Re: [PATCH] Fix off by one error in page_region_mask()

[Lachlan McIlroy <lachlan@xxxxxxx>]

Eric Sandeen wrote:
> Lachlan McIlroy wrote:
> > final is calculated to be the last bit to set (ie inclusive) but when we
> > do the mask shifting final really needs to be first bit not to set.
> >
> > For example if first and final are both bit 0 (ie only first bit to be set)
> > then mask is completely shifted and becomes all zeroes.
> >
> > Or if first is 0 and final is 63 then the mask is shifted one bit when it
> > shouldn't be shifted at all.
>
> Lachlan, what's the end result of this bug?  What's the broken behavior?

There was no observed bug - well nothing I can tie directly to this code.
I found this by inspection while investigating the page bitmap stuff.
We have a problem with ia64 going to 64K page size with filesystems that
use a filesystem sector size of 512 bytes - we don't have the granularity
we need in the bitmap.

I suppose it is possible this bug could indicate a page region is not up
to date when it actually is and we might re-read something from disk and
overwrite the more up to date in-memory version.

> Thanks,
> -Eric
>
> > --- xfs-fix.orig/fs/xfs/linux-2.6/xfs_buf.c
> > +++ xfs-fix/fs/xfs/linux-2.6/xfs_buf.c
> > @@ -129,15 +129,17 @@ page_region_mask(
> >         int             first, final;
> >
> >         first = BTOPR(offset);
> > -       final = BTOPRT(offset + length - 1);
> > -       first = min(first, final);
> > +       final = BTOPRT(offset + length);
> > +
> > +       if (first >= final)
> > +               return 0UL;
> >
> >         mask = ~0UL;
> >         mask <<= BITS_PER_LONG - (final - first);
> >         mask >>= BITS_PER_LONG - (final);
> >
> >         ASSERT(offset + length <= PAGE_CACHE_SIZE);
> > -       ASSERT((final - first) < BITS_PER_LONG && (final - first) >= 0);
> > +       ASSERT((final - first) <= BITS_PER_LONG && (final - first) > 0);
> >
> >         return mask;
> >   }
> >
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