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    SubjectRe: [PATCH v10 07/10] mm: Device exclusive memory access
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    On Friday, 11 June 2021 4:04:35 AM AEST Peter Xu wrote:
    > External email: Use caution opening links or attachments
    >
    >
    > On Thu, Jun 10, 2021 at 10:18:25AM +1000, Alistair Popple wrote:
    > > > > The main problem is split_huge_pmd_address() unconditionally calls a mmu
    > > > > notifier so I would need to plumb in passing an owner everywhere which could
    > > > > get messy.
    > > >
    > > > Could I ask why? split_huge_pmd_address() will notify with CLEAR, so I'm a bit
    > > > confused why we need to pass over the owner.
    > >
    > > Sure, it is the same reason we need to pass it for the exclusive notifier.
    > > Any invalidation during the make exclusive operation will break the mmu read
    > > side critical section forcing a retry of the operation. The owner field is what
    > > is used to filter out invalidations (such as the exclusive invalidation) that
    > > don't need to be retried.
    >
    > Do you mean the mmu_interval_read_begin|retry() calls?

    Yep.

    > Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit
    > call split_huge_pmd_address(), afaict. Since both of them use __split_huge_pmd()
    > internally which will generate that unwanted CLEAR notify.

    Agree that gup calls __split_huge_pmd() via split_huge_pmd_address()
    which will always CLEAR. However gup only calls split_huge_pmd_address() if it
    finds a thp pmd. In follow_pmd_mask() we have:

    if (likely(!pmd_trans_huge(pmdval)))
    return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);

    So I don't think we have a problem here.

    > If that's the case, I think it fails because split_huge_pmd_address() will
    > trigger that CLEAR notify unconditionally (even if it's not a thp; not sure
    > whether it should be optimized to not notify at all... definitely another
    > story), while FOLL_SPLIT_PMD will skip the notify as it calls split_huge_pmd()
    > instead, who checks the pmd before calling __split_huge_pmd().
    >
    > Does it also mean that if there's a real THP it won't really work? As then
    > FOLL_SPLIT_PMD will start to trigger that CLEAR notify too, I think..
    >
    > --
    > Peter Xu
    >




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