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

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Message-ID: <18adbf89-473e-7ba6-9a2b-522e1592bdc6@huawei.com>
Date:   Thu, 18 Aug 2022 09:55:32 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>,
        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 2022/8/18 9:14, Yin, Fengwei wrote:
> 
> 
> On 8/17/2022 7:21 PM, Muchun Song wrote:
>>
>>
>>> 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?
> This is my understanding also. Thanks.

That's also my understanding. Thanks both.

Thanks,
Miaohe Lin