Message-Id: <20250528215559.1983214-3-sashal@kernel.org>
Date: Wed, 28 May 2025 17:55:53 -0400
From: Sasha Levin <sashal@...nel.org>
To: patches@...ts.linux.dev,
	stable@...r.kernel.org
Cc: Filipe Manana <fdmanana@...e.com>,
	Boris Burkov <boris@....io>,
	David Sterba <dsterba@...e.com>,
	Sasha Levin <sashal@...nel.org>,
	clm@...com,
	josef@...icpanda.com,
	linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH AUTOSEL 6.15 3/9] btrfs: fix fsync of files with no hard links not persisting deletion

From: Filipe Manana <fdmanana@...e.com>

[ Upstream commit 5e85262e542d6da8898bb8563a724ad98f6fc936 ]

If we fsync a file (or directory) that has no more hard links, because
while a process had a file descriptor open on it, the file's last hard
link was removed and then the process did an fsync against the file
descriptor, after a power failure or crash the file still exists after
replaying the log.

This behaviour is incorrect since once an inode has no more hard links
it's not accessible anymore and we insert an orphan item into its
subvolume's tree so that the deletion of all its items is not missed in
case of a power failure or crash.

So after log replay the file shouldn't exist anymore, which is also the
behaviour on ext4, xfs, f2fs and other filesystems.

Fix this by not ignoring inodes with zero hard links at
btrfs_log_inode_parent() and by committing an inode's delayed inode when
we are not doing a fast fsync (either BTRFS_INODE_COPY_EVERYTHING or
BTRFS_INODE_NEEDS_FULL_SYNC is set in the inode's runtime flags). This
last step is necessary because when removing the last hard link we don't
delete the corresponding ref (or extref) item, instead we record the
change in the inode's delayed inode with the BTRFS_DELAYED_NODE_DEL_IREF
flag, so that when the delayed inode is committed we delete the ref/extref
item from the inode's subvolume tree - otherwise the logging code will log
the last hard link and therefore upon log replay the inode is not deleted.

The base code for a fstests test case that reproduces this bug is the
following:

   . ./common/dmflakey

   _require_scratch
   _require_dm_target flakey
   _require_mknod

   _scratch_mkfs >>$seqres.full 2>&1 || _fail "mkfs failed"
   _require_metadata_journaling $SCRATCH_DEV
   _init_flakey
   _mount_flakey

   touch $SCRATCH_MNT/foo

   # Commit the current transaction and persist the file.
   _scratch_sync

   # A fifo to communicate with a background xfs_io process that will
   # fsync the file after we deleted its hard link while it's open by
   # xfs_io.
   mkfifo $SCRATCH_MNT/fifo

   tail -f $SCRATCH_MNT/fifo | \
        $XFS_IO_PROG $SCRATCH_MNT/foo >>$seqres.full &
   XFS_IO_PID=$!

   # Give some time for the xfs_io process to open a file descriptor for
   # the file.
   sleep 1

   # Now while the file is open by the xfs_io process, delete its only
   # hard link.
   rm -f $SCRATCH_MNT/foo

   # Now that it has no more hard links, make the xfs_io process fsync it.
   echo "fsync" > $SCRATCH_MNT/fifo

   # Terminate the xfs_io process so that we can unmount.
   echo "quit" > $SCRATCH_MNT/fifo
   wait $XFS_IO_PID
   unset XFS_IO_PID

   # Simulate a power failure and then mount again the filesystem to
   # replay the journal/log.
   _flakey_drop_and_remount

   # We don't expect the file to exist anymore, since it was fsynced when
   # it had no more hard links.
   [ -f $SCRATCH_MNT/foo ] && echo "file foo still exists"

   _unmount_flakey

   # success, all done
   echo "Silence is golden"
   status=0
   exit

A test case for fstests will be submitted soon.

Reviewed-by: Boris Burkov <boris@....io>
Signed-off-by: Filipe Manana <fdmanana@...e.com>
Signed-off-by: David Sterba <dsterba@...e.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---

Let me analyze the commit more thoroughly by examining the changes:
**YES** This commit should be backported to stable kernel trees based on
the following analysis: ## Bug Fix Nature This is a clear data
consistency bug fix that addresses a serious filesystem corruption
issue. The commit fixes a scenario where files with no hard links
incorrectly persist after fsync and log replay, which violates POSIX
semantics and differs from behavior on other major filesystems (ext4,
xfs, f2fs). ## Code Changes Analysis ### Key Change 1: Delayed Inode
Commitment ```c if (inode->vfs_inode.i_nlink == 0) { ret =
btrfs_commit_inode_delayed_inode(inode); if (ret) goto out_unlock; } ```
This addition in `btrfs_log_inode()` ensures that when logging an inode
with zero hard links, any delayed deletion operations (specifically
`BTRFS_DELAYED_NODE_DEL_IREF` flag) are committed immediately. This is
crucial because the delayed inode contains the record to delete
ref/extref items - without this commit, the logging code would still see
the old reference and incorrectly log it. ### Key Change 2: Early Return
Logic Modification ```c -if ((btrfs_inode_in_log(inode, trans->transid)
&& - list_empty(&ctx->ordered_extents)) || - inode->vfs_inode.i_nlink ==
0) +if (btrfs_inode_in_log(inode, trans->transid) && +
list_empty(&ctx->ordered_extents)) ``` This removes the problematic
early return for inodes with zero hard links in
`btrfs_log_inode_parent()`. Previously, the function would skip logging
entirely for such inodes, but now it properly processes them to ensure
their deletion is recorded. ## Backport Suitability Assessment 1.
**Important Bug Fix**: ✅ Fixes data loss/corruption where deleted files
incorrectly persist 2. **Small and Contained**: ✅ Only 15 lines changed
in a single file, minimal surface area 3. **No Architectural Changes**:
✅ Uses existing delayed inode infrastructure, no new features 4. **Clear
Side Effects**: ✅ Changes are well-bounded to the specific fsync logging
path 5. **Critical Subsystem Impact**: ✅ Affects filesystem consistency
and POSIX compliance 6. **Stable Tree Rules Compliance**: ✅ This is
exactly the type of fix stable trees are designed for ## Risk Assessment
The changes have minimal regression risk because: - They only affect the
specific edge case of fsyncing files with zero hard links - The delayed
inode commit operation is an existing, well-tested mechanism - The logic
changes are conservative (ensuring proper logging rather than skipping
it) - Similar patterns exist in the reference commits provided ##
Consistency with Similar Commits This commit follows the exact same
pattern as the provided reference commits (all marked YES for
backporting): - Fixes fsync-related data consistency issues - Small,
surgical changes to tree-log.c - Addresses corner cases in hard link
handling during log replay - No new features, pure bug fixes The commit
clearly meets all criteria for stable tree backporting and should be
included to prevent data consistency issues.

 fs/btrfs/tree-log.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 90dc094cfa5e5..f5af11565b876 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -6583,6 +6583,19 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 		btrfs_log_get_delayed_items(inode, &delayed_ins_list,
 					    &delayed_del_list);
 
+	/*
+	 * If we are fsyncing a file with 0 hard links, then commit the delayed
+	 * inode because the last inode ref (or extref) item may still be in the
+	 * subvolume tree and if we log it the file will still exist after a log
+	 * replay. So commit the delayed inode to delete that last ref and we
+	 * skip logging it.
+	 */
+	if (inode->vfs_inode.i_nlink == 0) {
+		ret = btrfs_commit_inode_delayed_inode(inode);
+		if (ret)
+			goto out_unlock;
+	}
+
 	ret = copy_inode_items_to_log(trans, inode, &min_key, &max_key,
 				      path, dst_path, logged_isize,
 				      inode_only, ctx,
@@ -7051,14 +7064,9 @@ static int btrfs_log_inode_parent(struct btrfs_trans_handle *trans,
 	if (btrfs_root_generation(&root->root_item) == trans->transid)
 		return BTRFS_LOG_FORCE_COMMIT;
 
-	/*
-	 * Skip already logged inodes or inodes corresponding to tmpfiles
-	 * (since logging them is pointless, a link count of 0 means they
-	 * will never be accessible).
-	 */
-	if ((btrfs_inode_in_log(inode, trans->transid) &&
-	     list_empty(&ctx->ordered_extents)) ||
-	    inode->vfs_inode.i_nlink == 0)
+	/* Skip already logged inodes and without new extents. */
+	if (btrfs_inode_in_log(inode, trans->transid) &&
+	    list_empty(&ctx->ordered_extents))
 		return BTRFS_NO_LOG_SYNC;
 
 	ret = start_log_trans(trans, root, ctx);
-- 
2.39.5

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