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Date:   Tue, 14 Jan 2020 12:57:22 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
cc:     Michal Hocko <mhocko@kernel.org>,
        Wei Yang <richardw.yang@linux.intel.com>, hannes@cmpxchg.org,
        vdavydov.dev@gmail.com, akpm@linux-foundation.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com,
        yang.shi@linux.alibaba.com, alexander.duyck@gmail.com
Subject: Re: [Patch v2] mm: thp: grab the lock before manipulation defer
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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.