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Subject: Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before
 set_pte_at()
To:     Muchun Song <muchun.song@linux.dev>,
        "Yin, Fengwei" <fengwei.yin@intel.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Linux MM <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20220816130553.31406-1-linmiaohe@huawei.com>
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From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <615c8ec8-6977-2ce0-f049-d2ec1619245c@huawei.com>
Date:   Thu, 18 Aug 2022 20:58:54 +0800
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On 2022/8/18 17:18, Muchun Song wrote:
> 
> 
>> On Aug 18, 2022, at 16:54, Yin, Fengwei <fengwei.yin@intel.com> wrote:
>>
>>
>>
>> On 8/18/2022 4:40 PM, Muchun Song wrote:
>>>
>>>
>>>> On Aug 18, 2022, at 16:32, Yin, Fengwei <fengwei.yin@intel.com> wrote:
>>>>
>>>>
>>>>
>>>> On 8/18/2022 3:59 PM, Muchun Song wrote:
>>>>>
>>>>>
>>>>>> On Aug 18, 2022, at 15:52, Miaohe Lin <linmiaohe@huawei.com> wrote:
>>>>>>
>>>>>> On 2022/8/18 10:47, Muchun Song wrote:
>>>>>>>
>>>>>>>
>>>>>>>> On Aug 18, 2022, at 10:00, Yin, Fengwei <fengwei.yin@intel.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().

Will it be easier to construct the memory reorder manually like below?

vmemmap_restore_pte()
	...
	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
	/* might a delay. */
	copy_page(to, (void *)walk->reuse_addr);
	reset_struct_pages(to);

And another thread detects whether it can see a tail page with some invalid fields? If so,
it seems the problem will always trigger? If not, we depend on the observed meory reorder
and set_pte_at doesn't contain a memory barrier?

Thanks,
Miaohe Lin