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

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Message-ID: <87zf98xq20.fsf@linux.dev>
Date: Thu, 30 Oct 2025 10:22:31 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Song Liu <song@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
  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@...ck.org,
  cgroups@...r.kernel.org,  bpf@...r.kernel.org,  Martin KaFai Lau
 <martin.lau@...nel.org>,  Kumar Kartikeya Dwivedi <memxor@...il.com>,
  Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops
 to cgroups

Song Liu <song@...nel.org> writes:

> On Mon, Oct 27, 2025 at 4:17 PM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> [...]
>>  struct bpf_struct_ops_value {
>>         struct bpf_struct_ops_common_value common;
>> @@ -1359,6 +1360,18 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
>>         }
>>         bpf_link_init(&link->link, BPF_LINK_TYPE_STRUCT_OPS, &bpf_struct_ops_map_lops, NULL,
>>                       attr->link_create.attach_type);
>> +#ifdef CONFIG_CGROUPS
>> +       if (attr->link_create.cgroup.relative_fd) {
>> +               struct cgroup *cgrp;
>> +
>> +               cgrp = cgroup_get_from_fd(attr->link_create.cgroup.relative_fd);
>
> We should use "target_fd" here, not relative_fd.
>
> Also, 0 is a valid fd, so we cannot use target_fd == 0 to attach to
> global memcg.

Yep, but then we need somehow signal there is a cgroup fd passed,
so that struct ops'es which are not attached to cgroups keep working
as previously. And we can't use link_create.attach_type.

Should I use link_create.flags? E.g. something like add new flag

@@ -1224,6 +1224,7 @@ enum bpf_perf_event_type {
 #define BPF_F_AFTER		(1U << 4)
 #define BPF_F_ID		(1U << 5)
 #define BPF_F_PREORDER		(1U << 6)
+#define BPF_F_CGROUP		(1U << 7)
 #define BPF_F_LINK		BPF_F_LINK /* 1 << 13 */
 
 /* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the

and then do something like this:

int bpf_struct_ops_link_create(union bpf_attr *attr)
{
	<...>
	if (attr->link_create.flags & BPF_F_CGROUP) {
		struct cgroup *cgrp;

		cgrp = cgroup_get_from_fd(attr->link_create.target_fd);
		if (IS_ERR(cgrp)) {
			err = PTR_ERR(cgrp);
			goto err_out;
		}

		link->cgroup_id = cgroup_id(cgrp);
		cgroup_put(cgrp);
	}

Does it sound right?

Thanks