text: // SPDX-License-Identifier: GPL-2.0
  /*
   * Copyright (C) 2010 Red Hat, Inc.
   * Copyright (C) 2016-2019 Christoph Hellwig.
   */
  #include <linux/module.h>
  #include <linux/compiler.h>
  #include <linux/fs.h>
  #include <linux/iomap.h>
  #include <linux/pagemap.h>
  #include <linux/uio.h>
  #include <linux/buffer_head.h>
  #include <linux/dax.h>
  #include <linux/writeback.h>
  #include <linux/list_sort.h>
  #include <linux/swap.h>
  #include <linux/bio.h>
  #include <linux/sched/signal.h>
  #include <linux/migrate.h>
  #include "trace.h"
  
  #include "../internal.h"
  
  /*
   * Structure allocated for each page or THP when block size < page size
   * to track sub-page uptodate status and I/O completions.
   */
  struct iomap_page {
  	atomic_t		read_bytes_pending;
  	atomic_t		write_bytes_pending;
  	spinlock_t		uptodate_lock;
  	unsigned long		uptodate[];
  };
  
  static inline struct iomap_page *to_iomap_page(struct page *page)
  {
  	/*
  	 * per-block data is stored in the head page.  Callers should
  	 * not be dealing with tail pages (and if they are, they can
  	 * call thp_head() first.
  	 */
  	VM_BUG_ON_PGFLAGS(PageTail(page), page);
  
  	if (page_has_private(page))
  		return (struct iomap_page *)page_private(page);
  	return NULL;
  }
  
  static struct bio_set iomap_ioend_bioset;
  
  static struct iomap_page *
  iomap_page_create(struct inode *inode, struct page *page)
  {
  	struct iomap_page *iop = to_iomap_page(page);
  	unsigned int nr_blocks = i_blocks_per_page(inode, page);
  
  	if (iop || nr_blocks <= 1)
  		return iop;
  
  	iop = kzalloc(struct_size(iop, uptodate, BITS_TO_LONGS(nr_blocks)),
  			GFP_NOFS | __GFP_NOFAIL);
  	spin_lock_init(&iop->uptodate_lock);
  	if (PageUptodate(page))
  		bitmap_fill(iop->uptodate, nr_blocks);
  	attach_page_private(page, iop);
  	return iop;
  }
  
  static void
  iomap_page_release(struct page *page)
  {
  	struct iomap_page *iop = detach_page_private(page);
  	unsigned int nr_blocks = i_blocks_per_page(page->mapping->host, page);
  
  	if (!iop)
  		return;
  	WARN_ON_ONCE(atomic_read(&iop->read_bytes_pending));
  	WARN_ON_ONCE(atomic_read(&iop->write_bytes_pending));
  	WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
  			PageUptodate(page));
  	kfree(iop);
  }
  
  /*
   * Calculate the range inside the page that we actually need to read.
   */
  static void
  iomap_adjust_read_range(struct inode *inode, struct iomap_page *iop,
  		loff_t *pos, loff_t length, unsigned *offp, unsigned *lenp)
  {
  	loff_t orig_pos = *pos;
  	loff_t isize = i_size_read(inode);
  	unsigned block_bits = inode->i_blkbits;
  	unsigned block_size = (1 << block_bits);
  	unsigned poff = offset_in_page(*pos);
  	unsigned plen = min_t(loff_t, PAGE_SIZE - poff, length);
  	unsigned first = poff >> block_bits;
  	unsigned last = (poff + plen - 1) >> block_bits;
  
  	/*
  	 * If the block size is smaller than the page size we need to check the
  	 * per-block uptodate status and adjust the offset and length if needed
  	 * to avoid reading in already uptodate ranges.
  	 */
  	if (iop) {
  		unsigned int i;
  
  		/* move forward for each leading block marked uptodate */
  		for (i = first; i <= last; i++) {
  			if (!test_bit(i, iop->uptodate))
  				break;
  			*pos += block_size;
  			poff += block_size;
  			plen -= block_size;
  			first++;
  		}
  
  		/* truncate len if we find any trailing uptodate block(s) */
  		for ( ; i <= last; i++) {
  			if (test_bit(i, iop->uptodate)) {
  				plen -= (last - i + 1) * block_size;
  				last = i - 1;
  				break;
  			}
  		}
  	}
  
  	/*
  	 * If the extent spans the block that contains the i_size we need to
  	 * handle both halves separately so that we properly zero data in the
  	 * page cache for blocks that are entirely outside of i_size.
  	 */
  	if (orig_pos <= isize && orig_pos + length > isize) {
  		unsigned end = offset_in_page(isize - 1) >> block_bits;
  
  		if (first <= end && last > end)
  			plen -= (last - end) * block_size;
  	}
  
  	*offp = poff;
  	*lenp = plen;
  }
  
  static void
  iomap_iop_set_range_uptodate(struct page *page, unsigned off, unsigned len)
  {
  	struct iomap_page *iop = to_iomap_page(page);
  	struct inode *inode = page->mapping->host;
  	unsigned first = off >> inode->i_blkbits;
  	unsigned last = (off + len - 1) >> inode->i_blkbits;
  	unsigned long flags;
  
  	spin_lock_irqsave(&iop->uptodate_lock, flags);
  	bitmap_set(iop->uptodate, first, last - first + 1);
  	if (bitmap_full(iop->uptodate, i_blocks_per_page(inode, page)))
  		SetPageUptodate(page);
  	spin_unlock_irqrestore(&iop->uptodate_lock, flags);
  }
  
  static void
  iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
  {
  	if (PageError(page))
  		return;
  
  	if (page_has_private(page))
  		iomap_iop_set_range_uptodate(page, off, len);
  	else
  		SetPageUptodate(page);
  }
  
  static void
  iomap_read_page_end_io(struct bio_vec *bvec, int error)
  {
  	struct page *page = bvec->bv_page;
  	struct iomap_page *iop = to_iomap_page(page);
  
  	if (unlikely(error)) {
  		ClearPageUptodate(page);
  		SetPageError(page);
  	} else {
  		iomap_set_range_uptodate(page, bvec->bv_offset, bvec->bv_len);
  	}
  
  	if (!iop || atomic_sub_and_test(bvec->bv_len, &iop->read_bytes_pending))
  		unlock_page(page);
  }
  
  static void
  iomap_read_end_io(struct bio *bio)
  {
  	int error = blk_status_to_errno(bio->bi_status);
  	struct bio_vec *bvec;
  	struct bvec_iter_all iter_all;
  
  	bio_for_each_segment_all(bvec, bio, iter_all)
  		iomap_read_page_end_io(bvec, error);
  	bio_put(bio);
  }
  
  struct iomap_readpage_ctx {
  	struct page		*cur_page;
  	bool			cur_page_in_bio;
  	struct bio		*bio;
  	struct readahead_control *rac;
  };
  
  static void
  iomap_read_inline_data(struct inode *inode, struct page *page,
  		struct iomap *iomap)
  {
  	size_t size = i_size_read(inode);
  	void *addr;
  
  	if (PageUptodate(page))
  		return;
  
  	BUG_ON(page->index);
  	BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
  
  	addr = kmap_atomic(page);
  	memcpy(addr, iomap->inline_data, size);
  	memset(addr + size, 0, PAGE_SIZE - size);
  	kunmap_atomic(addr);
  	SetPageUptodate(page);
  }
  
  static inline bool iomap_block_needs_zeroing(struct inode *inode,
  		struct iomap *iomap, loff_t pos)
  {
  	return iomap->type != IOMAP_MAPPED ||
  		(iomap->flags & IOMAP_F_NEW) ||
  		pos >= i_size_read(inode);
  }
  
  static loff_t
  iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
  		struct iomap *iomap, struct iomap *srcmap)
  {
  	struct iomap_readpage_ctx *ctx = data;
  	struct page *page = ctx->cur_page;
  	struct iomap_page *iop = iomap_page_create(inode, page);
  	bool same_page = false, is_contig = false;
  	loff_t orig_pos = pos;
  	unsigned poff, plen;
  	sector_t sector;
  
  	if (iomap->type == IOMAP_INLINE) {
  		WARN_ON_ONCE(pos);
  		iomap_read_inline_data(inode, page, iomap);
  		return PAGE_SIZE;
  	}
  
  	/* zero post-eof blocks as the page may be mapped */
  	iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen);
  	if (plen == 0)
  		goto done;
  
  	if (iomap_block_needs_zeroing(inode, iomap, pos)) {
  		zero_user(page, poff, plen);
  		iomap_set_range_uptodate(page, poff, plen);
  		goto done;
  	}
  
  	ctx->cur_page_in_bio = true;
  	if (iop)
  		atomic_add(plen, &iop->read_bytes_pending);
  
  	/* Try to merge into a previous segment if we can */
  	sector = iomap_sector(iomap, pos);
  	if (ctx->bio && bio_end_sector(ctx->bio) == sector) {
  		if (__bio_try_merge_page(ctx->bio, page, plen, poff,
  				&same_page))
  			goto done;
  		is_contig = true;
  	}
  
  	if (!is_contig || bio_full(ctx->bio, plen)) {
  		gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
  		gfp_t orig_gfp = gfp;
  		int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT;
  
  		if (ctx->bio)
  			submit_bio(ctx->bio);
  
  		if (ctx->rac) /* same as readahead_gfp_mask */
  			gfp |= __GFP_NORETRY | __GFP_NOWARN;
  		ctx->bio = bio_alloc(gfp, min(BIO_MAX_PAGES, nr_vecs));
  		/*
  		 * If the bio_alloc fails, try it again for a single page to
  		 * avoid having to deal with partial page reads.  This emulates
  		 * what do_mpage_readpage does.
  		 */
  		if (!ctx->bio)
  			ctx->bio = bio_alloc(orig_gfp, 1);
  		ctx->bio->bi_opf = REQ_OP_READ;
  		if (ctx->rac)
  			ctx->bio->bi_opf |= REQ_RAHEAD;
  		ctx->bio->bi_iter.bi_sector = sector;
  		bio_set_dev(ctx->bio, iomap->bdev);
  		ctx->bio->bi_end_io = iomap_read_end_io;
  	}
  
  	bio_add_page(ctx->bio, page, plen, poff);
  done:
  	/*
  	 * Move the caller beyond our range so that it keeps making progress.
  	 * For that we have to include any leading non-uptodate ranges, but
  	 * we can skip trailing ones as they will be handled in the next
  	 * iteration.
  	 */
  	return pos - orig_pos + plen;
  }
  
  int
  iomap_readpage(struct page *page, const struct iomap_ops *ops)
  {
  	struct iomap_readpage_ctx ctx = { .cur_page = page };
  	struct inode *inode = page->mapping->host;
  	unsigned poff;
  	loff_t ret;
  
  	trace_iomap_readpage(page->mapping->host, 1);
  
  	for (poff = 0; poff < PAGE_SIZE; poff += ret) {
  		ret = iomap_apply(inode, page_offset(page) + poff,
  				PAGE_SIZE - poff, 0, ops, &ctx,
  				iomap_readpage_actor);
  		if (ret <= 0) {
  			WARN_ON_ONCE(ret == 0);
  			SetPageError(page);
  			break;
  		}
  	}
  
  	if (ctx.bio) {
  		submit_bio(ctx.bio);
  		WARN_ON_ONCE(!ctx.cur_page_in_bio);
  	} else {
  		WARN_ON_ONCE(ctx.cur_page_in_bio);
  		unlock_page(page);
  	}
  
  	/*
  	 * Just like mpage_readahead and block_read_full_page we always
  	 * return 0 and just mark the page as PageError on errors.  This
  	 * should be cleaned up all through the stack eventually.
  	 */
  	return 0;
  }
  EXPORT_SYMBOL_GPL(iomap_readpage);
  
  static loff_t
  iomap_readahead_actor(struct inode *inode, loff_t pos, loff_t length,
  		void *data, struct iomap *iomap, struct iomap *srcmap)
  {
  	struct iomap_readpage_ctx *ctx = data;
  	loff_t done, ret;
  
  	for (done = 0; done < length; done += ret) {
  		if (ctx->cur_page && offset_in_page(pos + done) == 0) {
  			if (!ctx->cur_page_in_bio)
  				unlock_page(ctx->cur_page);
  			put_page(ctx->cur_page);
  			ctx->cur_page = NULL;
  		}
  		if (!ctx->cur_page) {
  			ctx->cur_page = readahead_page(ctx->rac);
  			ctx->cur_page_in_bio = false;
  		}
  		ret = iomap_readpage_actor(inode, pos + done, length - done,
  				ctx, iomap, srcmap);
  	}
  
  	return done;
  }
  
  /**
   * iomap_readahead - Attempt to read pages from a file.
   * @rac: Describes the pages to be read.
   * @ops: The operations vector for the filesystem.
   *
   * This function is for filesystems to call to implement their readahead
   * address_space operation.
   *
   * Context: The @ops callbacks may submit I/O (eg to read the addresses of
   * blocks from disc), and may wait for it.  The caller may be trying to
   * access a different page, and so sleeping excessively should be avoided.
   * It may allocate memory, but should avoid costly allocations.  This
   * function is called with memalloc_nofs set, so allocations will not cause
   * the filesystem to be reentered.
   */
  void iomap_readahead(struct readahead_control *rac, const struct iomap_ops *ops)
  {
  	struct inode *inode = rac->mapping->host;
  	loff_t pos = readahead_pos(rac);
  	loff_t length = readahead_length(rac);
  	struct iomap_readpage_ctx ctx = {
  		.rac	= rac,
  	};
  
  	trace_iomap_readahead(inode, readahead_count(rac));
  
  	while (length > 0) {
  		loff_t ret = iomap_apply(inode, pos, length, 0, ops,
  				&ctx, iomap_readahead_actor);
  		if (ret <= 0) {
  			WARN_ON_ONCE(ret == 0);
  			break;
  		}
  		pos += ret;
  		length -= ret;
  	}
  
  	if (ctx.bio)
  		submit_bio(ctx.bio);
  	if (ctx.cur_page) {
  		if (!ctx.cur_page_in_bio)
  			unlock_page(ctx.cur_page);
  		put_page(ctx.cur_page);
  	}
  }
  EXPORT_SYMBOL_GPL(iomap_readahead);
  
  /*
   * iomap_is_partially_uptodate checks whether blocks within a page are
   * uptodate or not.
   *
   * Returns true if all blocks which correspond to a file portion
   * we want to read within the page are uptodate.
   */
  int
  iomap_is_partially_uptodate(struct page *page, unsigned long from,
  		unsigned long count)
  {
  	struct iomap_page *iop = to_iomap_page(page);
  	struct inode *inode = page->mapping->host;
  	unsigned len, first, last;
  	unsigned i;
  
  	/* Limit range to one page */
  	len = min_t(unsigned, PAGE_SIZE - from, count);
  
  	/* First and last blocks in range within page */
  	first = from >> inode->i_blkbits;
  	last = (from + len - 1) >> inode->i_blkbits;
  
  	if (iop) {
  		for (i = first; i <= last; i++)
  			if (!test_bit(i, iop->uptodate))
  				return 0;
  		return 1;
  	}
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
  
  int
  iomap_releasepage(struct page *page, gfp_t gfp_mask)
  {
  	trace_iomap_releasepage(page->mapping->host, page_offset(page),
  			PAGE_SIZE);
  
  	/*
  	 * mm accommodates an old ext3 case where clean pages might not have had
  	 * the dirty bit cleared. Thus, it can send actual dirty pages to
  	 * ->releasepage() via shrink_active_list(), skip those here.
  	 */
  	if (PageDirty(page) || PageWriteback(page))
  		return 0;
  	iomap_page_release(page);
  	return 1;
  }
  EXPORT_SYMBOL_GPL(iomap_releasepage);
  
  void
  iomap_invalidatepage(struct page *page, unsigned int offset, unsigned int len)
  {
  	trace_iomap_invalidatepage(page->mapping->host, offset, len);
  
  	/*
  	 * If we are invalidating the entire page, clear the dirty state from it
  	 * and release it to avoid unnecessary buildup of the LRU.
  	 */
  	if (offset == 0 && len == PAGE_SIZE) {
  		WARN_ON_ONCE(PageWriteback(page));
  		cancel_dirty_page(page);
  		iomap_page_release(page);
  	}
  }
  EXPORT_SYMBOL_GPL(iomap_invalidatepage);
  
  #ifdef CONFIG_MIGRATION
  int
  iomap_migrate_page(struct address_space *mapping, struct page *newpage,
  		struct page *page, enum migrate_mode mode)
  {
  	int ret;
  
  	ret = migrate_page_move_mapping(mapping, newpage, page, 0);
  	if (ret != MIGRATEPAGE_SUCCESS)
  		return ret;
  
  	if (page_has_private(page))
  		attach_page_private(newpage, detach_page_private(page));
  
  	if (mode != MIGRATE_SYNC_NO_COPY)
  		migrate_page_copy(newpage, page);
  	else
  		migrate_page_states(newpage, page);
  	return MIGRATEPAGE_SUCCESS;
  }
  EXPORT_SYMBOL_GPL(iomap_migrate_page);
  #endif /* CONFIG_MIGRATION */
  
  enum {
  	IOMAP_WRITE_F_UNSHARE		= (1 << 0),
  };
  
  static void
  iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
  {
  	loff_t i_size = i_size_read(inode);
  
  	/*
  	 * Only truncate newly allocated pages beyoned EOF, even if the
  	 * write started inside the existing inode size.
  	 */
  	if (pos + len > i_size)
  		truncate_pagecache_range(inode, max(pos, i_size), pos + len);
  }
  
  static int
  iomap_read_page_sync(loff_t block_start, struct page *page, unsigned poff,
  		unsigned plen, struct iomap *iomap)
  {
  	struct bio_vec bvec;
  	struct bio bio;
  
  	bio_init(&bio, &bvec, 1);
  	bio.bi_opf = REQ_OP_READ;
  	bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
  	bio_set_dev(&bio, iomap->bdev);
  	__bio_add_page(&bio, page, plen, poff);
  	return submit_bio_wait(&bio);
  }
  
  static int
  __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
  		struct page *page, struct iomap *srcmap)
  {
  	struct iomap_page *iop = iomap_page_create(inode, page);
  	loff_t block_size = i_blocksize(inode);
  	loff_t block_start = round_down(pos, block_size);
  	loff_t block_end = round_up(pos + len, block_size);
  	unsigned from = offset_in_page(pos), to = from + len, poff, plen;
  
  	if (PageUptodate(page))
  		return 0;
  	ClearPageError(page);
  
  	do {
  		iomap_adjust_read_range(inode, iop, &block_start,
  				block_end - block_start, &poff, &plen);
  		if (plen == 0)
  			break;
  
  		if (!(flags & IOMAP_WRITE_F_UNSHARE) &&
  		    (from <= poff || from >= poff + plen) &&
  		    (to <= poff || to >= poff + plen))
  			continue;
  
  		if (iomap_block_needs_zeroing(inode, srcmap, block_start)) {
  			if (WARN_ON_ONCE(flags & IOMAP_WRITE_F_UNSHARE))
  				return -EIO;
  			zero_user_segments(page, poff, from, to, poff + plen);
  		} else {
  			int status = iomap_read_page_sync(block_start, page,
  					poff, plen, srcmap);
  			if (status)
  				return status;
  		}
  		iomap_set_range_uptodate(page, poff, plen);
  	} while ((block_start += plen) < block_end);
  
  	return 0;
  }
  
  static int
  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
  		struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
  {
  	const struct iomap_page_ops *page_ops = iomap->page_ops;
  	struct page *page;
  	int status = 0;
  
  	BUG_ON(pos + len > iomap->offset + iomap->length);
  	if (srcmap != iomap)
  		BUG_ON(pos + len > srcmap->offset + srcmap->length);
  
  	if (fatal_signal_pending(current))
  		return -EINTR;
  
  	if (page_ops && page_ops->page_prepare) {
  		status = page_ops->page_prepare(inode, pos, len, iomap);
  		if (status)
  			return status;
  	}
  
  	page = grab_cache_page_write_begin(inode->i_mapping, pos >> PAGE_SHIFT,
  			AOP_FLAG_NOFS);
  	if (!page) {
  		status = -ENOMEM;
  		goto out_no_page;
  	}
  
  	if (srcmap->type == IOMAP_INLINE)
  		iomap_read_inline_data(inode, page, srcmap);
  	else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
  		status = __block_write_begin_int(page, pos, len, NULL, srcmap);
  	else
  		status = __iomap_write_begin(inode, pos, len, flags, page,
  				srcmap);
  
  	if (unlikely(status))
  		goto out_unlock;
  
  	*pagep = page;
  	return 0;
  
  out_unlock:
  	unlock_page(page);
  	put_page(page);
  	iomap_write_failed(inode, pos, len);
  
  out_no_page:
  	if (page_ops && page_ops->page_done)
  		page_ops->page_done(inode, pos, 0, NULL, iomap);
  	return status;
  }
  
  int
  iomap_set_page_dirty(struct page *page)
  {
  	struct address_space *mapping = page_mapping(page);
  	int newly_dirty;
  
  	if (unlikely(!mapping))
  		return !TestSetPageDirty(page);
  
  	/*
  	 * Lock out page->mem_cgroup migration to keep PageDirty
  	 * synchronized with per-memcg dirty page counters.
  	 */
  	lock_page_memcg(page);
  	newly_dirty = !TestSetPageDirty(page);
  	if (newly_dirty)
  		__set_page_dirty(page, mapping, 0);
  	unlock_page_memcg(page);
  
  	if (newly_dirty)
  		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
  	return newly_dirty;
  }
  EXPORT_SYMBOL_GPL(iomap_set_page_dirty);
  
  static size_t __iomap_write_end(struct inode *inode, loff_t pos, size_t len,
  		size_t copied, struct page *page)
  {
  	flush_dcache_page(page);
  
  	/*
  	 * The blocks that were entirely written will now be uptodate, so we
  	 * don't have to worry about a readpage reading them and overwriting a
  	 * partial write.  However if we have encountered a short write and only
  	 * partially written into a block, it will not be marked uptodate, so a
  	 * readpage might come in and destroy our partial write.
  	 *
  	 * Do the simplest thing, and just treat any short write to a non
  	 * uptodate page as a zero-length write, and force the caller to redo
  	 * the whole thing.
  	 */
  	if (unlikely(copied < len && !PageUptodate(page)))
  		return 0;
  	iomap_set_range_uptodate(page, offset_in_page(pos), len);
  	iomap_set_page_dirty(page);
  	return copied;
  }
  
  static size_t iomap_write_end_inline(struct inode *inode, struct page *page,
  		struct iomap *iomap, loff_t pos, size_t copied)
  {
  	void *addr;
  
  	WARN_ON_ONCE(!PageUptodate(page));
  	BUG_ON(pos + copied > PAGE_SIZE - offset_in_page(iomap->inline_data));
  
  	flush_dcache_page(page);
  	addr = kmap_atomic(page);
  	memcpy(iomap->inline_data + pos, addr + pos, copied);
  	kunmap_atomic(addr);
  
  	mark_inode_dirty(inode);
  	return copied;
  }
  
  /* Returns the number of bytes copied.  May be 0.  Cannot be an errno. */
  static size_t iomap_write_end(struct inode *inode, loff_t pos, size_t len,
  		size_t copied, struct page *page, struct iomap *iomap,
  		struct iomap *srcmap)
  {
  	const struct iomap_page_ops *page_ops = iomap->page_ops;
  	loff_t old_size = inode->i_size;
  	size_t ret;
  
  	if (srcmap->type == IOMAP_INLINE) {
  		ret = iomap_write_end_inline(inode, page, iomap, pos, copied);
  	} else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
  		ret = block_write_end(NULL, inode->i_mapping, pos, len, copied,
  				page, NULL);
  	} else {
  		ret = __iomap_write_end(inode, pos, len, copied, page);
  	}
  
  	/*
  	 * Update the in-memory inode size after copying the data into the page
  	 * cache.  It's up to the file system to write the updated size to disk,
  	 * preferably after I/O completion so that no stale data is exposed.
  	 */
  	if (pos + ret > old_size) {
  		i_size_write(inode, pos + ret);
  		iomap->flags |= IOMAP_F_SIZE_CHANGED;
  	}
  	unlock_page(page);
  
  	if (old_size < pos)
  		pagecache_isize_extended(inode, old_size, pos);
  	if (page_ops && page_ops->page_done)
  		page_ops->page_done(inode, pos, ret, page, iomap);
  	put_page(page);
  
  	if (ret < len)
  		iomap_write_failed(inode, pos, len);
  	return ret;
  }
  
  static loff_t
  iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
  		struct iomap *iomap, struct iomap *srcmap)
  {
  	struct iov_iter *i = data;
  	long status = 0;
  	ssize_t written = 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 */
  
  		offset = offset_in_page(pos);
  		bytes = min_t(unsigned long, PAGE_SIZE - offset,
  						iov_iter_count(i));
  again:
  		if (bytes > length)
  			bytes = length;
  
  		/*
  		 * 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 = iomap_write_begin(inode, pos, bytes, 0, &page, iomap,
  				srcmap);
  		if (unlikely(status))
  			break;
  
  		if (mapping_writably_mapped(inode->i_mapping))
  			flush_dcache_page(page);
  
  		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
  
  		copied = iomap_write_end(inode, pos, bytes, copied, page, iomap,
  				srcmap);
  
  		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;
  		length -= copied;
  
  		balance_dirty_pages_ratelimited(inode->i_mapping);
  	} while (iov_iter_count(i) && length);
  
  	return written ? written : status;
  }
  
  ssize_t
  iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *iter,
  		const struct iomap_ops *ops)
  {
  	struct inode *inode = iocb->ki_filp->f_mapping->host;
  	loff_t pos = iocb->ki_pos, ret = 0, written = 0;
  
  	while (iov_iter_count(iter)) {
  		ret = iomap_apply(inode, pos, iov_iter_count(iter),
  				IOMAP_WRITE, ops, iter, iomap_write_actor);
  		if (ret <= 0)
  			break;
  		pos += ret;
  		written += ret;
  	}
  
  	return written ? written : ret;
  }
  EXPORT_SYMBOL_GPL(iomap_file_buffered_write);
  
  static loff_t
  iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
  		struct iomap *iomap, struct iomap *srcmap)
  {
  	long status = 0;
  	loff_t written = 0;
  
  	/* don't bother with blocks that are not shared to start with */
  	if (!(iomap->flags & IOMAP_F_SHARED))
  		return length;
  	/* don't bother with holes or unwritten extents */
  	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
  		return length;
  
  	do {
  		unsigned long offset = offset_in_page(pos);
  		unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
  		struct page *page;
  
  		status = iomap_write_begin(inode, pos, bytes,
  				IOMAP_WRITE_F_UNSHARE, &page, iomap, srcmap);
  		if (unlikely(status))
  			return status;
  
  		status = iomap_write_end(inode, pos, bytes, bytes, page, iomap,
  				srcmap);
  		if (WARN_ON_ONCE(status == 0))
  			return -EIO;
  
  		cond_resched();
  
  		pos += status;
  		written += status;
  		length -= status;
  
  		balance_dirty_pages_ratelimited(inode->i_mapping);
  	} while (length);
  
  	return written;
  }
  
  int
  iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
  		const struct iomap_ops *ops)
  {
  	loff_t ret;
  
  	while (len) {
  		ret = iomap_apply(inode, pos, len, IOMAP_WRITE, ops, NULL,
  				iomap_unshare_actor);
  		if (ret <= 0)
  			return ret;
  		pos += ret;
  		len -= ret;
  	}
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(iomap_file_unshare);
  
  static s64 iomap_zero(struct inode *inode, loff_t pos, u64 length,
  		struct iomap *iomap, struct iomap *srcmap)
  {
  	struct page *page;
  	int status;
  	unsigned offset = offset_in_page(pos);
  	unsigned bytes = min_t(u64, PAGE_SIZE - offset, length);
  
  	status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap, srcmap);
  	if (status)
  		return status;
  
  	zero_user(page, offset, bytes);
  	mark_page_accessed(page);
  
  	return iomap_write_end(inode, pos, bytes, bytes, page, iomap, srcmap);
  }
  
  static loff_t iomap_zero_range_actor(struct inode *inode, loff_t pos,
  		loff_t length, void *data, struct iomap *iomap,
  		struct iomap *srcmap)
  {
  	bool *did_zero = data;
  	loff_t written = 0;
  
  	/* already zeroed?  we're done. */
  	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
  		return length;
  
  	do {
  		s64 bytes;
  
  		if (IS_DAX(inode))
  			bytes = dax_iomap_zero(pos, length, iomap);
  		else
  			bytes = iomap_zero(inode, pos, length, iomap, srcmap);
  		if (bytes < 0)
  			return bytes;
  
  		pos += bytes;
  		length -= bytes;
  		written += bytes;
  		if (did_zero)
  			*did_zero = true;
  	} while (length > 0);
  
  	return written;
  }
  
  int
  iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
  		const struct iomap_ops *ops)
  {
  	loff_t ret;
  
  	while (len > 0) {
  		ret = iomap_apply(inode, pos, len, IOMAP_ZERO,
  				ops, did_zero, iomap_zero_range_actor);
  		if (ret <= 0)
  			return ret;
  
  		pos += ret;
  		len -= ret;
  	}
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(iomap_zero_range);
  
  int
  iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
  		const struct iomap_ops *ops)
  {
  	unsigned int blocksize = i_blocksize(inode);
  	unsigned int off = pos & (blocksize - 1);
  
  	/* Block boundary? Nothing to do */
  	if (!off)
  		return 0;
  	return iomap_zero_range(inode, pos, blocksize - off, did_zero, ops);
  }
  EXPORT_SYMBOL_GPL(iomap_truncate_page);
  
  static loff_t
  iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
  		void *data, struct iomap *iomap, struct iomap *srcmap)
  {
  	struct page *page = data;
  	int ret;
  
  	if (iomap->flags & IOMAP_F_BUFFER_HEAD) {
  		ret = __block_write_begin_int(page, pos, length, NULL, iomap);
  		if (ret)
  			return ret;
  		block_commit_write(page, 0, length);
  	} else {
  		WARN_ON_ONCE(!PageUptodate(page));
  		iomap_page_create(inode, page);
  		set_page_dirty(page);
  	}
  
  	return length;
  }
  
  vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
  {
  	struct page *page = vmf->page;
  	struct inode *inode = file_inode(vmf->vma->vm_file);
  	unsigned long length;
  	loff_t offset;
  	ssize_t ret;
  
  	lock_page(page);
  	ret = page_mkwrite_check_truncate(page, inode);
  	if (ret < 0)
  		goto out_unlock;
  	length = ret;
  
  	offset = page_offset(page);
  	while (length > 0) {
  		ret = iomap_apply(inode, offset, length,
  				IOMAP_WRITE | IOMAP_FAULT, ops, page,
  				iomap_page_mkwrite_actor);
  		if (unlikely(ret <= 0))
  			goto out_unlock;
  		offset += ret;
  		length -= ret;
  	}
  
  	wait_for_stable_page(page);
  	return VM_FAULT_LOCKED;
  out_unlock:
  	unlock_page(page);
  	return block_page_mkwrite_return(ret);
  }
  EXPORT_SYMBOL_GPL(iomap_page_mkwrite);
  
  static void
  iomap_finish_page_writeback(struct inode *inode, struct page *page,
  		int error, unsigned int len)
  {
  	struct iomap_page *iop = to_iomap_page(page);
  
  	if (error) {
  		SetPageError(page);
  		mapping_set_error(inode->i_mapping, error);
  	}
  
  	WARN_ON_ONCE(i_blocks_per_page(inode, page) > 1 && !iop);
  	WARN_ON_ONCE(iop && atomic_read(&iop->write_bytes_pending) <= 0);
  
  	if (!iop || atomic_sub_and_test(len, &iop->write_bytes_pending))
  		end_page_writeback(page);
  }
  
  /*
   * We're now finished for good with this ioend structure.  Update the page
   * state, release holds on bios, and finally free up memory.  Do not use the
   * ioend after this.
   */
  static void
  iomap_finish_ioend(struct iomap_ioend *ioend, int error)
  {
  	struct inode *inode = ioend->io_inode;
  	struct bio *bio = &ioend->io_inline_bio;
  	struct bio *last = ioend->io_bio, *next;
  	u64 start = bio->bi_iter.bi_sector;
  	loff_t offset = ioend->io_offset;
  	bool quiet = bio_flagged(bio, BIO_QUIET);
  
  	for (bio = &ioend->io_inline_bio; bio; bio = next) {
  		struct bio_vec *bv;
  		struct bvec_iter_all iter_all;
  
  		/*
  		 * For the last bio, bi_private points to the ioend, so we
  		 * need to explicitly end the iteration here.
  		 */
  		if (bio == last)
  			next = NULL;
  		else
  			next = bio->bi_private;
  
  		/* walk each page on bio, ending page IO on them */
  		bio_for_each_segment_all(bv, bio, iter_all)
  			iomap_finish_page_writeback(inode, bv->bv_page, error,
  					bv->bv_len);
  		bio_put(bio);
  	}
  	/* The ioend has been freed by bio_put() */
  
  	if (unlikely(error && !quiet)) {
  		printk_ratelimited(KERN_ERR
  "%s: writeback error on inode %lu, offset %lld, sector %llu",
  			inode->i_sb->s_id, inode->i_ino, offset, start);
  	}
  }
  
  void
  iomap_finish_ioends(struct iomap_ioend *ioend, int error)
  {
  	struct list_head tmp;
  
  	list_replace_init(&ioend->io_list, &tmp);
  	iomap_finish_ioend(ioend, error);
  
  	while (!list_empty(&tmp)) {
  		ioend = list_first_entry(&tmp, struct iomap_ioend, io_list);
  		list_del_init(&ioend->io_list);
  		iomap_finish_ioend(ioend, error);
  	}
  }
  EXPORT_SYMBOL_GPL(iomap_finish_ioends);
  
  /*
   * We can merge two adjacent ioends if they have the same set of work to do.
   */
  static bool
  iomap_ioend_can_merge(struct iomap_ioend *ioend, struct iomap_ioend *next)
  {
  	if (ioend->io_bio->bi_status != next->io_bio->bi_status)
  		return false;
  	if ((ioend->io_flags & IOMAP_F_SHARED) ^
  	    (next->io_flags & IOMAP_F_SHARED))
  		return false;
  	if ((ioend->io_type == IOMAP_UNWRITTEN) ^
  	    (next->io_type == IOMAP_UNWRITTEN))
  		return false;
  	if (ioend->io_offset + ioend->io_size != next->io_offset)
  		return false;
  	return true;
  }
  
  void
  iomap_ioend_try_merge(struct iomap_ioend *ioend, struct list_head *more_ioends,
  		void (*merge_private)(struct iomap_ioend *ioend,
  				struct iomap_ioend *next))
  {
  	struct iomap_ioend *next;
  
  	INIT_LIST_HEAD(&ioend->io_list);
  
  	while ((next = list_first_entry_or_null(more_ioends, struct iomap_ioend,
  			io_list))) {
  		if (!iomap_ioend_can_merge(ioend, next))
  			break;
  		list_move_tail(&next->io_list, &ioend->io_list);
  		ioend->io_size += next->io_size;
  		if (next->io_private && merge_private)
  			merge_private(ioend, next);
  	}
  }
  EXPORT_SYMBOL_GPL(iomap_ioend_try_merge);
  
  static int
  iomap_ioend_compare(void *priv, const struct list_head *a,
  		const struct list_head *b)
  {
  	struct iomap_ioend *ia = container_of(a, struct iomap_ioend, io_list);
  	struct iomap_ioend *ib = container_of(b, struct iomap_ioend, io_list);
  
  	if (ia->io_offset < ib->io_offset)
  		return -1;
  	if (ia->io_offset > ib->io_offset)
  		return 1;
  	return 0;
  }
  
  void
  iomap_sort_ioends(struct list_head *ioend_list)
  {
  	list_sort(NULL, ioend_list, iomap_ioend_compare);
  }
  EXPORT_SYMBOL_GPL(iomap_sort_ioends);
  
  static void iomap_writepage_end_bio(struct bio *bio)
  {
  	struct iomap_ioend *ioend = bio->bi_private;
  
  	iomap_finish_ioend(ioend, blk_status_to_errno(bio->bi_status));
  }
  
  /*
   * Submit the final bio for an ioend.
   *
   * If @error is non-zero, it means that we have a situation where some part of
   * the submission process has failed after we have marked paged for writeback
   * and unlocked them.  In this situation, we need to fail the bio instead of
   * submitting it.  This typically only happens on a filesystem shutdown.
   */
  static int
  iomap_submit_ioend(struct iomap_writepage_ctx *wpc, struct iomap_ioend *ioend,
  		int error)
  {
  	ioend->io_bio->bi_private = ioend;
  	ioend->io_bio->bi_end_io = iomap_writepage_end_bio;
  
  	if (wpc->ops->prepare_ioend)
  		error = wpc->ops->prepare_ioend(ioend, error);
  	if (error) {
  		/*
  		 * If we are failing the IO now, just mark the ioend with an
  		 * error and finish it.  This will run IO completion immediately
  		 * as there is only one reference to the ioend at this point in
  		 * time.
  		 */
  		ioend->io_bio->bi_status = errno_to_blk_status(error);
  		bio_endio(ioend->io_bio);
  		return error;
  	}
  
  	submit_bio(ioend->io_bio);
  	return 0;
  }
  
  static struct iomap_ioend *
  iomap_alloc_ioend(struct inode *inode, struct iomap_writepage_ctx *wpc,
  		loff_t offset, sector_t sector, struct writeback_control *wbc)
  {
  	struct iomap_ioend *ioend;
  	struct bio *bio;
  
  	bio = bio_alloc_bioset(GFP_NOFS, BIO_MAX_PAGES, &iomap_ioend_bioset);
  	bio_set_dev(bio, wpc->iomap.bdev);
  	bio->bi_iter.bi_sector = sector;
  	bio->bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
  	bio->bi_write_hint = inode->i_write_hint;
  	wbc_init_bio(wbc, bio);
  
  	ioend = container_of(bio, struct iomap_ioend, io_inline_bio);
  	INIT_LIST_HEAD(&ioend->io_list);
  	ioend->io_type = wpc->iomap.type;
  	ioend->io_flags = wpc->iomap.flags;
  	ioend->io_inode = inode;
  	ioend->io_size = 0;
  	ioend->io_offset = offset;
  	ioend->io_private = NULL;
  	ioend->io_bio = bio;
  	return ioend;
  }
  
  /*
   * Allocate a new bio, and chain the old bio to the new one.
   *
   * Note that we have to do perform the chaining in this unintuitive order
   * so that the bi_private linkage is set up in the right direction for the
   * traversal in iomap_finish_ioend().
   */
  static struct bio *
  iomap_chain_bio(struct bio *prev)
  {
  	struct bio *new;
  
  	new = bio_alloc(GFP_NOFS, BIO_MAX_PAGES);
  	bio_copy_dev(new, prev);/* also copies over blkcg information */
  	new->bi_iter.bi_sector = bio_end_sector(prev);
  	new->bi_opf = prev->bi_opf;
  	new->bi_write_hint = prev->bi_write_hint;
  
  	bio_chain(prev, new);
  	bio_get(prev);		/* for iomap_finish_ioend */
  	submit_bio(prev);
  	return new;
  }
  
  static bool
  iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset,
  		sector_t sector)
  {
  	if ((wpc->iomap.flags & IOMAP_F_SHARED) !=
  	    (wpc->ioend->io_flags & IOMAP_F_SHARED))
  		return false;
  	if (wpc->iomap.type != wpc->ioend->io_type)
  		return false;
  	if (offset != wpc->ioend->io_offset + wpc->ioend->io_size)
  		return false;
  	if (sector != bio_end_sector(wpc->ioend->io_bio))
  		return false;
  	return true;
  }
  
  /*
   * Test to see if we have an existing ioend structure that we could append to
   * first, otherwise finish off the current ioend and start another.
   */
  static void
  iomap_add_to_ioend(struct inode *inode, loff_t offset, struct page *page,
  		struct iomap_page *iop, struct iomap_writepage_ctx *wpc,
  		struct writeback_control *wbc, struct list_head *iolist)
  {
  	sector_t sector = iomap_sector(&wpc->iomap, offset);
  	unsigned len = i_blocksize(inode);
  	unsigned poff = offset & (PAGE_SIZE - 1);
  	bool merged, same_page = false;
  
  	if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, offset, sector)) {
  		if (wpc->ioend)
  			list_add(&wpc->ioend->io_list, iolist);
  		wpc->ioend = iomap_alloc_ioend(inode, wpc, offset, sector, wbc);
  	}
  
  	merged = __bio_try_merge_page(wpc->ioend->io_bio, page, len, poff,
  			&same_page);
  	if (iop)
  		atomic_add(len, &iop->write_bytes_pending);
  
  	if (!merged) {
  		if (bio_full(wpc->ioend->io_bio, len)) {
  			wpc->ioend->io_bio =
  				iomap_chain_bio(wpc->ioend->io_bio);
  		}
  		bio_add_page(wpc->ioend->io_bio, page, len, poff);
  	}
  
  	wpc->ioend->io_size += len;
  	wbc_account_cgroup_owner(wbc, page, len);
  }
  
  /*
   * We implement an immediate ioend submission policy here to avoid needing to
   * chain multiple ioends and hence nest mempool allocations which can violate
   * forward progress guarantees we need to provide. The current ioend we are
   * adding blocks to is cached on the writepage context, and if the new block
   * does not append to the cached ioend it will create a new ioend and cache that
   * instead.
   *
   * If a new ioend is created and cached, the old ioend is returned and queued
   * locally for submission once the entire page is processed or an error has been
   * detected.  While ioends are submitted immediately after they are completed,
   * batching optimisations are provided by higher level block plugging.
   *
   * At the end of a writeback pass, there will be a cached ioend remaining on the
   * writepage context that the caller will need to submit.
   */
  static int
  iomap_writepage_map(struct iomap_writepage_ctx *wpc,
  		struct writeback_control *wbc, struct inode *inode,
  		struct page *page, u64 end_offset)
  {
  	struct iomap_page *iop = to_iomap_page(page);
  	struct iomap_ioend *ioend, *next;
  	unsigned len = i_blocksize(inode);
  	u64 file_offset; /* file offset of page */
  	int error = 0, count = 0, i;
  	LIST_HEAD(submit_list);
  
  	WARN_ON_ONCE(i_blocks_per_page(inode, page) > 1 && !iop);
  	WARN_ON_ONCE(iop && atomic_read(&iop->write_bytes_pending) != 0);
  
  	/*
  	 * Walk through the page to find areas to write back. If we run off the
  	 * end of the current map or find the current map invalid, grab a new
  	 * one.
  	 */
  	for (i = 0, file_offset = page_offset(page);
  	     i < (PAGE_SIZE >> inode->i_blkbits) && file_offset < end_offset;
  	     i++, file_offset += len) {
  		if (iop && !test_bit(i, iop->uptodate))
  			continue;
  
  		error = wpc->ops->map_blocks(wpc, inode, file_offset);
  		if (error)
  			break;
  		if (WARN_ON_ONCE(wpc->iomap.type == IOMAP_INLINE))
  			continue;
  		if (wpc->iomap.type == IOMAP_HOLE)
  			continue;
  		iomap_add_to_ioend(inode, file_offset, page, iop, wpc, wbc,
  				 &submit_list);
  		count++;
  	}
  
  	WARN_ON_ONCE(!wpc->ioend && !list_empty(&submit_list));
  	WARN_ON_ONCE(!PageLocked(page));
  	WARN_ON_ONCE(PageWriteback(page));
  	WARN_ON_ONCE(PageDirty(page));
  
  	/*
  	 * We cannot cancel the ioend directly here on error.  We may have
  	 * already set other pages under writeback and hence we have to run I/O
  	 * completion to mark the error state of the pages under writeback
  	 * appropriately.
  	 */
  	if (unlikely(error)) {
  		/*
  		 * Let the filesystem know what portion of the current page
  		 * failed to map. If the page wasn't been added to ioend, it
  		 * won't be affected by I/O completion and we must unlock it
  		 * now.
  		 */
  		if (wpc->ops->discard_page)
  			wpc->ops->discard_page(page, file_offset);
  		if (!count) {
  			ClearPageUptodate(page);
  			unlock_page(page);
  			goto done;
  		}
  	}
  
  	set_page_writeback(page);
  	unlock_page(page);
  
  	/*
  	 * Preserve the original error if there was one, otherwise catch
  	 * submission errors here and propagate into subsequent ioend
  	 * submissions.
  	 */
  	list_for_each_entry_safe(ioend, next, &submit_list, io_list) {
  		int error2;
  
  		list_del_init(&ioend->io_list);
  		error2 = iomap_submit_ioend(wpc, ioend, error);
  		if (error2 && !error)
  			error = error2;
  	}
  
  	/*
  	 * We can end up here with no error and nothing to write only if we race
  	 * with a partial page truncate on a sub-page block sized filesystem.
  	 */
  	if (!count)
  		end_page_writeback(page);
  done:
  	mapping_set_error(page->mapping, error);
  	return error;
  }
  
  /*
   * Write out a dirty page.
   *
   * For delalloc space on the page we need to allocate space and flush it.
   * For unwritten space on the page we need to start the conversion to
   * regular allocated space.
   */
  static int
  iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
  {
  	struct iomap_writepage_ctx *wpc = data;
  	struct inode *inode = page->mapping->host;
  	pgoff_t end_index;
  	u64 end_offset;
  	loff_t offset;
  
  	trace_iomap_writepage(inode, page_offset(page), PAGE_SIZE);
  
  	/*
  	 * Refuse to write the page out if we are called from reclaim context.
  	 *
  	 * This avoids stack overflows when called from deeply used stacks in
  	 * random callers for direct reclaim or memcg reclaim.  We explicitly
  	 * allow reclaim from kswapd as the stack usage there is relatively low.
  	 *
  	 * This should never happen except in the case of a VM regression so
  	 * warn about it.
  	 */
  	if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
  			PF_MEMALLOC))
  		goto redirty;
  
  	/*
  	 * Given that we do not allow direct reclaim to call us, we should
  	 * never be called in a recursive filesystem reclaim context.
  	 */
  	if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
  		goto redirty;
  
  	/*
  	 * Is this page beyond the end of the file?
  	 *
  	 * The page index is less than the end_index, adjust the end_offset
  	 * to the highest offset that this page should represent.
  	 * -----------------------------------------------------
  	 * |			file mapping	       | <EOF> |
  	 * -----------------------------------------------------
  	 * | Page ... | Page N-2 | Page N-1 |  Page N  |       |
  	 * ^--------------------------------^----------|--------
  	 * |     desired writeback range    |      see else    |
  	 * ---------------------------------^------------------|
  	 */
  	offset = i_size_read(inode);
  	end_index = offset >> PAGE_SHIFT;
  	if (page->index < end_index)
  		end_offset = (loff_t)(page->index + 1) << PAGE_SHIFT;
  	else {
  		/*
  		 * Check whether the page to write out is beyond or straddles
  		 * i_size or not.
  		 * -------------------------------------------------------
  		 * |		file mapping		        | <EOF>  |
  		 * -------------------------------------------------------
  		 * | Page ... | Page N-2 | Page N-1 |  Page N   | Beyond |
  		 * ^--------------------------------^-----------|---------
  		 * |				    |      Straddles     |
  		 * ---------------------------------^-----------|--------|
  		 */
  		unsigned offset_into_page = offset & (PAGE_SIZE - 1);
  
  		/*
  		 * Skip the page if it is fully outside i_size, e.g. due to a
  		 * truncate operation that is in progress. We must redirty the
  		 * page so that reclaim stops reclaiming it. Otherwise
  		 * iomap_vm_releasepage() is called on it and gets confused.
  		 *
  		 * Note that the end_index is unsigned long, it would overflow
  		 * if the given offset is greater than 16TB on 32-bit system
  		 * and if we do check the page is fully outside i_size or not
  		 * via "if (page->index >= end_index + 1)" as "end_index + 1"
  		 * will be evaluated to 0.  Hence this page will be redirtied
  		 * and be written out repeatedly which would result in an
  		 * infinite loop, the user program that perform this operation
  		 * will hang.  Instead, we can verify this situation by checking
  		 * if the page to write is totally beyond the i_size or if it's
  		 * offset is just equal to the EOF.
  		 */
  		if (page->index > end_index ||
  		    (page->index == end_index && offset_into_page == 0))
  			goto redirty;
  
  		/*
  		 * The page straddles i_size.  It must be zeroed out on each
  		 * and every writepage invocation because it may be mmapped.
  		 * "A file is mapped in multiples of the page size.  For a file
  		 * that is not a multiple of the page size, the remaining
  		 * memory is zeroed when mapped, and writes to that region are
  		 * not written out to the file."
  		 */
  		zero_user_segment(page, offset_into_page, PAGE_SIZE);
  
  		/* Adjust the end_offset to the end of file */
  		end_offset = offset;
  	}
  
  	return iomap_writepage_map(wpc, wbc, inode, page, end_offset);
  
  redirty:
  	redirty_page_for_writepage(wbc, page);
  	unlock_page(page);
  	return 0;
  }
  
  int
  iomap_writepage(struct page *page, struct writeback_control *wbc,
  		struct iomap_writepage_ctx *wpc,
  		const struct iomap_writeback_ops *ops)
  {
  	int ret;
  
  	wpc->ops = ops;
  	ret = iomap_do_writepage(page, wbc, wpc);
  	if (!wpc->ioend)
  		return ret;
  	return iomap_submit_ioend(wpc, wpc->ioend, ret);
  }
  EXPORT_SYMBOL_GPL(iomap_writepage);
  
  int
  iomap_writepages(struct address_space *mapping, struct writeback_control *wbc,
  		struct iomap_writepage_ctx *wpc,
  		const struct iomap_writeback_ops *ops)
  {
  	int			ret;
  
  	wpc->ops = ops;
  	ret = write_cache_pages(mapping, wbc, iomap_do_writepage, wpc);
  	if (!wpc->ioend)
  		return ret;
  	return iomap_submit_ioend(wpc, wpc->ioend, ret);
  }
  EXPORT_SYMBOL_GPL(iomap_writepages);
  
  static int __init iomap_init(void)
  {
  	return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
  			   offsetof(struct iomap_ioend, io_inline_bio),
  			   BIOSET_NEED_BVECS);
  }
  fs_initcall(iomap_init);
  