Re: [PATCH] xfs: Fix rounding in xfs_alloc_fix_len()

[Brian Foster <bfoster@redhat.com>]

On Wed, Jun 04, 2014 at 05:10:34PM +0200, Jan Kara wrote:
> On Wed 04-06-14 09:35:51, Brian Foster wrote:
> > On Wed, Jun 04, 2014 at 11:48:13AM +0200, Jan Kara wrote:
> > > Rounding in xfs_alloc_fix_len() is wrong. As the comment states, the
> > > result should be a number of a form (k*prod+mod) however due to sign
> > > mistake the result is different. As a result allocations on raid arrays
> > > could be misaligned in some cases.
> > > 
> > > This also seems to fix occasional assertion failure:
> > > 	XFS_WANT_CORRUPTED_GOTO(rlen <= flen, error0)
> > > in xfs_alloc_ag_vextent_size().
> > 
> > Do you happen to have a reproducer for this?
>   No, IBM triggered this during their testing on powerPC. I can ask them if
> they can share the test if you are interested.
> 

I think it would be generally interesting, particularly to see if we
could create an xfstests test..?

> > The meaning of args->prod (the structure definition comment calls it the
> > prod value) is not clear to me. I see that we set it to an extent
> > size hint if one exists (in xfs_bmap_btalloc()), so I'll go with that.
> > args->mod then becomes the modulo of the file offset against that
> > alignment hint.
> > 
> > > Signed-off-by: Jan Kara <jack@suse.cz>
> > > ---
> > >  fs/xfs/xfs_alloc.c | 14 ++++++--------
> > >  1 file changed, 6 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/fs/xfs/xfs_alloc.c b/fs/xfs/xfs_alloc.c
> > > index c1cf6a336a72..6a0281b16451 100644
> > > --- a/fs/xfs/xfs_alloc.c
> > > +++ b/fs/xfs/xfs_alloc.c
> > > @@ -257,14 +257,12 @@ xfs_alloc_fix_len(
> > 
> > We get here and take the extent length, mod against the alignment and
> > compare to the mod of the offset. 
> > 
> > >  	k = rlen % args->prod;
> > >  	if (k == args->mod)
> > >  		return;
> > > -	if (k > args->mod) {
> > > -		if ((int)(rlen = rlen - k - args->mod) < (int)args->minlen)
> > > -			return;
> > > -	} else {
> > > -		if ((int)(rlen = rlen - args->prod - (args->mod - k)) <
> > > -		    (int)args->minlen)
> > > -			return;
> > > -	}
> > > +	if (k > args->mod)
> > > +		rlen = rlen - (k - args->mod);
> > 
> > If the length mod is greater than the offset mod, reduce the length by
> > the delta of the mods.
> > 
> > > +	else
> > > +		rlen = rlen - args->prod + (args->mod - k);
> > 
> > Otherwise (length mod is less than offset mod), reduce by a full
> > alignment size and add back the difference to match the offset mod.
> > 
> > This seems correct to me.
> > 
> > > +	if ((int)rlen < (int)args->minlen)
> > > +		return;
> > >  	ASSERT(rlen >= args->minlen);
> > >  	ASSERT(rlen <= args->maxlen);
> > 
> > The rlen >= minlen assert seems kind of pointless here, but what about
> > changing both instances of these two asserts to the following:
>   Well, rlen has been decreased so rlen >= minlen makes sense. rlen <=
> maxlen seems to be the obvious one to me.
> 

That was more a commentary on the fact that the assert now immediately
follows a check for the negation of the assert, where we return. The
assert below seems a bit more generic and just makes it stand out a
little less (to me). Not really a big deal.

> > 	ASSERT(rlen >= args->minlen && rlen <= args->maxlen);
> > 
> > ... and add a new one after the length adjustment along the lines of:
> > 
> > 	ASSERT((rlen % args->prod) == args->mod);
> > 
> > Thoughts? Would this have caught the problem you've found earlier?
>   Yes, this would have caught the bug. Should I add this assertion an
> resend?

Yeah, if you don't mind. I think that one is definitely beneficial.

Brian

> 
> 								Honza
> 
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR

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