linux-kernel - Re: [PATCH 02/16] mm/huge_memory: access vm_page_prot with READ_ONCE in remove_migration

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Message-ID: <20220623031401.wdyt5ylin4aijzhh@box.shutemov.name>
Date:   Thu, 23 Jun 2022 06:14:01 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, shy828301@...il.com,
        willy@...radead.org, zokeefe@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/16] mm/huge_memory: access vm_page_prot with READ_ONCE
 in remove_migration_pmd

On Thu, Jun 23, 2022 at 01:06:13AM +0800, Miaohe Lin wrote:
> vma->vm_page_prot is read lockless from the rmap_walk, it may be updated
> concurrently. Using READ_ONCE to prevent the risk of reading intermediate
> values.

Have you checked all other vm_page_prot reads that they hold mmap_lock?

I think the right fix would be to provide a helper to read vm_page_prot
which does READ_ONCE() and use it everywhere. This seems more sustainable.

-- 
 Kirill A. Shutemov