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.