[PATCH RFC v2 04/13] ext4: use sb_find_get_block_nonatomic() in ext4_get_inode_loc_noio()

[Christian Brauner <brauner@kernel.org>]

ext4_get_inode_loc_noio() looks the inode table block up with
sb_find_get_block(). That's the atomic lookup. It serializes against
try_to_free_buffers() with i_private_lock instead of the folio lock and
gives up if the folio is being migrated. So it can report the block as
not cached while it is.

For ext4_fc_snapshot_inode() that only means falling back to a full
commit. But ext4_sync_inode_metadata() is going to use
ext4_get_inode_loc_noio() to find the inode block that fsync() has to
write out and there a dirty block must not be skipped.

ext4_fc_snapshot_inode() is the only caller and it can sleep. It runs
under jbd2_journal_lock_updates() and allocates the snapshot with
GFP_NOFS. So use sb_find_get_block_nonatomic() which takes the folio
lock and waits for the migration instead.

Suggested-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org # v7.3
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/ext4/inode.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b4d581aa5ee1..4484c2383eaf 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5876,7 +5876,8 @@ int ext4_get_inode_loc(struct inode *inode, struct ext4_iloc *iloc)
 /*
  * ext4_get_inode_loc_noio() is a best-effort variant of ext4_get_inode_loc().
  * It looks up the inode table block in the buffer cache and returns -EAGAIN if
- * the block is not present or not uptodate, without starting any I/O.
+ * the block is not present or not uptodate, without starting any I/O. It may
+ * sleep on the folio lock of the block.
  */
 int ext4_get_inode_loc_noio(struct inode *inode, struct ext4_iloc *iloc)
 {
@@ -5912,7 +5913,7 @@ int ext4_get_inode_loc_noio(struct inode *inode, struct ext4_iloc *iloc)
 	}
 	block += inode_offset / inodes_per_block;
 
-	bh = sb_find_get_block(sb, block);
+	bh = sb_find_get_block_nonatomic(sb, block);
 	if (!bh)
 		return -EAGAIN;
 	if (!ext4_buffer_uptodate(bh)) {

-- 
2.53.0