fat: Fix persisting directory entries on fsync(2) of the root directory

Buffers containing the directory entries of a directory's children are
tracked in the directory inode's metadata bh list. Before commit
525da4f40a7c ("fat: Fix missed inode writeback during fsync(2)")
fsync(2) of a directory wrote that list out unconditionally via
mmb_fsync_noflush(). Now the list is written by
fat_sync_inode_metadata() which __writeback_single_inode() only
invokes when the inode has I_METADATA_WRITEBACK set. The root inode
never gets I_METADATA_WRITEBACK - __fat_write_inode() returns early
for it since the root directory has no directory entry of its own -
and fat_sync_inode_metadata() returns early for it as well. Hence
fsync(2) on the root directory returns success without writing out the
directory entries of its children.

Set I_METADATA_WRITEBACK for the root inode in __fat_write_inode() and
make fat_sync_inode_metadata() only skip the nonexistent directory
entry for the root inode but still sync the metadata bh list.

Fixes: 525da4f40a7c ("fat: Fix missed inode writeback during fsync(2)")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index ef1f826179cd..5ea6f74a2a3f 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -634,11 +634,12 @@ static int fat_sync_inode_metadata(struct inode *inode,
 	sector_t blocknr;
 	int offset;
 
+	/* The root directory has no directory entry of its own. */
 	if (inode->i_ino == MSDOS_ROOT_INO)
-		return 0;
+		goto sync_bhs;
 	i_pos = fat_i_pos_read(sbi, inode);
 	if (!i_pos)
-		return 0;
+		goto sync_bhs;
 
 	fat_get_blknr_offset(sbi, i_pos, &blocknr, &offset);
 	bh = sb_find_get_block_nonatomic(inode->i_sb, blocknr);
@@ -654,6 +655,7 @@ static int fat_sync_inode_metadata(struct inode *inode,
 		}
 	}
 	brelse(bh);
+sync_bhs:
 	return mmb_sync(&MSDOS_I(inode)->i_metadata_bhs);
 }
 
@@ -897,8 +899,11 @@ static int __fat_write_inode(struct inode *inode)
 	sector_t blocknr;
 	int offset;
 
-	if (inode->i_ino == MSDOS_ROOT_INO)
+	if (inode->i_ino == MSDOS_ROOT_INO) {
+		/* No entry to update but the metadata bh list may need syncing. */
+		set_inode_metadata_writeback(inode);
 		return 0;
+	}
 
 retry:
 	i_pos = fat_i_pos_read(sbi, inode);