[PATCH] btrfs: fix unnecessary flush on close when truncating zero-sized files

[Dave Chen <davechen@synology.com>]

In btrfs_setsize(), when a file is truncated to size 0, the
BTRFS_INODE_FLUSH_ON_CLOSE flag is unconditionally set to ensure
pending writes get flushed on close. This flag was designed to protect
the "truncate-then-rewrite" pattern, where an application truncates a
file with existing data down to zero and writes new content, ensuring
the new data reaches disk on close.

However, when a file already has a size of 0 (e.g., a newly created
file opened with O_CREAT | O_TRUNC), oldsize and newsize are both 0.
In this case, setting BTRFS_INODE_FLUSH_ON_CLOSE is unnecessary because
no "good data" was truncated away. The subsequent filemap_flush() in
btrfs_release_file() then triggers avoidable writeback that disrupts
the normal delayed writeback batching, adding I/O overhead.

Fixes: a41ad394a03b ("Btrfs: convert to the new truncate sequence")
Signed-off-by: Dave Chen <davechen@synology.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
---
 fs/btrfs/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index a6da98435ef7c..73902056c1c50 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -5434,7 +5434,7 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
 		 * zero. Make sure any new writes to the file get on disk
 		 * on close.
 		 */
-		if (newsize == 0)
+		if (newsize == 0 && oldsize != 0)
 			set_bit(BTRFS_INODE_FLUSH_ON_CLOSE,
 				&BTRFS_I(inode)->runtime_flags);
 
-- 
2.43.0


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