Re: [PATCH v4 RESEND 2/2] buffer: record blockdev write errors in super_block that it backs

On Tue, 2020-04-14 at 18:26 +0200, Jan Kara wrote:

On Tue 14-04-20 08:04:09, Jeff Layton wrote:

quoted

From: Jeff Layton <redacted>

When syncing out a block device (a'la __sync_blockdev), any error
encountered will only be recorded in the bd_inode's mapping. When the
blockdev contains a filesystem however, we'd like to also record the
error in the super_block that's stored there.

Make mark_buffer_write_io_error also record the error in the
corresponding super_block when a writeback error occurs and the block
device contains a mounted superblock.

Signed-off-by: Jeff Layton <jlayton@kernel.org>

The patch looks good to me. I'd just note that bh->b_bdev->bd_super
dereference is safe only because we will flush all dirty data when
unmounting a filesystem which is somewhat tricky. Maybe that warrants a
comment? Otherwise feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Oh, hmm...now that I look again, I'm not sure this is actually safe.

bh->b_bdev gets cleared out as we discard the buffer, so I don't think
that could end up getting zeroed while we're still using it.

The bd_super pointer gets zeroed out in kill_block_super, and after that
point it calls sync_blockdev(). Could writeback error processing race
with kill_block_super such that bd_inode gets set to NULL after we test
it but before we dereference it?

Thanks,
-- 
Jeff Layton [off-list ref]