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Date: Mon, 29 Jan 2018 15:48:02 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "Zi Yan" <zi.yan@cs.rutgers.edu>
Cc: "Michal Hocko" <mhocko@kernel.org>,
        "Naoya Horiguchi" <n-horiguchi@ah.jp.nec.com>,
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        LKML <linux-kernel@vger.kernel.org>, "Michal Hocko" <mhocko@suse.com>
Subject: Re: [PATCH 1/3] mm, numa: rework do_pages_move
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On Mon, 29 Jan 2018 18:39:01 -0500 "Zi Yan" <zi.yan@cs.rutgers.edu> wrote:

> On 29 Jan 2018, at 17:35, Andrew Morton wrote:
> 
> > On Mon, 29 Jan 2018 17:06:14 -0500 "Zi Yan" <zi.yan@cs.rutgers.edu> wrote:
> >
> >> I discover that this patch does not hold mmap_sem while migrating pages in
> >> do_move_pages_to_node().
> >>
> >> A simple fix below moves mmap_sem from add_page_for_migration()
> >> to the outmost do_pages_move():
> >
> > I'm not surprised.  Why does do_move_pages_to_node() need mmap_sem
> > and how is a reader to discover that fact???
> 
> do_move_pages_to_node() calls migrate_pages(), which requires down_read(&mmap_sem).
> 
> In the outmost do_pages_move(), both add_page_for_migration() and
> do_move_pages_to_node() inside it need to hold read lock of mmap_sem.
> 
> Do we need to add comments for both functions?

Just for migrate_pages(), I guess.  Let's include a description of
*why* mmap_sem is needed.  What it is protecting.