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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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