linux-kernel - Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YHbAzL9ls6EBsB3L@dhcp22.suse.cz>
Date:   Wed, 14 Apr 2021 12:15:40 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     guro@...com, hannes@...xchg.org, akpm@...ux-foundation.org,
        shakeelb@...gle.com, vdavydov.dev@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, fam.zheng@...edance.com
Subject: Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when
 !mm in get_mem_cgroup_from_mm

On Wed 14-04-21 18:04:35, Muchun Song wrote:
> On Wed, Apr 14, 2021 at 5:24 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Tue 13-04-21 14:51:48, Muchun Song wrote:
> > > When mm is NULL, we do not need to hold rcu lock and call css_tryget for
> > > the root memcg. And we also do not need to check !mm in every loop of
> > > while. So bail out early when !mm.
> >
> > mem_cgroup_charge and other callers unconditionally drop the reference
> > so how come this does not underflow reference count?
> 
> For the root memcg, the CSS_NO_REF flag is set, so css_get
> and css_put do not get or put reference.

Ohh, right you are. I must have forgot about that special case. I am
pretty sure I (and likely few more) will stumble over that in the future
again. A small comment explaining that the reference can be safely
ignore would be helpful.

Anyway
Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

-- 
Michal Hocko
SUSE Labs