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Date:   Mon, 20 Jan 2020 22:27:26 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     David Rientjes <rientjes@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Wei Yang <richardw.yang@linux.intel.com>, hannes@cmpxchg.org,
        vdavydov.dev@gmail.com, ktkhai@virtuozzo.com,
        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 v4] mm: thp: remove the defer list related code since
 this will not happen
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On Mon 20-01-20 13:10:56, David Rientjes wrote:
> On Mon, 20 Jan 2020, Michal Hocko wrote:
> 
> > > When migrating memcg charges of thp memory, there are two possibilities:
> > > 
> > >  (1) The underlying compound page is mapped by a pmd and thus does is not 
> > >      on a deferred split queue (it's mapped), or
> > > 
> > >  (2) The compound page is not mapped by a pmd and is awaiting split on a
> > >      deferred split queue.
> > > 
> > > The current charge migration implementation does *not* migrate charges for 
> > > thp memory on the deferred split queue, it only migrates charges for pages 
> > > that are mapped by a pmd.
> > > 
> > > Thus, to migrate charges, the underlying compound page cannot be on a 
> > > deferred split queue; no list manipulation needs to be done in 
> > > mem_cgroup_move_account().
> > > 
> > > With the current code, the underlying compound page is moved to the 
> > > deferred split queue of the memcg its memory is not charged to, so 
> > > susbequent reclaim will consider these pages for the wrong memcg.  Remove 
> > > the deferred split queue handling in mem_cgroup_move_account() entirely.
> > 
> > I believe this still doesn't describe the underlying problem to the full
> > extent. What happens with the page on the deferred list when it
> > shouldn't be there in fact? Unless I am missing something deferred_split_scan
> > will simply split that huge page. Which is a bit unfortunate but nothing
> > really critical. This should be mentioned in the changelog.
> > 
> 
> Are you referring to a compound page on the deferred split queue before a 
> task is moved?  I'm not sure this is within the scope of Wei's patch.. 
> this is simply preventing a page from being moved to the deferred split
> queue of a memcg that it is not charged to.  Is there a concern about why 
> this code can be removed or a suggestion on something else it should be 
> doing instead?

No, I do not have any concern about the patch itslef. It is that the
changelog doesn't decribe the user visible effect. All I am saying is
that the current code splits THPs of moved pages under memory pressure
even if that is not needed. And that is a clear bug.
-- 
Michal Hocko
SUSE Labs