Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages

[Martin Schwidefsky]

Hi Kirill,

On Wed, 14 Jun 2017 16:51:40 +0300
"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:

> Vlastimil noted that pmdp_invalidate() is not atomic and we can loose
> dirty and access bits if CPU sets them after pmdp dereference, but
> before set_pmd_at().
> 
> The bug doesn't lead to user-visible misbehaviour in current kernel, but
> fixing this would be critical for future work on THP: both huge-ext4 and THP
> swap out rely on proper dirty tracking.
> 
> Unfortunately, there's no way to address the issue in a generic way. We need to
> fix all architectures that support THP one-by-one.
> 
> All architectures that have THP supported have to provide atomic
> pmdp_invalidate(). If generic implementation of pmdp_invalidate() is used,
> architecture needs to provide atomic pmdp_mknonpresent().
> 
> I've fixed the issue for x86, but I need help with the rest.
> 
> So far THP is supported on 8 architectures. Power and S390 already provides
> atomic pmdp_invalidate(). x86 is fixed by this patches, so 5 architectures
> left:

For s390 the pmdp_invalidate() is atomic only in regard to the dirty and
referenced bits because we use a fault driven approach for this, no?

More specifically the update via the pmdp_xchg_direct() function is protected
by the page table lock, the update on the pmd entry itself does *not* have
to be atomic (for s390).

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.