Re: [Patch v3] mm: thp: grab the lock before manipulation defer list

[Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

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?