From: "Yin, Fengwei" <fengwei.yin@intel.com>
To: Muchun Song <muchun.song@linux.dev>
Cc: Miaohe Lin <linmiaohe@huawei.com>,
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>
Subject: Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before set_pte_at()
Date: Thu, 18 Aug 2022 16:54:44 +0800 [thread overview]
Message-ID: <7408156a-f708-5e73-d0a2-69b1acca9b96@intel.com> (raw)
In-Reply-To: <C6F3DC27-4819-43A4-B884-DD3D03A2DF90@linux.dev>
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.
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
>
next prev parent reply other threads:[~2022-08-18 10:12 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-16 13:05 [PATCH 0/6] A few fixup patches for hugetlb Miaohe Lin
2022-08-16 13:05 ` [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge_pages Miaohe Lin
2022-08-16 22:52 ` Mike Kravetz
2022-08-16 23:20 ` Andrew Morton
2022-08-16 23:34 ` Mike Kravetz
2022-08-17 1:53 ` Miaohe Lin
2022-08-17 2:28 ` Muchun Song
2022-08-16 13:05 ` [PATCH 2/6] mm/hugetlb: fix WARN_ON(!kobj) in sysfs_create_group() Miaohe Lin
2022-08-16 22:55 ` Mike Kravetz
2022-08-17 2:31 ` Muchun Song
2022-08-17 2:39 ` Miaohe Lin
2022-08-16 13:05 ` [PATCH 3/6] mm/hugetlb: fix missing call to restore_reserve_on_error() Miaohe Lin
2022-08-16 23:31 ` Mike Kravetz
2022-08-17 1:59 ` Miaohe Lin
2022-08-16 13:05 ` [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before set_pte_at() Miaohe Lin
2022-08-17 2:53 ` Muchun Song
2022-08-17 8:41 ` Miaohe Lin
2022-08-17 9:13 ` Yin, Fengwei
2022-08-17 11:21 ` Muchun Song
2022-08-18 1:14 ` Yin, Fengwei
2022-08-18 1:55 ` Miaohe Lin
2022-08-18 2:00 ` Yin, Fengwei
2022-08-18 2:47 ` Muchun Song
2022-08-18 7:52 ` Miaohe Lin
2022-08-18 7:59 ` Muchun Song
2022-08-18 8:32 ` Yin, Fengwei
2022-08-18 8:40 ` Muchun Song
2022-08-18 8:54 ` Yin, Fengwei [this message]
2022-08-18 9:18 ` Muchun Song
2022-08-18 12:58 ` Miaohe Lin
2022-08-18 23:53 ` Yin, Fengwei
2022-08-19 3:19 ` Muchun Song
2022-08-19 7:26 ` Miaohe Lin
2022-08-18 1:15 ` Yin, Fengwei
2022-08-20 8:12 ` Muchun Song
2022-08-22 8:45 ` Miaohe Lin
2022-08-22 10:23 ` Muchun Song
2022-08-23 1:42 ` Miaohe Lin
2022-08-16 13:05 ` [PATCH 5/6] mm/hugetlb: fix sysfs group leak in hugetlb_unregister_node() Miaohe Lin
2022-08-17 9:41 ` Yin, Fengwei
2022-08-18 1:00 ` Yin, Fengwei
2022-08-18 1:12 ` Yin, Fengwei
2022-08-16 13:05 ` [PATCH 6/6] mm/hugetlb: make detecting shared pte more reliable Miaohe Lin
2022-08-17 23:56 ` Mike Kravetz
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