Re: [PATCH 1/2] block: add BPF kfuncs to read blkcg io.stat

[Ziyang Men <ziyang.meme@gmail.com>]

Hi Tejun,

Thanks for your review and suggestions!

On Mon, Aug 10, 2026 at 10:00:55AM -1000, Tejun Heo wrote:
>Hello,
>
>On Fri, Aug 07, 2026 at 12:37:31PM -0700, Ziyang Men wrote:
>> +__bpf_kfunc struct blkcg *bpf_get_root_blkcg(void)
>> +__bpf_kfunc struct blkcg *bpf_get_blkcg(struct cgroup_subsys_state *css)
>> +__bpf_kfunc void bpf_put_blkcg(struct blkcg *blkcg)
>> +__bpf_kfunc void bpf_blkcg_flush_stats(struct blkcg *blkcg)
>
>I wonder whether making the above deal with css's would make more sense so
>that we don't end up with per-subsystem iterators. If casting to per-subsys
>type is necessary, we can provide RCU protected cast kfucns.
>

Good idea! These do nothing block specific and we already have some common
helpers for it such as the bpf_get_root_blkcg(). What is missing is a way to get
one controller's css from a cgroup with a reference held, and the cast. So
I would add in kernel/cgroup/

   struct cgroup_subsys_state *bpf_cgroup_css(struct cgroup *cgrp, int ssid);
   void bpf_css_release(struct cgroup_subsys_state *css);

where the @ssid is an enum cgroup_subsys_id value. Afterwards, the only piece for
blkcg left is:
   struct blkcg *bpf_css_to_blkcg(struct cgroup_subsys_state *css);

Note the bpf_blkcg_flush_stats may keep its current shape as it mirrors the
blkcg_print_stat() which skips the root.

With these helpers we can also replace the existing ones in
mm/bpf_memcontrol.c as well:
	bpf_get_mem_cgroup()
	bpf_put_mem_cgroup()
	bpf_mem_cgroup_flush_stats()

they map onto bpf_cgroup_css() plus a bpf_css_to_memcg() cast the same way.

Hi Roman, JP, Shakeel: Do you think it is worth applying above changes for
the memory controller as well? If you are happy with the direction I am glad to
send the conversion as a follow-up once the generic kfuncs are in. Would you
take it?

>> +__bpf_kfunc u64 bpf_blkg_iostat_bytes(struct blkcg_gq *blkg,
>> +				      enum blkg_iostat_type rw)
>> +__bpf_kfunc u64 bpf_blkg_iostat_ios(struct blkcg_gq *blkg,
>> +				    enum blkg_iostat_type rw)
>> +__bpf_kfunc u64 bpf_blkg_dev(struct blkcg_gq *blkg)
>
>Ignoring 32bit split reads (do we really care for BPF accesses?), the above
>can easily be done with BPF_CORE_READ(), right?

Agree, these parts will be removed in the next version.

>
>Thanks.
>
>-- 
>tejun

Please let me know whether the above ideas works. Thanks very much!

Best,
Ziyang