[PATCH 3/6] cgroup: introduce cgroup_taskset and use it in subsys->can_attach(), cancel_attach() and attach()

[KAMEZAWA Hiroyuki]

On Thu, 25 Aug 2011 10:40:06 +0200
Tejun Heo <tj at kernel.org> wrote:

> Hello,
> 
> On Thu, Aug 25, 2011 at 10:21 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> >> Hmmm... the above is basically identity transformation of the existing
> >> code.  If the above is broken, the current code is broken too.  Is it?
> >>
> > Current code is not broken.
> 
> Trust me.  If the posted code is broken, the current code is too. It
> is an IDENTITY transformation.
> 
> > mem_cgroup_can_attach(....., task) need to do real job only when task->mm->owner
> > == task. In this modification, you pass a set of task at once.
> 
> Before the change, cgroup would migrate multiple tasks all the same
> but memcg wouldn't have noticed it unless it opted in explicitly using
> [can_]attach_task(). When multiple tasks were moving, [can_]attach()
> would only be called with the leader whether the leader actually is
> changing cgroup or not. The interface sucked and it wasn't properly
> documented but that's what was happening. The interface change is just
> making the breakage obvious - +1 for the new interface. :)
> 
Thank you for clarification. Ok, current code is broken.


> > So, mem_cgroup_can_attach() need to check _all_ tasks in tset rather than a
> > first task in tset. please scan and find mm->owner.
> >
> > Anyway, if you merge this onto mm-tree first, I think I can have time to
> > write a fix up if complicated.
> 
> As for specific merging order, it shouldn't matter all that much but
> if you wanna backport fixes for -stable, maybe it would make more
> sense to sequence the fix before this change.
> 
> Thank you.

Sure. IIUC, the case thread_leader != mm->owner is uncommon. 
I'll consider a fix onto your fix, first.

I'll cosinder a fix for stable tree if someone requests.

Thanks,
-Kame