[PATCH RFC 4/9] ext4: don't read the inode block in ext4_sync_inode_metadata()

[Christian Brauner <brauner@kernel.org>]

In nojournal mode ext4_sync_inode_metadata() looks up the inode table
block 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 in the buffer cache is neither dirty nor under writeback. So
there's nothing to write or to wait for and reading it in is pointless.

It's also not something we can afford once inode eviction calls
->sync_inode_metadata() from reclaim. So look the block up with
sb_find_get_block_nonatomic() instead.

Note that ext4_get_inode_loc_noio() isn't usable here. It uses the
atomic lookup and that one gives up on a folio that is being migrated.
That's fine for a best-effort fast commit snapshot but not for a data
integrity sync.

Cc: stable@vger.kernel.org # v7.3
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/ext4/inode.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b34b99d15db6..cab26ec01aba 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6824,28 +6824,32 @@ int ext4_sync_inode_metadata(struct inode *inode, struct writeback_control *wbc)
 {
 	struct ext4_iloc iloc;
 	struct mapping_metadata_bhs *mmb;
+	ext4_fsblk_t block;
 	int err;
 
 	/* We should only get here in nojournal mode */
 	if (WARN_ON_ONCE(EXT4_SB(inode->i_sb)->s_journal))
 		return -EFSCORRUPTED;
 
-	err = __ext4_get_inode_loc_noinmem(inode, &iloc);
+	err = ext4_inode_loc_block(inode, &iloc, &block);
 	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. */
+	iloc.bh = sb_find_get_block_nonatomic(inode->i_sb, block);
+	if (!iloc.bh)
+		return 0;
 	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