[PATCH RFC v2 12/13] fat: sync inode metadata in fat_evict_inode()

[Christian Brauner <brauner@kernel.org>]

fat_write_inode() copies the inode into its directory entry, marks the
buffer dirty and sets I_METADATA_WRITEBACK so that a later fsync()
writes the entry out and waits for it. But the bit goes away with the
inode. If the inode is evicted and read back in, fsync() finds a clean
inode and returns while the directory entry might still be dirty or
under writeback.

Right now that can't happen because inode_lru_isolate() never lets go
of an inode with I_METADATA_WRITEBACK set. That is about to change.

So call fat_sync_inode_metadata() from fat_evict_inode() for a linked
inode that still has the bit set. It syncs the metadata buffers as well
so it replaces the mmb_sync() there, and it already looks the directory
entry up in the buffer cache only.

Do it before fat_free_eofblocks(). That one updates the directory entry
again for a file with unused fallocated clusters and only waits for it
on a sync mount. That's intentional (cf. [1]) and syncing after it
would turn every such eviction into a synchronous write.

[1]: https://lore.kernel.org/r/87v76co5dv.fsf@mail.parknet.co.jp

Suggested-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org # v7.3
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/fat/inode.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index d85c8fbdcca4..df4616636c65 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -693,7 +693,16 @@ static void fat_evict_inode(struct inode *inode)
 		inode->i_size = 0;
 		fat_truncate_blocks(inode, 0);
 	} else {
-		mmb_sync(&MSDOS_I(inode)->i_metadata_bhs);
+		/*
+		 * fsync() can't find what I_METADATA_WRITEBACK tracks once the
+		 * inode is gone. This syncs the metadata buffers as well.
+		 */
+		if (inode_state_read_once(inode) & I_METADATA_WRITEBACK)
+			fat_sync_inode_metadata(inode, &(struct writeback_control) {
+				.sync_mode = WB_SYNC_ALL,
+			});
+		else
+			mmb_sync(&MSDOS_I(inode)->i_metadata_bhs);
 		fat_free_eofblocks(inode);
 	}
 

-- 
2.53.0