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

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Message-Id: <15DD6DCA-39BC-4EA2-984F-D488E94CC4FF@linux.dev>
Date:   Thu, 18 Aug 2022 17:18:09 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>,
        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 Aug 18, 2022, at 16:54, Yin, Fengwei <fengwei.yin@...el.com> wrote:
> 
> 
> 
> 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.

I am not against this change. Just in order to make me get a better understanding of
hardware behavior.

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

Hi Miaohe,

Would you mind doing this test? One thread do vmemmap_restore_pte(), another thread
detect if it can see a tail page with PG_head after the previous thread has executed
set_pte_at().

Thanks.

> 
> 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