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 19:21:20 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Miaohe Lin <linmiaohe@...wei.com>
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 Aug 17, 2022, at 16:41, Miaohe Lin <linmiaohe@...wei.com> wrote:
> 
> 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);

IIUC, the case here we should make sure others (CPUs) can see new page’s
contents after they have saw PG_uptodate is set. I think commit 0ed361dec369
can tell us more details.

I also looked at commit 52f37629fd3c to see why we need a barrier before
set_pte_at(), but I didn’t find any info to explain why. I guess we want
to make sure the order between the page’s contents and subsequent memory
accesses using the corresponding virtual address, do you agree with this?

Thanks.

> 
> 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.
>>