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Subject: Re: [Patch v3] mm: thp: grab the lock before manipulation defer list
To:     David Rientjes <rientjes@google.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Wei Yang <richardw.yang@linux.intel.com>, hannes@cmpxchg.org,
        vdavydov.dev@gmail.com, akpm@linux-foundation.org,
        kirill.shutemov@linux.intel.com, yang.shi@linux.alibaba.com,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, alexander.duyck@gmail.com,
        stable@vger.kernel.org
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <11ba0af7-c2b2-83f9-ac55-7793cedb8028@virtuozzo.com>
Date:   Fri, 17 Jan 2020 12:42:05 +0300
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On 17.01.2020 12:32, David Rientjes wrote:
> On Fri, 17 Jan 2020, Kirill Tkhai wrote:
> 
>>>> I think that's a good point, especially considering that the current code 
>>>> appears to unconditionally place any compound page on the deferred split 
>>>> queue of the destination memcg.  The correct list that it should appear 
>>>> on, I believe, depends on whether the pmd has been split for the process 
>>>> being moved: note the MC_TARGET_PAGE caveat in 
>>>> mem_cgroup_move_charge_pte_range() that does not move the charge for 
>>>> compound pages with split pmds.  So when mem_cgroup_move_account() is 
>>>> called with compound == true, we're moving the charge of the entire 
>>>> compound page: why would it appear on that memcg's deferred split queue?
>>>
>>> I believe Kirill asked how do we know that the page should be actually
>>> added to the deferred list just from the list_empty check. In other
>>> words what if the page hasn't been split at all?
>>
>> Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
>> huge page to the deferred list, while we need to do that only for pages,
>> which are queued for splitting...
>>
> 
> Yup, and that appears broken before Wei's patch.  Since we only migrate 
> charges of entire compound pages (we have a mapping pmd, the underlying 
> page cannot be split), it should not appear on the deferred split queue 
> for any memcg, right?

Hm. Can't a huge page be mapped in two tasks:

1)the first task unmapped a part of page and initiated splitting,
2)the second task still refers the whole page,

then we move account for the second task?