Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Subject: Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before
 set_pte_at()
To:     Muchun Song <muchun.song@linux.dev>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Linux MM <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20220816130553.31406-1-linmiaohe@huawei.com>
 <20220816130553.31406-5-linmiaohe@huawei.com>
 <D8C00BDA-160D-40CE-AFBD-9488F85E76CE@linux.dev>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <ea67ab10-667e-f361-b80f-dafb13da4808@huawei.com>
Date:   Wed, 17 Aug 2022 16:41:40 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101
 Thunderbird/78.6.0
MIME-Version: 1.0
In-Reply-To: <D8C00BDA-160D-40CE-AFBD-9488F85E76CE@linux.dev>
Content-Type: text/plain; charset="utf-8"
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 2022/8/17 10:53, Muchun Song wrote:
> 
> 
>> On Aug 16, 2022, at 21:05, Miaohe Lin <linmiaohe@huawei.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.
>