Re: [PATCH v3 2/3] mm: Charge active memcg when no mm is set

From: Shakeel Butt <shakeelb@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>,
	Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
	Li Zefan <lizefan@huawei.com>, Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>, Roman Gushchin <guro@fb.com>,
	Chris Down <chris@chrisdown.name>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"open list:BLOCK LAYER" <linux-block@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
	"open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" 
	<linux-mm@kvack.org>
Subject: Re: [PATCH v3 2/3] mm: Charge active memcg when no mm is set
Date: Thu, 20 Feb 2020 13:14:17 -0800	[thread overview]
Message-ID: <CALvZod5_AgaS_=uvXsfsL1bthxMUk3DiD90Ach=cdMkaync5vQ@mail.gmail.com> (raw)
In-Reply-To: <20200220210344.GK54486@cmpxchg.org>

Hi Johannes,

On Thu, Feb 20, 2020 at 1:03 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> Hey Shakeel!
>
> On Thu, Feb 20, 2020 at 10:14:45AM -0800, Shakeel Butt wrote:
> > On Thu, Feb 20, 2020 at 8:52 AM Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
> > >
> > > memalloc_use_memcg() worked for kernel allocations but was silently
> > > ignored for user pages.
> > >
> > > This patch establishes a precedence order for who gets charged:
> > >
> > > 1. If there is a memcg associated with the page already, that memcg is
> > >    charged. This happens during swapin.
> > >
> > > 2. If an explicit mm is passed, mm->memcg is charged. This happens
> > >    during page faults, which can be triggered in remote VMs (eg gup).
> > >
> > > 3. Otherwise consult the current process context. If it has configured
> > >    a current->active_memcg, use that.
> >
> > What if css_tryget_online(current->active_memcg) in
> > get_mem_cgroup_from_current() fails? Do we want to change this to
> > css_tryget() and even if that fails should we fallback to
> > root_mem_cgroup or current->mm->memcg?
>
> Good questions.
>
> I think we can switch to css_tryget(). If a cgroup goes offline
> between issuing the IO and the loop layer executing that IO, the
> resources used could end up in the root instead of the closest
> ancestor of the offlined group. However, the risk of that actually
> happening and causing problems is probably pretty small, and the
> behavior isn't really worse than before Dan's patches.

Agreed.

>
> Would you mind sending a separate patch for this? AFAICS similar
> concerns apply to all users of foreign charging.

Sure and yes similar concerns apply to other users as well.

>
> As for tryget failing: can that actually happen? AFAICS, all current
> users acquire a reference first (get_memcg_from_somewhere()) that they
> assign to current->active_memcg. We should probably codify this rule
> and do WARN_ON(!css_tryget()) /* current->active_memcg must hold a ref */

Yes, we should WARN_ON().

Shakeel