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    SubjectRe: [RFC PATCH 04/26] hugetlb: make huge_pte_lockptr take an explicit shift argument.
    On 06/29/22 14:09, Muchun Song wrote:
    > On Mon, Jun 27, 2022 at 01:51:53PM -0700, Mike Kravetz wrote:
    > > On 06/24/22 17:36, James Houghton wrote:
    > > > This is needed to handle PTL locking with high-granularity mapping. We
    > > > won't always be using the PMD-level PTL even if we're using the 2M
    > > > hugepage hstate. It's possible that we're dealing with 4K PTEs, in which
    > > > case, we need to lock the PTL for the 4K PTE.
    > >
    > > I'm not really sure why this would be required.
    > > Why not use the PMD level lock for 4K PTEs? Seems that would scale better
    > > with less contention than using the more coarse mm lock.
    > >
    >
    > Your words make me thing of another question unrelated to this patch.
    > We __know__ that arm64 supports continues PTE HugeTLB. huge_pte_lockptr()
    > did not consider this case, in this case, those HugeTLB pages are contended
    > with mm lock. Seems we should optimize this case. Something like:
    >
    > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
    > index 0d790fa3f297..68a1e071bfc0 100644
    > --- a/include/linux/hugetlb.h
    > +++ b/include/linux/hugetlb.h
    > @@ -893,7 +893,7 @@ static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
    > static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
    > struct mm_struct *mm, pte_t *pte)
    > {
    > - if (huge_page_size(h) == PMD_SIZE)
    > + if (huge_page_size(h) <= PMD_SIZE)
    > return pmd_lockptr(mm, (pmd_t *) pte);
    > VM_BUG_ON(huge_page_size(h) == PAGE_SIZE);
    > return &mm->page_table_lock;
    >
    > I did not check if elsewhere needs to be changed as well. Just a primary
    > thought.

    That seems perfectly reasonable to me.

    Also unrelated, but using the pmd lock is REQUIRED for pmd sharing. The
    mm lock is process specific and does not synchronize shared access. I
    found that out the hard way. :)

    --
    Mike Kravetz

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