From: Michal Hocko <mhocko@suse.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Cgroups <cgroups@vger.kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page
Date: Fri, 5 Feb 2021 11:32:24 +0100 [thread overview]
Message-ID: <YB0euLiMU+T/9bMK@dhcp22.suse.cz> (raw)
In-Reply-To: <CAMZfGtVhBrwgkJVwiah6eDsppSf8fYp+uZ=tZmHBLDFeTmQX3w@mail.gmail.com>
On Fri 05-02-21 17:14:30, Muchun Song wrote:
> On Fri, Feb 5, 2021 at 4:36 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Fri 05-02-21 14:27:19, Muchun Song wrote:
> > > The get_mem_cgroup_from_page() is called under page lock, so the page
> > > memcg cannot be changed under us.
> >
> > Where is the page lock enforced?
>
> Because it is called from alloc_page_buffers(). This path is under
> page lock.
I do not see any page lock enforecement there. There is not even a
comment requiring that. Can we grow more users where this is not the
case? There is no actual relation between alloc_page_buffers and
get_mem_cgroup_from_page except that the former is the only _current_
existing user. I would be careful to dictate locking based solely on
that.
> > > 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.
> > > If we really want to make the get_mem_cgroup_from_page() suitable for
> > > arbitrary page, we should use page_memcg_rcu() instead of page_memcg()
> > > and call it after rcu_read_lock().
> >
> > What is the primary motivation to change this code? is the overhead of
> > tryget/RCU something that needs optimizing?
>
> Actually, the rcu_read_lock() is not necessary here. So it is better to
> remove it (indeed reduce some code).
Please state your reasoning in the changelog including benefits we get
from it.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-02-05 10:36 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-05 6:27 [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page Muchun Song
2021-02-05 8:36 ` Michal Hocko
2021-02-05 9:14 ` [External] " Muchun Song
2021-02-05 10:32 ` Michal Hocko [this message]
2021-02-05 12:56 ` Muchun Song
2021-02-05 15:59 ` Shakeel Butt
2021-02-05 16:06 ` Michal Hocko
2021-02-05 18:15 ` Johannes Weiner
2021-02-05 18:59 ` Shakeel Butt
2021-02-06 4:34 ` Muchun Song
2021-02-08 9:50 ` Michal Hocko
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