Re: [RFC PATCH bpf-next 6/9] cgroup: add v1 support to cgroup_get_from_id()

From: Yosry Ahmed <yosryahmed@google.com>
To: Tejun Heo <tj@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
	Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Shuah Khan <shuah@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Michal Hocko <mhocko@kernel.org>,
	Stanislav Fomichev <sdf@google.com>,
	David Rientjes <rientjes@google.com>,
	Greg Thelen <gthelen@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	cgroups@vger.kernel.org
Subject: Re: [RFC PATCH bpf-next 6/9] cgroup: add v1 support to cgroup_get_from_id()
Date: Tue, 10 May 2022 11:36:17 -0700	[thread overview]
Message-ID: <CAJD7tkb_fP=qTQRR7Os1UXSqFQvCEX+GYA9QHvbcoXyW1Kq48Q@mail.gmail.com> (raw)
In-Reply-To: <YnqwFuhncWiR3rjq@slm.duckdns.org>

On Tue, May 10, 2022 at 11:34 AM Tejun Heo <tj@kernel.org> wrote:
>
> On Tue, May 10, 2022 at 12:18:04AM +0000, Yosry Ahmed wrote:
> > The current implementation of cgroup_get_from_id() only searches the
> > default hierarchy for the given id. Make it compatible with cgroup v1 by
> > looking through all the roots instead.
> >
> > cgrp_dfl_root should be the first element in the list so there shouldn't
> > be a performance impact for cgroup v2 users (in the case of a valid id).
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >  kernel/cgroup/cgroup.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index af703cfcb9d2..12700cd21973 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -5970,10 +5970,16 @@ void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
> >   */
> >  struct cgroup *cgroup_get_from_id(u64 id)
> >  {
> > -     struct kernfs_node *kn;
> > +     struct kernfs_node *kn = NULL;
> >       struct cgroup *cgrp = NULL;
> > +     struct cgroup_root *root;
> > +
> > +     for_each_root(root) {
> > +             kn = kernfs_find_and_get_node_by_id(root->kf_root, id);
> > +             if (kn)
> > +                     break;
> > +     }
>
> I can't see how this can work. You're smashing together separate namespaces
> and the same IDs can exist across multiple of these hierarchies. You'd need
> a bigger surgery to make this work for cgroup1 which would prolly involve
> complications around 32bit ino's and file handle support too, which I'm not
> likely to ack, so please give it up on adding these things to cgroup1.
>
> Nacked-by: Tejun Heo <tj@kernel.org>
>
> Thanks.

Completely understandable. I sent this patch knowing that it likely
will not be accepted, with hopes of hearing feedback on whether this
can be done in a simple way or not. Looks like I got my answer, so
thanks for the info!

Will drop this patch in the incoming versions.

>
> --
> tejun