linux-kernel - Re: [PATCH v2] memcg: introduce non-blocking limit setting option

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Message-ID: <20250422-synergie-bauabschnitt-5f724f1d9866@brauner>
Date: Tue, 22 Apr 2025 11:48:23 +0200
From: Christian Brauner <brauner@...nel.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, 
	Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Yosry Ahmed <yosry.ahmed@...ux.dev>, Tejun Heo <tj@...nel.org>, 
	Greg Thelen <gthelen@...gle.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH v2] memcg: introduce non-blocking limit setting option

On Tue, Apr 22, 2025 at 11:31:23AM +0200, Michal Koutný wrote:
> On Tue, Apr 22, 2025 at 11:23:17AM +0200, Christian Brauner <brauner@...nel.org> wrote:
> > As written this isn't restricted to admin processes though, no? So any
> > unprivileged container can open that file O_NONBLOCK and avoid
> > synchronous reclaim?
> > 
> > Which might be fine I have no idea but it's something to explicitly
> > point out 
> 
> It occurred to me as well but I think this is fine -- changing the
> limits of a container is (should be) a privileged operation already
> (ensured by file permissions at opening).
> IOW, this doesn't allow bypassing the limits to anyone who couldn't have
> been able to change them already.

Hm, can you explain what you mean by a privileged operation here? If I
have nested containers with user namespaces with delegated cgroup tress,
i.e., chowned to them and then some PID 1 or privileged container
_within the user namespace_ lowers the limit and uses O_NONBLOCK then it
won't trigger synchronous reclaim. Again, this might all be fine I'm
just trying to understand.