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

Wei Yang
2020-01-15T01:19:27Z

On Tue, Jan 14, 2020 at 12:57:22PM -0800, David Rientjes wrote:
>On Tue, 14 Jan 2020, Kirill A. Shutemov wrote:
>
>> split_huge_page_to_list() has page lock taken.
>>
>> free_transhuge_page() is in the free path and doesn't susceptible to the
>> race.
>>
>> deferred_split_scan() is trickier. list_move() should be safe against
>> list_empty() as it will not produce false-positive list_empty().
>> list_del_init() *should* (correct me if I'm wrong) be safe because the page
>> is freeing and memcg will not touch the page anymore.
>>
>> deferred_split_huge_page() is a problematic one. It called from
>> page_remove_rmap() path witch does require page lock. I don't see any
>> obvious way to exclude race with mem_cgroup_move_account() here.
>> Anybody else?
>>
>> Wei, could you rewrite the commit message with deferred_split_huge_page()
>> as a race source instead of split_huge_page_to_list()?
>>
>
>I think describing the race in terms of deferred_split_huge_page() makes
>the most sense and I'd prefer a cc to stable for 5.4+. Even getting the
>split_queue_len, which is unsigned long, to underflow because of a
>list_empty(page_deferred_list()) check that is no longer accurate after
>the lock is taken would be a significant issue for shrinkers.

Oh, you are right. Even the list is not corrupted between
deferred_split_scan() and mem_cgroup_move_account(), split_queue_len would be
in a wrong state.

Hmm... to some extend, the page lock complicates the picture a little here,
even it helps in some cases.

--
Wei Yang
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