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 16:32:13 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Muchun Song <muchun.song@...ux.dev>,
        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 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
side memory barrier except alpha which has read side memory barrier.


Regards
Yin, Fengwei

> 
>>
>> Thanks,
>> Miaohe Lin
>