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

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Date:   Thu, 18 Aug 2022 10:47:19 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        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 18, 2022, at 10:00, Yin, Fengwei <fengwei.yin@...el.com> wrote:
> 
> 
> 
> On 8/18/2022 9:55 AM, Miaohe Lin wrote:
>>>>> 	/*
>>>>> 	 * 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.
> I have an unclear thing (not related with this patch directly): Who is response
> for the read barrier in the read side in this case?
> 
> For SetPageUptodate, there are paring write/read memory barrier.
> 

I have the same question. So I think the example proposed by Miaohe is a little
difference from the case (hugetlb_vmemmap) here.

> 
> Regards
> Yin, Fengwei
> 
>