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Date:   Fri, 1 Jun 2018 09:50:34 -0700
From:   Tejun Heo <tj@kernel.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>, peterz@infradead.org,
        viro@zeniv.linux.org.uk, mingo@kernel.org,
        paulmck@linux.vnet.ibm.com, keescook@chromium.org, riel@redhat.com,
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        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC][PATCH 1/2] memcg: Ensure every task that uses an mm is in
 the same memory cgroup
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Hello, Eric.

On Fri, Jun 01, 2018 at 09:53:09AM -0500, Eric W. Biederman wrote:
> 
> From a userspace perspective the cgroup of a mm is determined by which
> the cgroup the task belongs too.  As practically an mm can only belong to
> a single memory cgroup having multiple tasks with the same mm in different
> memory cgroups is not well defined.
> 
> Avoid the difficulties of dealing with crazy semantics and restrict all
> tasks that share a single mm to the same memory cgroup.
> 
> This is accomplished by adding a new function mem_cgroup_mm_can_attach
> that checks this condition with a straight forward algorithm.  In the worst
> case it is O(N^2).  In the common case it should be O(N) in the number of
> tasks being migrated.  As typically only a single process and thus a single
> process is being migrated and it is optimized for that case.
> 
> There are users of mmget such as proc that can create references to an
> mm this function can not find.  This results in an unnecessary
> migration failure.  It does not break the invariant that every task of
> an mm stays in the same memory cgroup.  So this condition is annoying
> but harmelss.
> 
> This requires multi-threaded mm's to be migrated using the procs file.
> 
> This effectively forbids process with mm's shared processes being migrated.
> Although enabling the control file might work.

So, I don't think we need to support putting tasks which share a mm in
different cgroups.  That said, if we're gonna put in that restriction,
I think it should be in cgroup core rather than memcg can_attach.  The
only thing we'd need to do is widening what cgroup migration considers
to be a process.

Thanks.

-- 
tejun