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.
