Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

Wei Yang
2020-01-13T00:44:57Z

On Mon, Jan 13, 2020 at 01:57:18AM +0300, Kirill A. Shutemov wrote:
>On Sun, Jan 12, 2020 at 10:28:58AM +0800, Wei Yang wrote:
>> On Sat, Jan 11, 2020 at 03:03:52AM +0300, Kirill A. Shutemov wrote:
>> >On Thu, Jan 09, 2020 at 10:30:54PM +0800, Wei Yang wrote:
>> >> As all the other places, we grab the lock before manipulate the defer list.
>> >> Current implementation may face a race condition.
>> >>
>> >> For example, the potential race would be:
>> >>
>> >> CPU1 CPU2
>> >> mem_cgroup_move_account split_huge_page_to_list
>> >> !list_empty
>> >> lock
>> >> !list_empty
>> >> list_del
>> >> unlock
>> >> lock
>> >> # !list_empty might not hold anymore
>> >> list_del_init
>> >> unlock
>> >
>> >I don't think this particular race is possible. Both parties take page
>> >lock before messing with deferred queue, but anytway:
>>
>> I am afraid not. Page lock is per page, while defer queue is per pgdate or
>> memcg.
>>
>> It is possible two page in the same pgdate or memcg grab page lock
>> respectively and then access the same defer queue concurrently.
>
>Look closer on the list_empty() argument. It's list_head local to the
>page. Too different pages can be handled in parallel without any problem
>in this particular scenario. As long as we as we modify it under the lock.
>
>Said that, page lock here was somewhat accidential and I still belive we
>need to move the check under the lock anyway.
>

If my understanding is correct, you agree with my statement?

>--
> Kirill A. Shutemov

--
Wei Yang
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