Re: [PATCH 1/3] mm, numa: rework do_pages_move

[Zi Yan]

On 29 Jan 2018, at 17:35, Andrew Morton wrote:

> On Mon, 29 Jan 2018 17:06:14 -0500 "Zi Yan" <zi.yan@xxxxxxxxxxxxxx> 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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