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Subject: Re: [PATCH 02/16] mm/huge_memory: access vm_page_prot with READ_ONCE
 in remove_migration_pmd
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
CC:     <akpm@linux-foundation.org>, <shy828301@gmail.com>,
        <willy@infradead.org>, <zokeefe@google.com>, <linux-mm@kvack.org>,
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References: <20220622170627.19786-1-linmiaohe@huawei.com>
 <20220622170627.19786-3-linmiaohe@huawei.com>
 <20220623031401.wdyt5ylin4aijzhh@box.shutemov.name>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <7ee4a597-d7cb-f387-5613-f51b17262745@huawei.com>
Date:   Thu, 23 Jun 2022 20:03:41 +0800
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On 2022/6/23 11:14, Kirill A. Shutemov wrote:
> 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 took a glance when I made this patch.

> 
> 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.
> 

This patch is inspired from the below commit
  6d2329f8872f ("mm: vm_page_prot: update with WRITE_ONCE/READ_ONCE")

It changed all the places that need to use READ_ONCE. But remove_migration_pmd
is missed due to it's introduced later. It looks fine to add a helper to read
vm_page_prot which does READ_ONCE() but READ_ONCE is unneeded while under the
mmap_lock, so might it be a little overkill to add a helper because the helper
is used iff mmap_lock is not held?

Thanks.