Re: [PATCH] btrfs: btrfs_wait_tree_block_writeback can be void return

[Liu Bo <bo.li.liu@oracle.com>]

On Fri, May 26, 2017 at 07:08:31PM +0200, David Sterba wrote:
> On Thu, May 25, 2017 at 06:39:52AM -0400, Jeff Layton wrote:
> > Nothing checks its return value.
> 
> I think we don't need to check the return value of
> filemap_fdatawait_range, because the errors are tracked by other means
> (attached to the btree_inode mapping that represents the metadata), in
> set_btree_ioerr.
> 
> > Signed-off-by: Jeff Layton <jlayton@redhat.com>
> > Reviewed-by: Jan Kara <jack@suse.cz>
> > Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
> 
> To Liu Bo:
> 
> May I ask if you did a review that matches the above understanding? It's
> trivial to see that nothing really checks the return value of
> btrfs_wait_tree_block_writeback but my question is if it's really safe
> not to so. Thanks.

About the question if it's safe not to do so, I think yes, it's used in
walk_log_tree which is called in two places, free_log_tree and log replay.  For
free_log_tree, it waits for any running writeback of the extent buffer under
freeing to finish in case we need to access the eb pointer from page->private,
and it's OK to not check the return value, while for log replay, it's doesn't
wait because wc->wait is not set. So neither cares about the writeback error.

thanks,
-liubo