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

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Message-Id: <C6F3DC27-4819-43A4-B884-DD3D03A2DF90@linux.dev>
Date:   Thu, 18 Aug 2022 16:40:38 +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 16:32, Yin, Fengwei <fengwei.yin@...el.com> wrote:
> 
> 
> 
> On 8/18/2022 3:59 PM, Muchun Song wrote:
>> 
>> 
>>> On Aug 18, 2022, at 15:52, Miaohe Lin <linmiaohe@...wei.com> wrote:
>>> 
>>> On 2022/8/18 10:47, Muchun Song wrote:
>>>> 
>>>> 
>>>>> 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.
>>> 
>>> Per my understanding, memory barrier in PageUptodate() is needed because user might access the
>>> page contents using page_address() (corresponding pagetable entry already exists) soon. But for
>>> the above proposed case, if user wants to access the page contents, the corresponding pagetable
>>> should be visible first or the page contents can't be accessed. So there should be a data dependency
>>> acting as memory barrier between pagetable entry is loaded and page contents is accessed.
>>> Or am I miss something?
>> 
>> Yep, it is a data dependency. The difference between hugetlb_vmemmap and PageUptodate() is that
>> the page table (a pointer to the mapped page frame) is loaded by MMU while PageUptodate() is
>> loaded by CPU. Seems like the data dependency should be inserted between the MMU access and the CPU
>> access. Maybe it is hardware’s guarantee?
> I just found the comment in pmd_install() explained why most arch has no read

I think pmd_install() is a little different as well. We should make sure the
page table walker (like GUP) see the correct PTE entry after they see the pmd
entry.

> side memory barrier except alpha which has read side memory barrier.

Right. Only alpha has data dependency barrier.

> 
> 
> Regards
> Yin, Fengwei
> 
>> 
>>> 
>>> Thanks,
>>> Miaohe Lin