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Subject[HELP-NEEDED, PATCHv2 0/3] Do not loose dirty bit on THP pages
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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() that returns previous value.

If generic implementation of pmdp_invalidate() is used, architecture needs to
provide atomic pmdp_estabish().

pmdp_estabish() is not used out-side generic implementation of
pmdp_invalidate() so far, but I think this can change in the future.

I've fixed the issue for x86, but I need help with the rest.

So far THP is supported on 7 architectures, beyond x86:

- arc;
- arm;
- arm64;
- mips;
- power;
- s390;
- sparc;

Please, help me with them.

v2:
- Introduce pmdp_estabish(), instead of pmdp_mknonpresent();
- Change pmdp_invalidate() to return previous value of the pmd;

arch/x86/include/asm/pgtable-3level.h | 18 ++++++++++++++++++
arch/x86/include/asm/pgtable.h | 14 ++++++++++++++
fs/proc/task_mmu.c | 8 ++++----
include/asm-generic/pgtable.h | 2 +-
mm/huge_memory.c | 29 ++++++++++++-----------------
mm/pgtable-generic.c | 9 +++++----
6 files changed, 54 insertions(+), 26 deletions(-)

--
2.11.0

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