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

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