[Devel] Re: [PATCH v7 10/10] Disable task moving when using kernel memory accounting

[KAMEZAWA Hiroyuki]

On Mon, 5 Dec 2011 07:18:37 -0200
Glauber Costa <glommer at parallels.com> wrote:

> On 12/05/2011 12:18 AM, KAMEZAWA Hiroyuki wrote:
> > On Fri, 2 Dec 2011 16:11:56 -0200
> > Glauber Costa<glommer at parallels.com>  wrote:
> >
> >> On 11/30/2011 12:22 AM, KAMEZAWA Hiroyuki wrote:
> >>> On Tue, 29 Nov 2011 21:57:01 -0200
> >>> Glauber Costa<glommer at parallels.com>   wrote:
> >>>
> >>>> Since this code is still experimental, we are leaving the exact
> >>>> details of how to move tasks between cgroups when kernel memory
> >>>> accounting is used as future work.
> >>>>
> >>>> For now, we simply disallow movement if there are any pending
> >>>> accounted memory.
> >>>>
> >>>> Signed-off-by: Glauber Costa<glommer at parallels.com>
> >>>> CC: Hiroyouki Kamezawa<kamezawa.hiroyu at jp.fujitsu.com>
> >>>> ---
> >>>>    mm/memcontrol.c |   23 ++++++++++++++++++++++-
> >>>>    1 files changed, 22 insertions(+), 1 deletions(-)
> >>>>
> >>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >>>> index a31a278..dd9a6d9 100644
> >>>> --- a/mm/memcontrol.c
> >>>> +++ b/mm/memcontrol.c
> >>>> @@ -5453,10 +5453,19 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
> >>>>    {
> >>>>    	int ret = 0;
> >>>>    	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgroup);
> >>>> +	struct mem_cgroup *from = mem_cgroup_from_task(p);
> >>>> +
> >>>> +#if defined(CONFIG_CGROUP_MEM_RES_CTLR_KMEM)&&   defined(CONFIG_INET)
> >>>> +	if (from != memcg&&   !mem_cgroup_is_root(from)&&
> >>>> +	    res_counter_read_u64(&from->tcp_mem.tcp_memory_allocated, RES_USAGE)) {
> >>>> +		printk(KERN_WARNING "Can't move tasks between cgroups: "
> >>>> +			"Kernel memory held.\n");
> >>>> +		return 1;
> >>>> +	}
> >>>> +#endif
> >>>
> >>> I wonder....reading all codes again, this is incorrect check.
> >>>
> >>> Hm, let me cralify. IIUC, in old code, "prevent moving" is because you hold
> >>> reference count of cgroup, which can cause trouble at rmdir() as leaking refcnt.
> >> right.
> >>
> >>> BTW, because socket is a shared resource between cgroup, changes in mm->owner
> >>> may cause task cgroup moving implicitly. So, if you allow leak of resource
> >>> here, I guess... you can take mem_cgroup_get() refcnt which is memcg-local and
> >>> allow rmdir(). Then, this limitation may disappear.
> >>
> >> Sorry, I didn't fully understand. Can you clarify further?
> >> If the task is implicitly moved, it will end up calling can_attach as
> >> well, right?
> >>
> > I'm sorry that my explanation is bad.
> >
> > You can take memory cgroup itself's reference count by mem_cgroup_put/get.
> > By getting this, memory cgroup object will continue to exist even after
> > its struct cgroup* is freed by rmdir().
> >
> > So, assume you do mem_cgroup_get()/put at socket attaching/detatching.
> >
> > 0) A task has a tcp socekts in memcg0.
> >
> > task(memcg0)
> >   +- socket0 -->  memcg0,usage=4096
> >
> > 1) move this task to memcg1
> >
> > task(memcg1)
> >   +- socket0 -->  memcg0,usage=4096
> >
> > 2) The task create a new socket.
> >
> > task(memcg1)
> >   +- socekt0 -->  memcg0,usage=4096
> >   +- socket1 -->  memcg1,usage=xxxx
> >
> > Here, the task will hold 4096bytes of usage in memcg0 implicitly.
> >
> > 3) an admin removes memcg0
> > task(memcg1)
> >   +- socket0 -->memcg0, usage=4096<-----(*)
> >   +- socket1 -->memcg1, usage=xxxx
> >
> > (*) is invisible to users....but this will not be very big problem.
> >
> Hi Kame,
> 
> Thanks for the explanation.
> 
> Hummm, Do you think that by doing it, we get rid of the need of moving 
> sockets to another memcg when the task is moved? So in my original 
> patchset, if you recall, I wanted to keep a socket forever in the same 
> cgroup. I didn't, because then rmdir would be blocked.
> 
> By using this memcg reference trick, both can be achieved. What do you 
> think ?

I think so. Using mem_cgroup_put/get is a way. Could you try ?

Thanks,
-Kame