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    SubjectRe: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages
    On Wed 29-08-18 14:14:25, Jerome Glisse wrote:
    > On Wed, Aug 29, 2018 at 10:24:44AM -0700, Mike Kravetz wrote:
    [...]
    > > What would be the best mmu notifier interface to use where there are no
    > > start/end calls?
    > > Or, is the best solution to add the start/end calls as is done in later
    > > versions of the code? If that is the suggestion, has there been any change
    > > in invalidate start/end semantics that we should take into account?
    >
    > start/end would be the one to add, 4.4 seems broken in respect to THP
    > and mmu notification. Another solution is to fix user of mmu notifier,
    > they were only a handful back then. For instance properly adjust the
    > address to match first address covered by pmd or pud and passing down
    > correct page size to mmu_notifier_invalidate_page() would allow to fix
    > this easily.
    >
    > This is ok because user of try_to_unmap_one() replace the pte/pmd/pud
    > with an invalid one (either poison, migration or swap) inside the
    > function. So anyone racing would synchronize on those special entry
    > hence why it is fine to delay mmu_notifier_invalidate_page() to after
    > dropping the page table lock.
    >
    > Adding start/end might the solution with less code churn as you would
    > only need to change try_to_unmap_one().

    What about dependencies? 369ea8242c0fb sounds like it needs work for all
    notifiers need to be updated as well.

    Anyway, I am wondering why we haven't see any bugs coming from
    incomplete range invalidation. How would those exhibit?
    --
    Michal Hocko
    SUSE Labs

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