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From:   "Zi Yan" <zi.yan@cs.rutgers.edu>
To:     "Andrew Morton" <akpm@linux-foundation.org>
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
Date:   Mon, 29 Jan 2018 18:39:01 -0500
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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?

--
Best Regards
Yan Zi

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