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

Jan Kara
2014-06-04T15:10:40Z

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.

> 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.

> 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?

Honza

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