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 16:54:44 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Muchun Song <muchun.song@...ux.dev>
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 8/18/2022 4:40 PM, Muchun Song wrote:
> 
> 
>> 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.

The difference I can see is that pmd/pte thing has both hardware page walker and
software page walker (like GUP) as read side. While the case here only has hardware
page walker as read side. But I suppose the memory barrier requirement still apply
here.

Maybe we could do a test: add large delay between reset_struct_page() and set_pte_at?

Regards
Yin, Fengwei 

> 
>> side memory barrier except alpha which has read side memory barrier.
> 
> Right. Only alpha has data dependency barrier.
> 
>>
>>
>> Regards
>> Yin, Fengwei
>>
>>>
>>>>
>>>> Thanks,
>>>> Miaohe Lin
>