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

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Message-ID: <9c791de0-b702-1bbe-38a4-30e87d9d1b95@intel.com>
Date:   Thu, 18 Aug 2022 10:00:47 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>,
        Muchun Song <muchun.song@...ux.dev>
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 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.


Regards
Yin, Fengwei