linux-kernel - [PATCH 2/4] xfs: convert delayed extents to unwritten when zeroing post eof blocks

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Date: Mon, 11 Mar 2024 20:22:53 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	djwong@...nel.org,
	hch@...radead.org,
	brauner@...nel.org,
	david@...morbit.com,
	tytso@....edu,
	jack@...e.cz,
	yi.zhang@...wei.com,
	yi.zhang@...weicloud.com,
	chengzhihao1@...wei.com,
	yukuai3@...wei.com
Subject: [PATCH 2/4] xfs: convert delayed extents to unwritten when zeroing post eof blocks

From: Zhang Yi <yi.zhang@...wei.com>

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 to pre-alloctions. If you write some data into a
file [A, B) (the position letters are increased one by one), and xfs
could pre-allocate some blocks, then we get a delayed extent [A, D).
Then the writeback path allocate blocks and convert this delayed extent
[A, C) since lack of enough contiguous physical blocks, so the extent
[C, D) is still delayed. After that, both the in-memory and the on-disk
file size are B. If we clone file range into [E, F) from another file,
xfs_reflink_zero_posteof() would call iomap_zero_range() to zero out the
range [B, E) beyond EOF and flush range. Since [C, D) is still a delayed
extent, it will be zeroed and the file's in-memory && on-disk size will
be updated to D after flushing and before doing the clone operation.
This is wrong, because user can user can see the size change and read
zeros in the middle of the clone operation.

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
invalidating any cached data over that range beyond EOF.

Suggested-by: Dave Chinner <david@...morbit.com>
Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
---
 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..2b2aace25355 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -957,6 +957,7 @@ xfs_buffered_write_iomap_begin(
 	struct xfs_mount	*mp = ip->i_mount;
 	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
 	xfs_fileoff_t		end_fsb = xfs_iomap_end_fsb(mp, offset, count);
+	xfs_fileoff_t		eof_fsb = XFS_B_TO_FSBT(mp, XFS_ISIZE(ip));
 	struct xfs_bmbt_irec	imap, cmap;
 	struct xfs_iext_cursor	icur, ccur;
 	xfs_fsblock_t		prealloc_blocks = 0;
@@ -1035,6 +1036,22 @@ xfs_buffered_write_iomap_begin(
 	}

 	if (imap.br_startoff <= offset_fsb) {
+		/*
+		 * For zeroing out delayed allocation extent, we trim it if
+		 * it's partial beyonds EOF block, or convert it to unwritten
+		 * extent if it's all beyonds EOF block.
+		 */
+		if ((flags & IOMAP_ZERO) &&
+		    isnullstartblock(imap.br_startblock)) {
+			if (offset_fsb > eof_fsb)
+				goto convert_delay;
+			if (end_fsb > eof_fsb) {
+				end_fsb = eof_fsb + 1;
+				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 +1175,18 @@ xfs_buffered_write_iomap_begin(
 	xfs_iunlock(ip, lockmode);
 	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags, 0, seq);

+convert_delay:
+	end_fsb = min(end_fsb, imap.br_startoff + imap.br_blockcount);
+	xfs_iunlock(ip, lockmode);
+	truncate_pagecache_range(inode, offset, XFS_FSB_TO_B(mp, end_fsb));
+	error = xfs_iomap_write_direct(ip, offset_fsb, end_fsb - offset_fsb,
+				       flags, &imap, &seq);
+	if (error)
+		return error;
+
+	trace_xfs_iomap_alloc(ip, offset, count, XFS_DATA_FORK, &imap);
+	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags, IOMAP_F_NEW, seq);
+
 found_cow:
 	seq = xfs_iomap_inode_sequence(ip, 0);
 	if (imap.br_startoff <= offset_fsb) {
-- 
2.39.2