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

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Message-Id: <F8381885-8B9F-405B-A860-AFB8A675D5A7@linux.dev>
Date:   Mon, 22 Aug 2022 18:23:27 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Miaohe Lin <linmiaohe@...wei.com>
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 Aug 22, 2022, at 16:45, Miaohe Lin <linmiaohe@...wei.com> wrote:
> 
> On 2022/8/20 16:12, Muchun Song wrote:
>> 
>> 
>>> On Aug 16, 2022, at 21:05, Miaohe Lin <linmiaohe@...wei.com> wrote:
>>> 
>>> The memory barrier smp_wmb() is needed to make sure that preceding stores
>>> to the page contents become visible before the below set_pte_at() write.
>> 
>> I found another place where is a similar case. See kasan_populate_vmalloc_pte() in
>> mm/kasan/shadow.c. 
> 
> Thanks for your report.
> 
>> 
>> Should we fix it as well?
> 
> I'm not familiar with kasan yet, but I think memory barrier is needed here or memory corrupt
> can't be detected until the contents are visible. smp_mb__after_atomic before set_pte_at should
> be enough? What's your opinion?

I didn’t see any atomic operation between set_pte_at() and memset(), I don’t think
smp_mb__after_atomic() is feasible if we really need to insert a barrier. I suggest
you to send a RFC patch to KASAN maintainers, they are more familiar with this than
us.

Thanks.

> 
> Thanks,
> Miaohe Lin
> 
>