linux-kernel - Re: [RFC PATCH 04/26] hugetlb: make huge_pte_lockptr take an explicit shift argument.

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Date:   Wed, 29 Jun 2022 14:03:56 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     James Houghton <jthoughton@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [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