[PATCH v5 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>
---
Changes since v4:

Move the delalloc converting hunk before searching the COW fork. Because
if the file has been reflinked and copied on write,
xfs_bmap_extsize_align() aligned the range of COW delalloc extent, after
the writeback, there might be some unwritten extents left over in the
COW fork that overlaps the delalloc extent we found in data fork.

  data fork  ...wwww|dddddddddd...
  cow fork          |uuuuuuuuuu...
                    ^
                  i_size

In my v4, we search the COW fork before checking the delalloc extent,
goto found_cow tag and return unconverted delalloc srcmap in the above
case, so the delayed extent in the data fork will have no chance to
convert to unwritten, it will lead to delalloc extent residue and break
generic/522 after merging patch 6.

 fs/xfs/xfs_iomap.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 236ee78aa75b..ab398cb3680a 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1022,6 +1022,23 @@ xfs_buffered_write_iomap_begin(
 		goto out_unlock;
 	}
 
+	/*
+	 * For zeroing, 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) && imap.br_startoff <= offset_fsb &&
+	    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);
+		}
+	}
+
 	/*
 	 * Search the COW fork extent list even if we did not find a data fork
 	 * extent.  This serves two purposes: first this implements the
@@ -1167,6 +1184,17 @@ 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