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    Subject[PATCHv3 10/11] mm: Do not loose dirty and access bits in pmdp_invalidate()
    Date
    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 patch change pmdp_invalidate() to make the entry non-present atomically and
    return previous value of the entry. This value can be used to check if
    CPU set dirty/accessed bits under us.

    The race window is very small and I haven't seen any reports that can be
    attributed to the bug. For this reason, I don't think backporting to
    stable trees needed.

    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Reported-by: Vlastimil Babka <vbabka@suse.cz>
    Cc: Hugh Dickins <hughd@google.com>
    ---
    include/asm-generic/pgtable.h | 2 +-
    mm/pgtable-generic.c | 6 +++---
    2 files changed, 4 insertions(+), 4 deletions(-)

    diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
    index bf0889eb774d..9df1da175fb0 100644
    --- a/include/asm-generic/pgtable.h
    +++ b/include/asm-generic/pgtable.h
    @@ -324,7 +324,7 @@ static inline pmd_t generic_pmdp_establish(struct vm_area_struct *vma,
    #endif

    #ifndef __HAVE_ARCH_PMDP_INVALIDATE
    -extern void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
    +extern pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
    pmd_t *pmdp);
    #endif

    diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
    index 1175f6a24fdb..3db8f2f76666 100644
    --- a/mm/pgtable-generic.c
    +++ b/mm/pgtable-generic.c
    @@ -180,12 +180,12 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)
    #endif

    #ifndef __HAVE_ARCH_PMDP_INVALIDATE
    -void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
    +pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
    pmd_t *pmdp)
    {
    - pmd_t entry = *pmdp;
    - set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry));
    + pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mknotpresent(*pmdp));
    flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
    + return old;
    }
    #endif

    --
    2.14.1
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