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Subject: Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before
 set_pte_at()
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <7be98c64-88a1-3bee-7f92-67bb1f4f495b@huawei.com>
Date:   Thu, 18 Aug 2022 15:59:43 +0800
Cc:     "Yin, Fengwei" <fengwei.yin@intel.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
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Message-Id: <3B1463C2-9DC4-43D0-93EC-2D2334A20502@linux.dev>
References: <20220816130553.31406-1-linmiaohe@huawei.com>
 <20220816130553.31406-5-linmiaohe@huawei.com>
 <D8C00BDA-160D-40CE-AFBD-9488F85E76CE@linux.dev>
 <ea67ab10-667e-f361-b80f-dafb13da4808@huawei.com>
 <0EAF1279-6A1C-41FA-9A32-414C36B3792A@linux.dev>
 <019c1272-9d01-9d51-91a0-2d656b25c318@intel.com>
 <18adbf89-473e-7ba6-9a2b-522e1592bdc6@huawei.com>
 <9c791de0-b702-1bbe-38a4-30e87d9d1b95@intel.com>
 <931536E2-3948-40AB-88A7-E36F67954AAA@linux.dev>
 <7be98c64-88a1-3bee-7f92-67bb1f4f495b@huawei.com>
To:     Miaohe Lin <linmiaohe@huawei.com>
Precedence: bulk


> On Aug 18, 2022, at 15:52, Miaohe Lin <linmiaohe@huawei.com> wrote:
>=20
> On 2022/8/18 10:47, Muchun Song wrote:
>>=20
>>=20
>>> On Aug 18, 2022, at 10:00, Yin, Fengwei <fengwei.yin@intel.com> =
wrote:
>>>=20
>>>=20
>>>=20
>>> 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=E2=80=99s
>>>>>> contents after they have saw PG_uptodate is set. I think commit =
0ed361dec369
>>>>>> can tell us more details.
>>>>>>=20
>>>>>> I also looked at commit 52f37629fd3c to see why we need a barrier =
before
>>>>>> set_pte_at(), but I didn=E2=80=99t find any info to explain why. =
I guess we want
>>>>>> to make sure the order between the page=E2=80=99s 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?
>>>=20
>>> For SetPageUptodate, there are paring write/read memory barrier.
>>>=20
>>=20
>> I have the same question. So I think the example proposed by Miaohe =
is a little
>> difference from the case (hugetlb_vmemmap) here.
>=20
> 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=E2=80=99s guarantee?

>=20
> Thanks,
> Miaohe Lin