From: Michal Hocko <mhocko@kernel.org>
To: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: Zi Yan <zi.yan@sent.com>,
Andrew Morton <akpm@linux-foundation.org>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] Lock mmap_sem when calling migrate_pages() in do_move_pages_to_node()
Date: Tue, 30 Jan 2018 17:10:25 +0100 [thread overview]
Message-ID: <20180130161025.GH21609@dhcp22.suse.cz> (raw)
In-Reply-To: <5A7094DA.4000804@cs.rutgers.edu>
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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next prev parent reply other threads:[~2018-01-30 16:10 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-30 3:00 Zi Yan
2018-01-30 4:10 ` Hugh Dickins
2018-01-30 15:56 ` Zi Yan
2018-01-30 8:14 ` Michal Hocko
2018-01-30 15:52 ` Zi Yan
2018-01-30 16:10 ` Michal Hocko [this message]
2018-01-30 19:12 ` Zi Yan
2018-01-31 8:01 ` Michal Hocko
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