[PATCH RFC v2 05/13] ext4: don't read the inode block in ext4_sync_inode_metadata()

[Christian Brauner <brauner@kernel.org>]

Without a journal ext4_sync_inode_metadata() looks the inode table
block up with __ext4_get_inode_loc_noinmem() which reads it from disk,
with readahead of the inode table on top, if it isn't cached. But a
block that isn't cached is neither dirty nor under writeback, so there's
nothing to write or to wait for.

That read is also something we can't afford once inode eviction calls
->sync_inode_metadata() from reclaim. So look the block up with
ext4_get_inode_loc_noio() instead and treat -EAGAIN as nothing to do.

Note that ext4_get_inode_loc_noio() also returns -EAGAIN for a block
that isn't uptodate. Such a block is being read or failed to be read
and can't be dirty either. A block whose write failed still counts as
uptodate for ext4_buffer_uptodate() so the write error still gets
reported.

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 | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 4484c2383eaf..18362580097f 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6815,22 +6815,24 @@ int ext4_sync_inode_metadata(struct inode *inode, struct writeback_control *wbc)
 	if (WARN_ON_ONCE(EXT4_SB(inode->i_sb)->s_journal))
 		return -EFSCORRUPTED;
 
-	err = __ext4_get_inode_loc_noinmem(inode, &iloc);
-	if (err)
-		return err;
 	mmb = READ_ONCE(EXT4_I(inode)->i_metadata_bhs);
 	if (mmb) {
 		err = mmb_sync(mmb);
 		if (err)
-			goto out;
+			return err;
 	}
+	/* A block that isn't cached is neither dirty nor under writeback. */
+	err = ext4_get_inode_loc_noio(inode, &iloc);
+	if (err == -EAGAIN)
+		return 0;
+	if (err)
+		return err;
 	sync_dirty_buffer(iloc.bh);
 	if (buffer_write_io_error(iloc.bh)) {
 		ext4_error_inode_block(inode, iloc.bh->b_blocknr, EIO,
 				       "IO error syncing inode");
 		err = -EIO;
 	}
-out:
 	brelse(iloc.bh);
 	return err;
 }

-- 
2.53.0