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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: <9c791de0-b702-1bbe-38a4-30e87d9d1b95@intel.com>
Date:   Thu, 18 Aug 2022 10:47:19 +0800
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
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Message-Id: <931536E2-3948-40AB-88A7-E36F67954AAA@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>
To:     "Yin, Fengwei" <fengwei.yin@intel.com>
Precedence: bulk


> 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

I have the same question. So I think the example proposed by Miaohe is a =
little
difference from the case (hugetlb_vmemmap) here.

>=20
> Regards
> Yin, Fengwei
>=20
>=20