linux-kernel - Re: bpf_st_ops and cgroups. Was: [PATCH v2 02/23] bpf: initial support for attaching struct ops to cgroups

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Message-ID: <CAADnVQLHT7DrqwNb_N_==vxCdtX3QvTyZKxZa4STw4cD-WKswQ@mail.gmail.com>
Date: Fri, 31 Oct 2025 10:37:19 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Amery Hung <ameryhung@...il.com>, Song Liu <song@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Andrii Nakryiko <andrii@...nel.org>, JP Kobryn <inwardvessel@...il.com>, 
	linux-mm <linux-mm@...ck.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...nel.org>, Kumar Kartikeya Dwivedi <memxor@...il.com>, Tejun Heo <tj@...nel.org>
Subject: Re: bpf_st_ops and cgroups. Was: [PATCH v2 02/23] bpf: initial
 support for attaching struct ops to cgroups

On Thu, Oct 30, 2025 at 4:24 PM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Thu, Oct 30, 2025 at 12:06 PM Roman Gushchin
> > <roman.gushchin@...ux.dev> wrote:
> >>
> >> Ok, let me summarize the options we discussed here:
> >>
> >> 1) Make the attachment details (e.g. cgroup_id) the part of struct ops
> >> itself. The attachment is happening at the reg() time.
> >>
> >>   +: It's convenient for complex stateful struct ops'es, because a
> >>       single entity represents a combination of code and data.
> >>   -: No way to attach a single struct ops to multiple entities.
> >>
> >> This approach is used by Tejun for per-cgroup sched_ext prototype.
> >
> > It's wrong. It should adopt bpf_struct_ops_link_create() approach
> > and use attr->link_create.cgroup.relative_fd to attach.
>
> This is basically what I have in v2, but Andrii and Song suggested that
> I should use attr->link_create.target_fd instead.

Yes. Of course.
link_create.cgroup.relative_fd actually points to a program.
We will need it if/when we add support for mprog style attach.

> I have a slight preference towards attr->link_create.cgroup.relative_fd
> because it makes it clear that fd is a cgroup fd and potentially opens
> a possibility to e.g. attach struct_ops to individual tasks and
> cgroups, but I'm fine with both options.

yeah. The name is confusing. It's not a cgroup fd.

> Also, as Song pointed out, fd==0 is in theory a valid target, so instead of
> using the "if (fd) {...}" check we might need a new flag. Idk if it
> really makes sense to complicate the code for it.

One option is to cgroup_get_from_fd(attr->link_create.target_fd)
and if it's not a cgroup, just ignore it in bpf_struct_ops_link_create()
But a new flag like BPF_F_CGROUP_FD maybe cleaner ?
If we ever attach st_ops to tasks there will be another BPF_F_PID_FD flag ?
Or we may try different supported kinds like bpf_fd_probe_obj() does
and don't bother with flags.

New attach_type-s are not necessary. The type of st_ops itself
reflects the purpose and hook location.