Re: [PATCH 1/2] btrfs: Add WARN_ON for qgroup reserved underflow

[David Sterba <dsterba@suse.cz>]

On Thu, Oct 20, 2016 at 09:05:34AM +0800, Qu Wenruo wrote:
> 
> 
> At 10/19/2016 10:31 PM, David Sterba wrote:
> > On Fri, Sep 30, 2016 at 09:15:36AM +0800, Qu Wenruo wrote:
> >> While the reason why qgroup reserved space may underflow is still under
> >> investigation, such WARN_ON will help us to expose the bug more easily,
> >> and for end-user we can detect and avoid underflow.
> >>
> >> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> >> ---
> >>  fs/btrfs/qgroup.c | 21 ++++++++++++++++-----
> >>  1 file changed, 16 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> >> index 8db2e29..8532587 100644
> >> --- a/fs/btrfs/qgroup.c
> >> +++ b/fs/btrfs/qgroup.c
> >> @@ -1061,8 +1061,12 @@ static int __qgroup_excl_accounting(struct btrfs_fs_info *fs_info,
> >>  	WARN_ON(sign < 0 && qgroup->excl < num_bytes);
> >>  	qgroup->excl += sign * num_bytes;
> >>  	qgroup->excl_cmpr += sign * num_bytes;
> >> -	if (sign > 0)
> >> -		qgroup->reserved -= num_bytes;
> >> +	if (sign > 0) {
> >> +		if (WARN_ON(qgroup->reserved < num_bytes))
> >
> > That's only partially helpful, you should also print the numbers,
> > ref_root and/or qgroup id.
> >
> 
> I'm OK to add more output.
> But normally it's not really help though.

So do you need the warning at all? The next patch adds tracepoints, I
assume you turn them on and look for the issues you're debugging. The
warning would otherwise appear for any user. Without the message it's
IME more scary, it's not obvious what happened without looking into the
code.