			int copy;
  
  			copy = min(bytes, iov->iov_len - base);
  			BUG_ON(!i->count || i->count < copy);
  			i->count -= copy;
  			bytes -= copy;
  			base += copy;
  			if (iov->iov_len == base) {
  				iov++;
  				nr_segs--;
  				base = 0;
  			}
  		}
  		i->iov = iov;
  		i->iov_offset = base;
  		i->nr_segs = nr_segs;
  	}
  }
  EXPORT_SYMBOL(iov_iter_advance);
  
  /*
   * Fault in the first iovec of the given iov_iter, to a maximum length
   * of bytes. Returns 0 on success, or non-zero if the memory could not be
   * accessed (ie. because it is an invalid address).
   *
   * writev-intensive code may want this to prefault several iovecs -- that
   * would be possible (callers must not rely on the fact that _only_ the
   * first iovec will be faulted with the current implementation).
   */
  int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes)
  {
  	char __user *buf = i->iov->iov_base + i->iov_offset;
  	bytes = min(bytes, i->iov->iov_len - i->iov_offset);
  	return fault_in_pages_readable(buf, bytes);
  }
  EXPORT_SYMBOL(iov_iter_fault_in_readable);
  
  /*
   * Return the count of just the current iov_iter segment.
   */
  size_t iov_iter_single_seg_count(const struct iov_iter *i)
  {
  	const struct iovec *iov = i->iov;
  	if (i->nr_segs == 1)
  		return i->count;
  	else
  		return min(i->count, iov->iov_len - i->iov_offset);
  }
  EXPORT_SYMBOL(iov_iter_single_seg_count);
  
  /*
   * 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 int generic_write_checks(struct file *file, loff_t *pos, size_t *count, int isblk)
  {
  	struct inode *inode = file->f_mapping->host;
  	unsigned long limit = rlimit(RLIMIT_FSIZE);
  
          if (unlikely(*pos < 0))
                  return -EINVAL;
  
  	if (!isblk) {
  		/* FIXME: this is for backwards compatibility with 2.4 */
  		if (file->f_flags & O_APPEND)
                          *pos = i_size_read(inode);
  
  		if (limit != RLIM_INFINITY) {
  			if (*pos >= limit) {
  				send_sig(SIGXFSZ, current, 0);
  				return -EFBIG;
  			}
  			if (*count > limit - (typeof(limit))*pos) {
  				*count = limit - (typeof(limit))*pos;
  			}
  		}
  	}
  
  	/*
  	 * LFS rule
  	 */
  	if (unlikely(*pos + *count > MAX_NON_LFS &&
  				!(file->f_flags & O_LARGEFILE))) {
  		if (*pos >= MAX_NON_LFS) {
  			return -EFBIG;
  		}
  		if (*count > MAX_NON_LFS - (unsigned long)*pos) {
  			*count = 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 (likely(!isblk)) {
  		if (unlikely(*pos >= inode->i_sb->s_maxbytes)) {
  			if (*count || *pos > inode->i_sb->s_maxbytes) {
  				return -EFBIG;
  			}
  			/* zero-length writes at ->s_maxbytes are OK */
  		}
  
  		if (unlikely(*pos + *count > inode->i_sb->s_maxbytes))
  			*count = inode->i_sb->s_maxbytes - *pos;
  	} else {
  #ifdef CONFIG_BLOCK
  		loff_t isize;
  		if (bdev_read_only(I_BDEV(inode)))
  			return -EPERM;
  		isize = i_size_read(inode);
  		if (*pos >= isize) {
  			if (*count || *pos > isize)
  				return -ENOSPC;
  		}
  
  		if (*pos + *count > isize)
  			*count = isize - *pos;
  #else
  		return -EPERM;
  #endif
  	}
  	return 0;
  }
  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;
  
  	mark_page_accessed(page);
  	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, const struct iovec *iov,
  		unsigned long *nr_segs, loff_t pos, loff_t *ppos,
  		size_t count, size_t ocount)
  {
  	struct file	*file = iocb->ki_filp;
  	struct address_space *mapping = file->f_mapping;
  	struct inode	*inode = mapping->host;
  	ssize_t		written;
  	size_t		write_len;
  	pgoff_t		end;
  
  	if (count != ocount)
  		*nr_segs = iov_shorten((struct iovec *)iov, *nr_segs, count);
  
  	write_len = iov_length(iov, *nr_segs);
  	end = (pos + write_len - 1) >> PAGE_CACHE_SHIFT;
  
  	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().
  	 */
  	if (mapping->nrpages) {
  		written = invalidate_inode_pages2_range(mapping,
  					pos >> PAGE_CACHE_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(WRITE, iocb, iov, pos, *nr_segs);
  
  	/*
  	 * 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...
  	 */
  	if (mapping->nrpages) {
  		invalidate_inode_pages2_range(mapping,
  					      pos >> PAGE_CACHE_SHIFT, end);
  	}
  
  	if (written > 0) {
  		pos += written;
  		if (pos > i_size_read(inode) && !S_ISBLK(inode->i_mode)) {
  			i_size_write(inode, pos);
  			mark_inode_dirty(inode);
  		}
  		*ppos = pos;
  	}
  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)
  {
  	int status;
  	gfp_t gfp_mask;
  	struct page *page;
  	gfp_t gfp_notmask = 0;
  
  	gfp_mask = mapping_gfp_mask(mapping);
  	if (mapping_cap_account_dirty(mapping))
  		gfp_mask |= __GFP_WRITE;
  	if (flags & AOP_FLAG_NOFS)
  		gfp_notmask = __GFP_FS;
  repeat:
  	page = find_lock_page(mapping, index);
  	if (page)
  		goto found;
  
  	page = __page_cache_alloc(gfp_mask & ~gfp_notmask);
  	if (!page)
  		return NULL;
  	status = add_to_page_cache_lru(page, mapping, index,
  						GFP_KERNEL & ~gfp_notmask);
  	if (unlikely(status)) {
  		page_cache_release(page);
  		if (status == -EEXIST)
  			goto repeat;
  		return NULL;
  	}
  found:
  	wait_for_stable_page(page);
  	return page;
  }
  EXPORT_SYMBOL(grab_cache_page_write_begin);
  
  static 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;
  
  	/*
  	 * Copies from kernel address space cannot fail (NFSD is a big user).
  	 */
  	if (segment_eq(get_fs(), KERNEL_DS))
  		flags |= AOP_FLAG_UNINTERRUPTIBLE;
  
  	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_CACHE_SIZE - 1));
  		bytes = min_t(unsigned long, PAGE_CACHE_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;
  		}
  
  		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
  						&page, &fsdata);
  		if (unlikely(status))
  			break;
  
  		if (mapping_writably_mapped(mapping))
  			flush_dcache_page(page);
  
  		pagefault_disable();
  		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
  		pagefault_enable();
  		flush_dcache_page(page);
  
  		mark_page_accessed(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_CACHE_SIZE - offset,
  						iov_iter_single_seg_count(i));
  			goto again;
  		}
  		pos += copied;
  		written += copied;
  
  		balance_dirty_pages_ratelimited(mapping);
  		if (fatal_signal_pending(current)) {
  			status = -EINTR;
  			break;
  		}
  	} while (iov_iter_count(i));
  
  	return written ? written : status;
  }
  
  ssize_t
  generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
  		unsigned long nr_segs, loff_t pos, loff_t *ppos,
  		size_t count, ssize_t written)
  {
  	struct file *file = iocb->ki_filp;
  	ssize_t status;
  	struct iov_iter i;
  
  	iov_iter_init(&i, iov, nr_segs, count, written);
  	status = generic_perform_write(file, &i, pos);
  
  	if (likely(status >= 0)) {
  		written += status;
  		*ppos = pos + status;
    	}
  	
  	return written ? written : status;
  }
  EXPORT_SYMBOL(generic_file_buffered_write);
  
  /**
   * __generic_file_aio_write - write data to a file
   * @iocb:	IO state structure (file, offset, etc.)
   * @iov:	vector with data to write
   * @nr_segs:	number of segments in the vector
   * @ppos:	position where 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_aio_write(struct kiocb *iocb, const struct iovec *iov,
  				 unsigned long nr_segs, loff_t *ppos)
  {
  	struct file *file = iocb->ki_filp;
  	struct address_space * mapping = file->f_mapping;
  	size_t ocount;		/* original count */
  	size_t count;		/* after file limit checks */
  	struct inode 	*inode = mapping->host;
  	loff_t		pos;
  	ssize_t		written;
  	ssize_t		err;
  
  	ocount = 0;
  	err = generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);
  	if (err)
  		return err;
  
  	count = ocount;
  	pos = *ppos;
  
  	/* We can write back this queue in page reclaim */
  	current->backing_dev_info = mapping->backing_dev_info;
  	written = 0;
  
  	err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
  	if (err)
  		goto out;
  
  	if (count == 0)
  		goto out;
  
  	err = file_remove_suid(file);
  	if (err)
  		goto out;
  
  	err = file_update_time(file);
  	if (err)
  		goto out;
  
  	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
  	if (unlikely(file->f_flags & O_DIRECT)) {
  		loff_t endbyte;
  		ssize_t written_buffered;
  
  		written = generic_file_direct_write(iocb, iov, &nr_segs, pos,
  							ppos, count, ocount);
  		if (written < 0 || written == count)
  			goto out;
  		/*
  		 * direct-io write to a hole: fall through to buffered I/O
  		 * for completing the rest of the request.
  		 */
  		pos += written;
  		count -= written;
  		written_buffered = generic_file_buffered_write(iocb, iov,
  						nr_segs, pos, ppos, count,
  						written);
  		/*
  		 * If generic_file_buffered_write() retuned 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 (written_buffered < 0) {
  			err = written_buffered;
  			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 + written_buffered - written - 1;
  		err = filemap_write_and_wait_range(file->f_mapping, pos, endbyte);
  		if (err == 0) {
  			written = written_buffered;
  			invalidate_mapping_pages(mapping,
  						 pos >> PAGE_CACHE_SHIFT,
  						 endbyte >> PAGE_CACHE_SHIFT);
  		} else {
  			/*
  			 * We don't know how much we wrote, so just return
  			 * the number of bytes which were direct-written
  			 */
  		}
  	} else {
  		written = generic_file_buffered_write(iocb, iov, nr_segs,
  				pos, ppos, count, written);
  	}
  out:
  	current->backing_dev_info = NULL;
  	return written ? written : err;
  }
  EXPORT_SYMBOL(__generic_file_aio_write);
  
  /**
   * generic_file_aio_write - write data to a file
   * @iocb:	IO state structure
   * @iov:	vector with data to write
   * @nr_segs:	number of segments in the vector
   * @pos:	position in file where to write
   *
   * This is a wrapper around __generic_file_aio_write() 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_aio_write(struct kiocb *iocb, const struct iovec *iov,
  		unsigned long nr_segs, loff_t pos)
  {
  	struct file *file = iocb->ki_filp;
  	struct inode *inode = file->f_mapping->host;
  	ssize_t ret;
  
  	BUG_ON(iocb->ki_pos != pos);
  
  	mutex_lock(&inode->i_mutex);
  	ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
  	mutex_unlock(&inode->i_mutex);
  
  	if (ret > 0) {
  		ssize_t err;
  
  		err = generic_write_sync(file, pos, ret);
  		if (err < 0 && ret > 0)
  			ret = err;
  	}
  	return ret;
  }
  EXPORT_SYMBOL(generic_file_aio_write);
  
  /**
   * 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_WAIT & __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);
  