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

[Shakeel Butt]

Hi Johannes,

On Thu, Feb 20, 2020 at 1:03 PM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxx> 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