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Date:   Fri, 17 Jan 2020 01:32:58 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.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
Subject: Re: [Patch v3] mm: thp: grab the lock before manipulation defer
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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?