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

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Message-ID: <Yrvsn66c+UI8ZQlO@FVFYT0MHHV2J.usts.net>
Date:   Wed, 29 Jun 2022 14:09:35 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.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 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.

Thanks.
 
> -- 
> Mike Kravetz
>