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    SubjectRe: [PATCH 02/16] mm/huge_memory: access vm_page_prot with READ_ONCE in remove_migration_pmd
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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.

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