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    SubjectRe: [PATCH] Lock mmap_sem when calling migrate_pages() in do_move_pages_to_node()
    On Tue 30-01-18 10:52:58, Zi Yan wrote:
    >
    >
    > Michal Hocko wrote:
    > > On Mon 29-01-18 22:00:11, Zi Yan wrote:
    > >> From: Zi Yan <zi.yan@cs.rutgers.edu>
    > >>
    > >> migrate_pages() requires at least down_read(mmap_sem) to protect
    > >> related page tables and VMAs from changing. Let's do it in
    > >> do_page_moves() for both do_move_pages_to_node() and
    > >> add_page_for_migration().
    > >>
    > >> Also add this lock requirement in the comment of migrate_pages().
    > >
    > > This doesn't make much sense to me, to be honest. We are holding
    > > mmap_sem for _read_ so we allow parallel updates like page faults
    > > or unmaps. Therefore we are isolating pages prior to the migration.
    > >
    > > The sole purpose of the mmap_sem in add_page_for_migration is to protect
    > > from vma going away _while_ need it to get the proper page.
    >
    > Then, I am wondering why we are holding mmap_sem when calling
    > migrate_pages() in existing code.
    > http://elixir.free-electrons.com/linux/latest/source/mm/migrate.c#L1576

    You mean in the original code? I strongly suspect this was to not take
    it for each page.
    --
    Michal Hocko
    SUSE Labs

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