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

[Kirill A. Shutemov]

Hi,

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:

 - arc;
 - arm;
 - arm64;
 - mips;
 - sparc -- it has custom pmdp_invalidate(), but it's racy too;

Please, help me with them.

Kirill A. Shutemov (3):
  x86/mm: Provide pmdp_mknotpresent() helper
  mm: Do not loose dirty and access bits in pmdp_invalidate()
  mm, thp: Do not loose dirty bit in __split_huge_pmd_locked()

 arch/x86/include/asm/pgtable-3level.h | 17 +++++++++++++++++
 arch/x86/include/asm/pgtable.h        | 13 +++++++++++++
 mm/huge_memory.c                      | 13 +++++++++----
 mm/pgtable-generic.c                  |  3 +--
 4 files changed, 40 insertions(+), 6 deletions(-)

-- 
2.11.0