linux-kernel - Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes help unified flushes

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: <ZQBFeHxzO49gv987@dhcp22.suse.cz>
Date:   Tue, 12 Sep 2023 13:03:20 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes
 help unified flushes

On Tue 05-09-23 08:54:46, Yosry Ahmed wrote:
> On Tue, Sep 5, 2023 at 7:10 AM Michal Koutný <mkoutny@...e.com> wrote:
> >
> > On Mon, Sep 04, 2023 at 05:41:10PM +0200, Michal Hocko <mhocko@...e.com> wrote:
> > > So it also creates an undocumented but userspace visible behavior.
> > > Something that userspace might start depending on, right?
> >
> > Yes but -
> > - depending on undocumented behavior is a mistake,
> > - breaking the dependency would manifest (in the case I imagine) as a
> >   performance regression (and if there are some users, the future can
> >   allow them configuring periodic kernel flush to compensate for that).
> 
> I think I am missing something. This change basically makes userspace
> readers (for the root memcg) help out unified flushers, which are
> in-kernel readers (e.g. reclaim) -- not the other way around.
> 
> How would that create a userspace visible behavior that a dependency
> can be formed on? Users expecting reclaim to be faster right after
> reading root stats? I would guess that would be too flaky to cause a
> behavior that people can depend on tbh.

Flaky or not, it might cause behavior difference and a subtle one. I can
imagine nohz and similar workloads wanting to (ab)use this to reduce
kernel footprint. If we really need this then make it obvious in the
changelog at least.

-- 
Michal Hocko
SUSE Labs