linux-kernel - Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before set_pte

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Date:   Wed, 17 Aug 2022 16:41:40 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Muchun Song <muchun.song@...ux.dev>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Linux MM <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before
 set_pte_at()

On 2022/8/17 10:53, Muchun Song wrote:
> 
> 
>> On Aug 16, 2022, at 21:05, Miaohe Lin <linmiaohe@...wei.com> wrote:
>>
>> The memory barrier smp_wmb() is needed to make sure that preceding stores
>> to the page contents become visible before the below set_pte_at() write.
> 
> I’m not sure if you are right. I think it is set_pte_at()’s responsibility.

Maybe not. There're many call sites do the similar things:

hugetlb_mcopy_atomic_pte
__do_huge_pmd_anonymous_page
collapse_huge_page
do_anonymous_page
migrate_vma_insert_page
mcopy_atomic_pte

Take do_anonymous_page as an example:

	/*
	 * The memory barrier inside __SetPageUptodate makes sure that
	 * preceding stores to the page contents become visible before
	 * the set_pte_at() write.
	 */
	__SetPageUptodate(page);

So I think a memory barrier is needed before the set_pte_at() write. Or am I miss something?

Thanks,
Miaohe Lin

> Take arm64 (since it is a Relaxed Memory Order model) as an example (the
> following code snippet is set_pte()), I see a barrier guarantee. So I am
> curious what issues you are facing. So I want to know the basis for you to
> do this change.
> 
>  static inline void set_pte(pte_t *ptep, pte_t pte)
>  {
>         *ptep = pte;
> 
>         /*
>          * Only if the new pte is valid and kernel, otherwise TLB maintenance
>          * or update_mmu_cache() have the necessary barriers.
>          */
>         if (pte_valid_not_user(pte)) {
>                dsb(ishst);
>                isb();
>         }
>  }
> 
> Thanks.
>