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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>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        "Vlastimil Babka" <vbabka@suse.cz>,
        "Andrea Reale" <ar@linux.vnet.ibm.com>,
        "Anshuman Khandual" <khandual@linux.vnet.ibm.com>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
        "Michal Hocko" <mhocko@suse.com>
Subject: Re: [PATCH 1/3] mm, numa: rework do_pages_move
Message-Id: <20180129154802.594025d081789b6620f001bd@linux-foundation.org>
In-Reply-To: <07425013-A7A9-4BB8-8FAA-9581D966A29B@cs.rutgers.edu>
References: <20180103082555.14592-1-mhocko@kernel.org>
        <20180103082555.14592-2-mhocko@kernel.org>
        <8ECFD324-D8A0-47DC-A6FD-B9F7D29445DC@cs.rutgers.edu>
        <20180129143522.68a5332ae80d28461441a6be@linux-foundation.org>
        <07425013-A7A9-4BB8-8FAA-9581D966A29B@cs.rutgers.edu>
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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.