linux-kernel - Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from

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Message-ID: <YB1s+puR+zPnkIFe@dhcp22.suse.cz>
Date:   Fri, 5 Feb 2021 17:06:18 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <guro@...com>,
        Muchun Song <songmuchun@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from
 get_mem_cgroup_from_page

On Fri 05-02-21 07:59:06, Shakeel Butt wrote:
> +Cc Roman
> 
> On Fri, Feb 5, 2021 at 2:49 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> [snip]
> > > > > Also, css_get is enough because page
> > > > > has a reference to the memcg.
> > > >
> > > > tryget used to be there to guard against offlined memcg but we have
> > > > concluded this is impossible in this path. tryget stayed there to catch
> > > > some unexpected cases IIRC.
> > >
> > > Yeah, it can catch some unexpected cases. But why is this path
> > > special so that we need a tryget?
> >
> > I do not remember details and the changelog of that change is not
> > explicit but I suspect it was just because this one could trigger as
> > there are external callers to memcg. Is this protection needed? I am not
> > sure, this is for you to justify if you want to remove it.
> >
> 
> It used to be css_tryget_online() which was changed to css_tryget()
> and from the discussion at [1], it seemed css_get() would be enough
> but we took a safer route.
> 
> Anyways, I think we can either take the page_memcg_rcu() route or put
> explicit restrictions with page lock or lock_page_memcg() to guarantee
> page and memcg binding. I don't have a strong opinion either way but I
> think removing restrictions in future for new use-cases will be much
> harder, so, page_memcg_rcu() approach seems more appropriate at least
> for now.

Yeah, I would like to not have very special locking requirements here.
Definitely not page_lock as that one is too overloaded already.

> 
> [1] https://lore.kernel.org/linux-mm/CALvZod5pAv=u8L2Tgk0hDY7XAiiF2dvjC1omQ5BSfzFu_2zSXA@mail.gmail.com/

-- 
Michal Hocko
SUSE Labs