Re: [Xen-devel] [PATCH] thp: avoid atomic64_read in pmd_read_atomic for 32bit PAE\

[Andrew Jones]

----- Original Message -----
> On Thu, Jun 07, 2012 at 11:00:33PM +0200, Andrea Arcangeli wrote:
> > In the x86 32bit PAE CONFIG_TRANSPARENT_HUGEPAGE=y case while
> > holding
> > the mmap_sem for reading, cmpxchg8b cannot be used to read pmd
> > contents under Xen.
> > 
> > So instead of dealing only with "consistent" pmdvals in
> > pmd_none_or_trans_huge_or_clear_bad() (which would be conceptually
> > simpler) we let pmd_none_or_trans_huge_or_clear_bad() deal with
> > pmdvals
> > where the low 32bit and high 32bit could be inconsistent (to avoid
> > having to use cmpxchg8b).
> 
> <nods>
> > 
> > The only guarantee we get from pmd_read_atomic is that if the low
> > part
> > of the pmd was found null, the high part will be null too (so the
> > pmd
> > will be considered unstable). And if the low part of the pmd is
> > found
> > "stable" later, then it means the whole pmd was read atomically
> > (because after a pmd is stable, neither MADV_DONTNEED nor page
> > faults
> > can alter it anymore, and we read the high part after the low
> > part).
> > 
> > In the 32bit PAE x86 case, it is enough to read the low part of the
> > pmdval atomically to declare the pmd as "stable" and that's true
> > for
> > THP and no THP, furthermore in the THP case we also have a
> > barrier()
> > that will prevent any inconsistent pmdvals to be cached by a later
> > re-read of the *pmd.
> 
> Nice. Andrew, any chane you could test this patch on the affected
> Xen hypervisors? Was it as easy to reproduce this on a RHEL5 (U1?)
> hypervisor or is it really only on Linode and Amazon EC2?
> 

Originally, I was able to reproduce the issue easily with a RHEL5
host. Now, with this patch it's fixed.

Drew
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