[Devel] Re: [PATCH 17/23] kmem controller charge/uncharge infrastructure

[David Rientjes]

On Tue, 24 Apr 2012, Glauber Costa wrote:

> > Yes, for user memory, I see charging to p->mm->owner as allowing that
> > process to eventually move and be charged to a different memcg and there's
> > no way to do proper accounting if the charge is split amongst different
> > memcgs because of thread membership to a set of memcgs.  This is
> > consistent with charges for shared memory being moved when a thread
> > mapping it moves to a new memcg, as well.
> 
> But that's the problem.
> 
> When we are dealing with kernel memory, we are allocating a whole slab page.
> It is essentially impossible to track, given a page, which task allocated
> which object.
> 

Right, so you have to make the distinction that slab charges cannot be 
migrated by memory.move_charge_at_immigrate (and it's not even specified 
to do anything beyond user pages in Documentation/cgroups/memory.txt), but 
it would be consistent to charge the same memcg for a process's slab 
allocations as the process's user allocations.

My response was why we shouldn't be charging user pages to 
mem_cgroup_from_task(current) rather than 
mem_cgroup_from_task(current->mm->owner) which is what is currently 
implemented.

If that can't be changed so that we can still migrate user memory amongst 
memcgs for memory.move_charge_at_immigrate, then it seems consistent to 
have all allocations done by a task to be charged to the same memcg.  
Hence, I suggested current->mm->owner for slab charging as well.