Re: [RFC][PATCH 1/2] memcg: Ensure every task that uses an mm is in the same memory cgroup

[Tejun Heo]

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