[PATCH v4 4/9] xfs: convert delayed extents to unwritten when zeroing post eof blocks

[Zhang Yi]

From: Zhang Yi <yi.zhang@xxxxxxxxxx>

Current clone operation could be non-atomic if the destination of a file
is beyond EOF, user could get a file with corrupted (zeroed) data on
crash.

The problem is about preallocations. If you write some data into a file:

	[A...B)

and XFS decides to preallocate some post-eof blocks, then it can create
a delayed allocation reservation:

	[A.........D)

The writeback path tries to convert delayed extents to real ones by
allocating blocks. If there aren't enough contiguous free space, we can
end up with two extents, the first real and the second still delalloc:

	[A....C)[C.D)

After that, both the in-memory and the on-disk file sizes are still B.
If we clone into the range [E...F) from another file:

	[A....C)[C.D)      [E...F)

then xfs_reflink_zero_posteof() calls iomap_zero_range() to zero out the
range [B, E) beyond EOF and flush it. Since [C, D) is still a delalloc
extent, its pagecache will be zeroed and both the in-memory and on-disk
size will be updated to D after flushing but before cloning. This is
wrong, because the user can see the size change and read the zeroes
while the clone operation is ongoing.

We need to keep the in-memory and on-disk size before the clone
operation starts, so instead of writing zeroes through the page cache
for delayed ranges beyond EOF, we convert these ranges to unwritten and
invalidate any cached data over that range beyond EOF.

Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
 fs/xfs/xfs_iomap.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index ccf83e72d8ca..334860f780ff 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1035,6 +1035,24 @@ xfs_buffered_write_iomap_begin(
 	}
 
 	if (imap.br_startoff <= offset_fsb) {
+		/*
+		 * Trim a delalloc extent that extends beyond the EOF block.
+		 * If it starts beyond the EOF block, convert it to an unwritten
+		 * extent.
+		 */
+		if ((flags & IOMAP_ZERO) &&
+		    isnullstartblock(imap.br_startblock)) {
+			xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
+
+			if (offset_fsb >= eof_fsb)
+				goto convert_delay;
+			if (end_fsb > eof_fsb) {
+				end_fsb = eof_fsb;
+				xfs_trim_extent(&imap, offset_fsb,
+						end_fsb - offset_fsb);
+			}
+		}
+
 		/*
 		 * For reflink files we may need a delalloc reservation when
 		 * overwriting shared extents.   This includes zeroing of
@@ -1158,6 +1176,17 @@ xfs_buffered_write_iomap_begin(
 	xfs_iunlock(ip, lockmode);
 	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags, 0, seq);
 
+convert_delay:
+	xfs_iunlock(ip, lockmode);
+	truncate_pagecache(inode, offset);
+	error = xfs_bmapi_convert_delalloc(ip, XFS_DATA_FORK, offset,
+					   iomap, NULL);
+	if (error)
+		return error;
+
+	trace_xfs_iomap_alloc(ip, offset, count, XFS_DATA_FORK, &imap);
+	return 0;
+
 found_cow:
 	seq = xfs_iomap_inode_sequence(ip, 0);
 	if (imap.br_startoff <= offset_fsb) {
-- 
2.39.2