xref: /openbmc/linux/fs/iomap/buffered-io.c (revision 83b975b5)

// 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"

#define IOEND_BATCH_SIZE	4096

/*
 * Structure allocated for each folio when block size < folio size
 * to track sub-folio 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 folio *folio)
{
	if (folio_test_private(folio))
		return folio_get_private(folio);
	return NULL;
}

static struct bio_set iomap_ioend_bioset;

static struct iomap_page *
iomap_page_create(struct inode *inode, struct folio *folio, unsigned int flags)
{
	struct iomap_page *iop = to_iomap_page(folio);
	unsigned int nr_blocks = i_blocks_per_folio(inode, folio);
	gfp_t gfp;

	if (iop || nr_blocks <= 1)
		return iop;

	if (flags & IOMAP_NOWAIT)
		gfp = GFP_NOWAIT;
	else
		gfp = GFP_NOFS | __GFP_NOFAIL;

	iop = kzalloc(struct_size(iop, uptodate, BITS_TO_LONGS(nr_blocks)),
		      gfp);
	if (iop) {
		spin_lock_init(&iop->uptodate_lock);
		if (folio_test_uptodate(folio))
			bitmap_fill(iop->uptodate, nr_blocks);
		folio_attach_private(folio, iop);
	}
	return iop;
}

static void iomap_page_release(struct folio *folio)
{
	struct iomap_page *iop = folio_detach_private(folio);
	struct inode *inode = folio->mapping->host;
	unsigned int nr_blocks = i_blocks_per_folio(inode, folio);

	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) !=
			folio_test_uptodate(folio));
	kfree(iop);
}

/*
 * Calculate the range inside the folio that we actually need to read.
 */
static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
		loff_t *pos, loff_t length, size_t *offp, size_t *lenp)
{
	struct iomap_page *iop = to_iomap_page(folio);
	loff_t orig_pos = *pos;
	loff_t isize = i_size_read(inode);
	unsigned block_bits = inode->i_blkbits;
	unsigned block_size = (1 << block_bits);
	size_t poff = offset_in_folio(folio, *pos);
	size_t plen = min_t(loff_t, folio_size(folio) - 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_folio(folio, 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 folio *folio,
		struct iomap_page *iop, size_t off, size_t len)
{
	struct inode *inode = folio->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_folio(inode, folio)))
		folio_mark_uptodate(folio);
	spin_unlock_irqrestore(&iop->uptodate_lock, flags);
}

static void iomap_set_range_uptodate(struct folio *folio,
		struct iomap_page *iop, size_t off, size_t len)
{
	if (iop)
		iomap_iop_set_range_uptodate(folio, iop, off, len);
	else
		folio_mark_uptodate(folio);
}

static void iomap_finish_folio_read(struct folio *folio, size_t offset,
		size_t len, int error)
{
	struct iomap_page *iop = to_iomap_page(folio);

	if (unlikely(error)) {
		folio_clear_uptodate(folio);
		folio_set_error(folio);
	} else {
		iomap_set_range_uptodate(folio, iop, offset, len);
	}

	if (!iop || atomic_sub_and_test(len, &iop->read_bytes_pending))
		folio_unlock(folio);
}

static void iomap_read_end_io(struct bio *bio)
{
	int error = blk_status_to_errno(bio->bi_status);
	struct folio_iter fi;

	bio_for_each_folio_all(fi, bio)
		iomap_finish_folio_read(fi.folio, fi.offset, fi.length, error);
	bio_put(bio);
}

struct iomap_readpage_ctx {
	struct folio		*cur_folio;
	bool			cur_folio_in_bio;
	struct bio		*bio;
	struct readahead_control *rac;
};

/**
 * iomap_read_inline_data - copy inline data into the page cache
 * @iter: iteration structure
 * @folio: folio to copy to
 *
 * Copy the inline data in @iter into @folio and zero out the rest of the folio.
 * Only a single IOMAP_INLINE extent is allowed at the end of each file.
 * Returns zero for success to complete the read, or the usual negative errno.
 */
static int iomap_read_inline_data(const struct iomap_iter *iter,
		struct folio *folio)
{
	struct iomap_page *iop;
	const struct iomap *iomap = iomap_iter_srcmap(iter);
	size_t size = i_size_read(iter->inode) - iomap->offset;
	size_t poff = offset_in_page(iomap->offset);
	size_t offset = offset_in_folio(folio, iomap->offset);
	void *addr;

	if (folio_test_uptodate(folio))
		return 0;

	if (WARN_ON_ONCE(size > PAGE_SIZE - poff))
		return -EIO;
	if (WARN_ON_ONCE(size > PAGE_SIZE -
			 offset_in_page(iomap->inline_data)))
		return -EIO;
	if (WARN_ON_ONCE(size > iomap->length))
		return -EIO;
	if (offset > 0)
		iop = iomap_page_create(iter->inode, folio, iter->flags);
	else
		iop = to_iomap_page(folio);

	addr = kmap_local_folio(folio, offset);
	memcpy(addr, iomap->inline_data, size);
	memset(addr + size, 0, PAGE_SIZE - poff - size);
	kunmap_local(addr);
	iomap_set_range_uptodate(folio, iop, offset, PAGE_SIZE - poff);
	return 0;
}

static inline bool iomap_block_needs_zeroing(const struct iomap_iter *iter,
		loff_t pos)
{
	const struct iomap *srcmap = iomap_iter_srcmap(iter);

	return srcmap->type != IOMAP_MAPPED ||
		(srcmap->flags & IOMAP_F_NEW) ||
		pos >= i_size_read(iter->inode);
}

static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
		struct iomap_readpage_ctx *ctx, loff_t offset)
{
	const struct iomap *iomap = &iter->iomap;
	loff_t pos = iter->pos + offset;
	loff_t length = iomap_length(iter) - offset;
	struct folio *folio = ctx->cur_folio;
	struct iomap_page *iop;
	loff_t orig_pos = pos;
	size_t poff, plen;
	sector_t sector;

	if (iomap->type == IOMAP_INLINE)
		return iomap_read_inline_data(iter, folio);

	/* zero post-eof blocks as the page may be mapped */
	iop = iomap_page_create(iter->inode, folio, iter->flags);
	iomap_adjust_read_range(iter->inode, folio, &pos, length, &poff, &plen);
	if (plen == 0)
		goto done;

	if (iomap_block_needs_zeroing(iter, pos)) {
		folio_zero_range(folio, poff, plen);
		iomap_set_range_uptodate(folio, iop, poff, plen);
		goto done;
	}

	ctx->cur_folio_in_bio = true;
	if (iop)
		atomic_add(plen, &iop->read_bytes_pending);

	sector = iomap_sector(iomap, pos);
	if (!ctx->bio ||
	    bio_end_sector(ctx->bio) != sector ||
	    !bio_add_folio(ctx->bio, folio, plen, poff)) {
		gfp_t gfp = mapping_gfp_constraint(folio->mapping, GFP_KERNEL);
		gfp_t orig_gfp = gfp;
		unsigned int nr_vecs = DIV_ROUND_UP(length, PAGE_SIZE);

		if (ctx->bio)
			submit_bio(ctx->bio);

		if (ctx->rac) /* same as readahead_gfp_mask */
			gfp |= __GFP_NORETRY | __GFP_NOWARN;
		ctx->bio = bio_alloc(iomap->bdev, bio_max_segs(nr_vecs),
				     REQ_OP_READ, gfp);
		/*
		 * 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_read_folio does.
		 */
		if (!ctx->bio) {
			ctx->bio = bio_alloc(iomap->bdev, 1, REQ_OP_READ,
					     orig_gfp);
		}
		if (ctx->rac)
			ctx->bio->bi_opf |= REQ_RAHEAD;
		ctx->bio->bi_iter.bi_sector = sector;
		ctx->bio->bi_end_io = iomap_read_end_io;
		bio_add_folio(ctx->bio, folio, 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_read_folio(struct folio *folio, const struct iomap_ops *ops)
{
	struct iomap_iter iter = {
		.inode		= folio->mapping->host,
		.pos		= folio_pos(folio),
		.len		= folio_size(folio),
	};
	struct iomap_readpage_ctx ctx = {
		.cur_folio	= folio,
	};
	int ret;

	trace_iomap_readpage(iter.inode, 1);

	while ((ret = iomap_iter(&iter, ops)) > 0)
		iter.processed = iomap_readpage_iter(&iter, &ctx, 0);

	if (ret < 0)
		folio_set_error(folio);

	if (ctx.bio) {
		submit_bio(ctx.bio);
		WARN_ON_ONCE(!ctx.cur_folio_in_bio);
	} else {
		WARN_ON_ONCE(ctx.cur_folio_in_bio);
		folio_unlock(folio);
	}

	/*
	 * Just like mpage_readahead and block_read_full_folio, we always
	 * return 0 and just set the folio error flag on errors.  This
	 * should be cleaned up throughout the stack eventually.
	 */
	return 0;
}
EXPORT_SYMBOL_GPL(iomap_read_folio);

static loff_t iomap_readahead_iter(const struct iomap_iter *iter,
		struct iomap_readpage_ctx *ctx)
{
	loff_t length = iomap_length(iter);
	loff_t done, ret;

	for (done = 0; done < length; done += ret) {
		if (ctx->cur_folio &&
		    offset_in_folio(ctx->cur_folio, iter->pos + done) == 0) {
			if (!ctx->cur_folio_in_bio)
				folio_unlock(ctx->cur_folio);
			ctx->cur_folio = NULL;
		}
		if (!ctx->cur_folio) {
			ctx->cur_folio = readahead_folio(ctx->rac);
			ctx->cur_folio_in_bio = false;
		}
		ret = iomap_readpage_iter(iter, ctx, done);
		if (ret <= 0)
			return ret;
	}

	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 iomap_iter iter = {
		.inode	= rac->mapping->host,
		.pos	= readahead_pos(rac),
		.len	= readahead_length(rac),
	};
	struct iomap_readpage_ctx ctx = {
		.rac	= rac,
	};

	trace_iomap_readahead(rac->mapping->host, readahead_count(rac));

	while (iomap_iter(&iter, ops) > 0)
		iter.processed = iomap_readahead_iter(&iter, &ctx);

	if (ctx.bio)
		submit_bio(ctx.bio);
	if (ctx.cur_folio) {
		if (!ctx.cur_folio_in_bio)
			folio_unlock(ctx.cur_folio);
	}
}
EXPORT_SYMBOL_GPL(iomap_readahead);

/*
 * iomap_is_partially_uptodate checks whether blocks within a folio are
 * uptodate or not.
 *
 * Returns true if all blocks which correspond to the specified part
 * of the folio are uptodate.
 */
bool iomap_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
{
	struct iomap_page *iop = to_iomap_page(folio);
	struct inode *inode = folio->mapping->host;
	unsigned first, last, i;

	if (!iop)
		return false;

	/* Caller's range may extend past the end of this folio */
	count = min(folio_size(folio) - from, count);

	/* First and last blocks in range within folio */
	first = from >> inode->i_blkbits;
	last = (from + count - 1) >> inode->i_blkbits;

	for (i = first; i <= last; i++)
		if (!test_bit(i, iop->uptodate))
			return false;
	return true;
}
EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);

bool iomap_release_folio(struct folio *folio, gfp_t gfp_flags)
{
	trace_iomap_release_folio(folio->mapping->host, folio_pos(folio),
			folio_size(folio));

	/*
	 * mm accommodates an old ext3 case where clean folios might
	 * not have had the dirty bit cleared.  Thus, it can send actual
	 * dirty folios to ->release_folio() via shrink_active_list();
	 * skip those here.
	 */
	if (folio_test_dirty(folio) || folio_test_writeback(folio))
		return false;
	iomap_page_release(folio);
	return true;
}
EXPORT_SYMBOL_GPL(iomap_release_folio);

void iomap_invalidate_folio(struct folio *folio, size_t offset, size_t len)
{
	trace_iomap_invalidate_folio(folio->mapping->host,
					folio_pos(folio) + offset, len);

	/*
	 * If we're invalidating the entire folio, clear the dirty state
	 * from it and release it to avoid unnecessary buildup of the LRU.
	 */
	if (offset == 0 && len == folio_size(folio)) {
		WARN_ON_ONCE(folio_test_writeback(folio));
		folio_cancel_dirty(folio);
		iomap_page_release(folio);
	} else if (folio_test_large(folio)) {
		/* Must release the iop so the page can be split */
		WARN_ON_ONCE(!folio_test_uptodate(folio) &&
			     folio_test_dirty(folio));
		iomap_page_release(folio);
	}
}
EXPORT_SYMBOL_GPL(iomap_invalidate_folio);

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 - 1);
}

static int iomap_read_folio_sync(loff_t block_start, struct folio *folio,
		size_t poff, size_t plen, const struct iomap *iomap)
{
	struct bio_vec bvec;
	struct bio bio;

	bio_init(&bio, iomap->bdev, &bvec, 1, REQ_OP_READ);
	bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
	bio_add_folio(&bio, folio, plen, poff);
	return submit_bio_wait(&bio);
}

static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
		size_t len, struct folio *folio)
{
	const struct iomap *srcmap = iomap_iter_srcmap(iter);
	struct iomap_page *iop;
	loff_t block_size = i_blocksize(iter->inode);
	loff_t block_start = round_down(pos, block_size);
	loff_t block_end = round_up(pos + len, block_size);
	unsigned int nr_blocks = i_blocks_per_folio(iter->inode, folio);
	size_t from = offset_in_folio(folio, pos), to = from + len;
	size_t poff, plen;

	if (folio_test_uptodate(folio))
		return 0;
	folio_clear_error(folio);

	iop = iomap_page_create(iter->inode, folio, iter->flags);
	if ((iter->flags & IOMAP_NOWAIT) && !iop && nr_blocks > 1)
		return -EAGAIN;

	do {
		iomap_adjust_read_range(iter->inode, folio, &block_start,
				block_end - block_start, &poff, &plen);
		if (plen == 0)
			break;

		if (!(iter->flags & IOMAP_UNSHARE) &&
		    (from <= poff || from >= poff + plen) &&
		    (to <= poff || to >= poff + plen))
			continue;

		if (iomap_block_needs_zeroing(iter, block_start)) {
			if (WARN_ON_ONCE(iter->flags & IOMAP_UNSHARE))
				return -EIO;
			folio_zero_segments(folio, poff, from, to, poff + plen);
		} else {
			int status;

			if (iter->flags & IOMAP_NOWAIT)
				return -EAGAIN;

			status = iomap_read_folio_sync(block_start, folio,
					poff, plen, srcmap);
			if (status)
				return status;
		}
		iomap_set_range_uptodate(folio, iop, poff, plen);
	} while ((block_start += plen) < block_end);

	return 0;
}

static int iomap_write_begin_inline(const struct iomap_iter *iter,
		struct folio *folio)
{
	/* needs more work for the tailpacking case; disable for now */
	if (WARN_ON_ONCE(iomap_iter_srcmap(iter)->offset != 0))
		return -EIO;
	return iomap_read_inline_data(iter, folio);
}

static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
		size_t len, struct folio **foliop)
{
	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
	const struct iomap *srcmap = iomap_iter_srcmap(iter);
	struct folio *folio;
	unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
	int status = 0;

	if (iter->flags & IOMAP_NOWAIT)
		fgp |= FGP_NOWAIT;

	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
	if (srcmap != &iter->iomap)
		BUG_ON(pos + len > srcmap->offset + srcmap->length);

	if (fatal_signal_pending(current))
		return -EINTR;

	if (!mapping_large_folio_support(iter->inode->i_mapping))
		len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));

	if (page_ops && page_ops->page_prepare) {
		status = page_ops->page_prepare(iter->inode, pos, len);
		if (status)
			return status;
	}

	folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
			fgp, mapping_gfp_mask(iter->inode->i_mapping));
	if (!folio) {
		status = (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
		goto out_no_page;
	}
	if (pos + len > folio_pos(folio) + folio_size(folio))
		len = folio_pos(folio) + folio_size(folio) - pos;

	if (srcmap->type == IOMAP_INLINE)
		status = iomap_write_begin_inline(iter, folio);
	else if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
		status = __block_write_begin_int(folio, pos, len, NULL, srcmap);
	else
		status = __iomap_write_begin(iter, pos, len, folio);

	if (unlikely(status))
		goto out_unlock;

	*foliop = folio;
	return 0;

out_unlock:
	folio_unlock(folio);
	folio_put(folio);
	iomap_write_failed(iter->inode, pos, len);

out_no_page:
	if (page_ops && page_ops->page_done)
		page_ops->page_done(iter->inode, pos, 0, NULL);
	return status;
}

static size_t __iomap_write_end(struct inode *inode, loff_t pos, size_t len,
		size_t copied, struct folio *folio)
{
	struct iomap_page *iop = to_iomap_page(folio);
	flush_dcache_folio(folio);

	/*
	 * The blocks that were entirely written will now be uptodate, so we
	 * don't have to worry about a read_folio reading them and overwriting a
	 * partial write.  However, if we've encountered a short write and only
	 * partially written into a block, it will not be marked uptodate, so a
	 * read_folio 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 && !folio_test_uptodate(folio)))
		return 0;
	iomap_set_range_uptodate(folio, iop, offset_in_folio(folio, pos), len);
	filemap_dirty_folio(inode->i_mapping, folio);
	return copied;
}

static size_t iomap_write_end_inline(const struct iomap_iter *iter,
		struct folio *folio, loff_t pos, size_t copied)
{
	const struct iomap *iomap = &iter->iomap;
	void *addr;

	WARN_ON_ONCE(!folio_test_uptodate(folio));
	BUG_ON(!iomap_inline_data_valid(iomap));

	flush_dcache_folio(folio);
	addr = kmap_local_folio(folio, pos);
	memcpy(iomap_inline_data(iomap, pos), addr, copied);
	kunmap_local(addr);

	mark_inode_dirty(iter->inode);
	return copied;
}

/* Returns the number of bytes copied.  May be 0.  Cannot be an errno. */
static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
		size_t copied, struct folio *folio)
{
	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
	const struct iomap *srcmap = iomap_iter_srcmap(iter);
	loff_t old_size = iter->inode->i_size;
	size_t ret;

	if (srcmap->type == IOMAP_INLINE) {
		ret = iomap_write_end_inline(iter, folio, pos, copied);
	} else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
		ret = block_write_end(NULL, iter->inode->i_mapping, pos, len,
				copied, &folio->page, NULL);
	} else {
		ret = __iomap_write_end(iter->inode, pos, len, copied, folio);
	}

	/*
	 * 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(iter->inode, pos + ret);
		iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
	}
	folio_unlock(folio);

	if (old_size < pos)
		pagecache_isize_extended(iter->inode, old_size, pos);
	if (page_ops && page_ops->page_done)
		page_ops->page_done(iter->inode, pos, ret, &folio->page);
	folio_put(folio);

	if (ret < len)
		iomap_write_failed(iter->inode, pos + ret, len - ret);
	return ret;
}

static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
{
	loff_t length = iomap_length(iter);
	loff_t pos = iter->pos;
	ssize_t written = 0;
	long status = 0;
	struct address_space *mapping = iter->inode->i_mapping;
	unsigned int bdp_flags = (iter->flags & IOMAP_NOWAIT) ? BDP_ASYNC : 0;

	do {
		struct folio *folio;
		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:
		status = balance_dirty_pages_ratelimited_flags(mapping,
							       bdp_flags);
		if (unlikely(status))
			break;

		if (bytes > length)
			bytes = length;

		/*
		 * Bring in the user page that we'll 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.
		 *
		 * For async buffered writes the assumption is that the user
		 * page has already been faulted in. This can be optimized by
		 * faulting the user page.
		 */
		if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
			status = -EFAULT;
			break;
		}

		status = iomap_write_begin(iter, pos, bytes, &folio);
		if (unlikely(status))
			break;

		page = folio_file_page(folio, pos >> PAGE_SHIFT);
		if (mapping_writably_mapped(mapping))
			flush_dcache_page(page);

		copied = copy_page_from_iter_atomic(page, offset, bytes, i);

		status = iomap_write_end(iter, pos, bytes, copied, folio);

		if (unlikely(copied != status))
			iov_iter_revert(i, copied - status);

		cond_resched();
		if (unlikely(status == 0)) {
			/*
			 * A short copy made iomap_write_end() reject the
			 * thing entirely.  Might be memory poisoning
			 * halfway through, might be a race with munmap,
			 * might be severe memory pressure.
			 */
			if (copied)
				bytes = copied;
			goto again;
		}
		pos += status;
		written += status;
		length -= status;
	} while (iov_iter_count(i) && length);

	if (status == -EAGAIN) {
		iov_iter_revert(i, written);
		return -EAGAIN;
	}
	return written ? written : status;
}

ssize_t
iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
		const struct iomap_ops *ops)
{
	struct iomap_iter iter = {
		.inode		= iocb->ki_filp->f_mapping->host,
		.pos		= iocb->ki_pos,
		.len		= iov_iter_count(i),
		.flags		= IOMAP_WRITE,
	};
	int ret;

	if (iocb->ki_flags & IOCB_NOWAIT)
		iter.flags |= IOMAP_NOWAIT;

	while ((ret = iomap_iter(&iter, ops)) > 0)
		iter.processed = iomap_write_iter(&iter, i);
	if (iter.pos == iocb->ki_pos)
		return ret;
	return iter.pos - iocb->ki_pos;
}
EXPORT_SYMBOL_GPL(iomap_file_buffered_write);

static loff_t iomap_unshare_iter(struct iomap_iter *iter)
{
	struct iomap *iomap = &iter->iomap;
	const struct iomap *srcmap = iomap_iter_srcmap(iter);
	loff_t pos = iter->pos;
	loff_t length = iomap_length(iter);
	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 folio *folio;

		status = iomap_write_begin(iter, pos, bytes, &folio);
		if (unlikely(status))
			return status;

		status = iomap_write_end(iter, pos, bytes, bytes, folio);
		if (WARN_ON_ONCE(status == 0))
			return -EIO;

		cond_resched();

		pos += status;
		written += status;
		length -= status;

		balance_dirty_pages_ratelimited(iter->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)
{
	struct iomap_iter iter = {
		.inode		= inode,
		.pos		= pos,
		.len		= len,
		.flags		= IOMAP_WRITE | IOMAP_UNSHARE,
	};
	int ret;

	while ((ret = iomap_iter(&iter, ops)) > 0)
		iter.processed = iomap_unshare_iter(&iter);
	return ret;
}
EXPORT_SYMBOL_GPL(iomap_file_unshare);

static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
{
	const struct iomap *srcmap = iomap_iter_srcmap(iter);
	loff_t pos = iter->pos;
	loff_t length = iomap_length(iter);
	loff_t written = 0;

	/* already zeroed?  we're done. */
	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
		return length;

	do {
		struct folio *folio;
		int status;
		size_t offset;
		size_t bytes = min_t(u64, SIZE_MAX, length);

		status = iomap_write_begin(iter, pos, bytes, &folio);
		if (status)
			return status;

		offset = offset_in_folio(folio, pos);
		if (bytes > folio_size(folio) - offset)
			bytes = folio_size(folio) - offset;

		folio_zero_range(folio, offset, bytes);
		folio_mark_accessed(folio);

		bytes = iomap_write_end(iter, pos, bytes, bytes, folio);
		if (WARN_ON_ONCE(bytes == 0))
			return -EIO;

		pos += bytes;
		length -= bytes;
		written += bytes;
	} while (length > 0);

	if (did_zero)
		*did_zero = true;
	return written;
}

int
iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
		const struct iomap_ops *ops)
{
	struct iomap_iter iter = {
		.inode		= inode,
		.pos		= pos,
		.len		= len,
		.flags		= IOMAP_ZERO,
	};
	int ret;

	while ((ret = iomap_iter(&iter, ops)) > 0)
		iter.processed = iomap_zero_iter(&iter, did_zero);
	return ret;
}
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_folio_mkwrite_iter(struct iomap_iter *iter,
		struct folio *folio)
{
	loff_t length = iomap_length(iter);
	int ret;

	if (iter->iomap.flags & IOMAP_F_BUFFER_HEAD) {
		ret = __block_write_begin_int(folio, iter->pos, length, NULL,
					      &iter->iomap);
		if (ret)
			return ret;
		block_commit_write(&folio->page, 0, length);
	} else {
		WARN_ON_ONCE(!folio_test_uptodate(folio));
		folio_mark_dirty(folio);
	}

	return length;
}

vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
{
	struct iomap_iter iter = {
		.inode		= file_inode(vmf->vma->vm_file),
		.flags		= IOMAP_WRITE | IOMAP_FAULT,
	};
	struct folio *folio = page_folio(vmf->page);
	ssize_t ret;

	folio_lock(folio);
	ret = folio_mkwrite_check_truncate(folio, iter.inode);
	if (ret < 0)
		goto out_unlock;
	iter.pos = folio_pos(folio);
	iter.len = ret;
	while ((ret = iomap_iter(&iter, ops)) > 0)
		iter.processed = iomap_folio_mkwrite_iter(&iter, folio);

	if (ret < 0)
		goto out_unlock;
	folio_wait_stable(folio);
	return VM_FAULT_LOCKED;
out_unlock:
	folio_unlock(folio);
	return block_page_mkwrite_return(ret);
}
EXPORT_SYMBOL_GPL(iomap_page_mkwrite);

static void iomap_finish_folio_write(struct inode *inode, struct folio *folio,
		size_t len, int error)
{
	struct iomap_page *iop = to_iomap_page(folio);

	if (error) {
		folio_set_error(folio);
		mapping_set_error(inode->i_mapping, error);
	}

	WARN_ON_ONCE(i_blocks_per_folio(inode, folio) > 1 && !iop);
	WARN_ON_ONCE(iop && atomic_read(&iop->write_bytes_pending) <= 0);

	if (!iop || atomic_sub_and_test(len, &iop->write_bytes_pending))
		folio_end_writeback(folio);
}

/*
 * 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 u32
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);
	u32 folio_count = 0;

	for (bio = &ioend->io_inline_bio; bio; bio = next) {
		struct folio_iter fi;

		/*
		 * 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 all folios in bio, ending page IO on them */
		bio_for_each_folio_all(fi, bio) {
			iomap_finish_folio_write(inode, fi.folio, fi.length,
					error);
			folio_count++;
		}
		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);
	}
	return folio_count;
}

/*
 * Ioend completion routine for merged bios. This can only be called from task
 * contexts as merged ioends can be of unbound length. Hence we have to break up
 * the writeback completions into manageable chunks to avoid long scheduler
 * holdoffs. We aim to keep scheduler holdoffs down below 10ms so that we get
 * good batch processing throughput without creating adverse scheduler latency
 * conditions.
 */
void
iomap_finish_ioends(struct iomap_ioend *ioend, int error)
{
	struct list_head tmp;
	u32 completions;

	might_sleep();

	list_replace_init(&ioend->io_list, &tmp);
	completions = iomap_finish_ioend(ioend, error);

	while (!list_empty(&tmp)) {
		if (completions > IOEND_BATCH_SIZE * 8) {
			cond_resched();
			completions = 0;
		}
		ioend = list_first_entry(&tmp, struct iomap_ioend, io_list);
		list_del_init(&ioend->io_list);
		completions += 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;
	/*
	 * Do not merge physically discontiguous ioends. The filesystem
	 * completion functions will have to iterate the physical
	 * discontiguities even if we merge the ioends at a logical level, so
	 * we don't gain anything by merging physical discontiguities here.
	 *
	 * We cannot use bio->bi_iter.bi_sector here as it is modified during
	 * submission so does not point to the start sector of the bio at
	 * completion.
	 */
	if (ioend->io_sector + (ioend->io_size >> 9) != next->io_sector)
		return false;
	return true;
}

void
iomap_ioend_try_merge(struct iomap_ioend *ioend, struct list_head *more_ioends)
{
	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;
	}
}
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've marked pages 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're 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(wpc->iomap.bdev, BIO_MAX_VECS,
			       REQ_OP_WRITE | wbc_to_write_flags(wbc),
			       GFP_NOFS, &iomap_ioend_bioset);
	bio->bi_iter.bi_sector = sector;
	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_folios = 0;
	ioend->io_offset = offset;
	ioend->io_bio = bio;
	ioend->io_sector = sector;
	return ioend;
}

/*
 * Allocate a new bio, and chain the old bio to the new one.
 *
 * Note that we have to 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(prev->bi_bdev, BIO_MAX_VECS, prev->bi_opf, GFP_NOFS);
	bio_clone_blkg_association(new, prev);
	new->bi_iter.bi_sector = bio_end_sector(prev);

	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;
	/*
	 * Limit ioend bio chain lengths to minimise IO completion latency. This
	 * also prevents long tight loops ending page writeback on all the
	 * folios in the ioend.
	 */
	if (wpc->ioend->io_folios >= IOEND_BATCH_SIZE)
		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 pos, struct folio *folio,
		struct iomap_page *iop, struct iomap_writepage_ctx *wpc,
		struct writeback_control *wbc, struct list_head *iolist)
{
	sector_t sector = iomap_sector(&wpc->iomap, pos);
	unsigned len = i_blocksize(inode);
	size_t poff = offset_in_folio(folio, pos);

	if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, pos, sector)) {
		if (wpc->ioend)
			list_add(&wpc->ioend->io_list, iolist);
		wpc->ioend = iomap_alloc_ioend(inode, wpc, pos, sector, wbc);
	}

	if (!bio_add_folio(wpc->ioend->io_bio, folio, len, poff)) {
		wpc->ioend->io_bio = iomap_chain_bio(wpc->ioend->io_bio);
		bio_add_folio(wpc->ioend->io_bio, folio, len, poff);
	}

	if (iop)
		atomic_add(len, &iop->write_bytes_pending);
	wpc->ioend->io_size += len;
	wbc_account_cgroup_owner(wbc, &folio->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
 * the forward progress guarantees we need to provide. The current ioend we're
 * adding blocks to is cached in the writepage context, and if the new block
 * doesn't 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 folio *folio, u64 end_pos)
{
	struct iomap_page *iop = iomap_page_create(inode, folio, 0);
	struct iomap_ioend *ioend, *next;
	unsigned len = i_blocksize(inode);
	unsigned nblocks = i_blocks_per_folio(inode, folio);
	u64 pos = folio_pos(folio);
	int error = 0, count = 0, i;
	LIST_HEAD(submit_list);

	WARN_ON_ONCE(iop && atomic_read(&iop->write_bytes_pending) != 0);

	/*
	 * Walk through the folio 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; i < nblocks && pos < end_pos; i++, pos += len) {
		if (iop && !test_bit(i, iop->uptodate))
			continue;

		error = wpc->ops->map_blocks(wpc, inode, pos);
		if (error)
			break;
		trace_iomap_writepage_map(inode, &wpc->iomap);
		if (WARN_ON_ONCE(wpc->iomap.type == IOMAP_INLINE))
			continue;
		if (wpc->iomap.type == IOMAP_HOLE)
			continue;
		iomap_add_to_ioend(inode, pos, folio, iop, wpc, wbc,
				 &submit_list);
		count++;
	}
	if (count)
		wpc->ioend->io_folios++;

	WARN_ON_ONCE(!wpc->ioend && !list_empty(&submit_list));
	WARN_ON_ONCE(!folio_test_locked(folio));
	WARN_ON_ONCE(folio_test_writeback(folio));
	WARN_ON_ONCE(folio_test_dirty(folio));

	/*
	 * 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 hasn't been added to ioend, it
		 * won't be affected by I/O completion and we must unlock it
		 * now.
		 */
		if (wpc->ops->discard_folio)
			wpc->ops->discard_folio(folio, pos);
		if (!count) {
			folio_unlock(folio);
			goto done;
		}
	}

	folio_start_writeback(folio);
	folio_unlock(folio);

	/*
	 * Preserve the original error if there was one; 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)
		folio_end_writeback(folio);
done:
	mapping_set_error(inode->i_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 folio *folio = page_folio(page);
	struct iomap_writepage_ctx *wpc = data;
	struct inode *inode = folio->mapping->host;
	u64 end_pos, isize;

	trace_iomap_writepage(inode, folio_pos(folio), folio_size(folio));

	/*
	 * Refuse to write the folio out if we're 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;

	/*
	 * Is this folio beyond the end of the file?
	 *
	 * The folio index is less than the end_index, adjust the end_pos
	 * to the highest offset that this folio should represent.
	 * -----------------------------------------------------
	 * |			file mapping	       | <EOF> |
	 * -----------------------------------------------------
	 * | Page ... | Page N-2 | Page N-1 |  Page N  |       |
	 * ^--------------------------------^----------|--------
	 * |     desired writeback range    |      see else    |
	 * ---------------------------------^------------------|
	 */
	isize = i_size_read(inode);
	end_pos = folio_pos(folio) + folio_size(folio);
	if (end_pos > isize) {
		/*
		 * 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     |
		 * ---------------------------------^-----------|--------|
		 */
		size_t poff = offset_in_folio(folio, isize);
		pgoff_t end_index = isize >> PAGE_SHIFT;

		/*
		 * Skip the page if it's fully outside i_size, e.g.
		 * due to a truncate operation that's in progress.  We've
		 * cleaned this page and truncate will finish things off for
		 * us.
		 *
		 * Note that the end_index is unsigned long.  If the given
		 * offset is greater than 16TB on a 32-bit system then if we
		 * checked if the page is fully outside i_size with
		 * "if (page->index >= end_index + 1)", "end_index + 1" would
		 * overflow and evaluate to 0.  Hence this page would be
		 * redirtied and written out repeatedly, which would result in
		 * an infinite loop; the user program performing this operation
		 * would hang.  Instead, we can detect this situation by
		 * checking if the page is totally beyond i_size or if its
		 * offset is just equal to the EOF.
		 */
		if (folio->index > end_index ||
		    (folio->index == end_index && poff == 0))
			goto unlock;

		/*
		 * 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."
		 */
		folio_zero_segment(folio, poff, folio_size(folio));
		end_pos = isize;
	}

	return iomap_writepage_map(wpc, wbc, inode, folio, end_pos);

redirty:
	folio_redirty_for_writepage(wbc, folio);
unlock:
	folio_unlock(folio);
	return 0;
}

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);