Re: [PATCH v2 1/2] block: add BPF kfuncs to read blkcg io.stat

Hi Tejun,

On Wed, Aug 19, 2026 at 10:43:51AM -1000, Tejun Heo wrote:
>Hello,
>
>On Tue, Aug 18, 2026 at 04:01:58PM -0700, Ziyang Men wrote:
>> On Tue, Aug 18, 2026 at 12:42:54PM -1000, Tejun Heo wrote:
>> > On Tue, Aug 18, 2026 at 03:31:29PM -0700, Ziyang Men wrote:
>> > > Ok I will add the casts bpf_get_blkcg() and bpf_put_blkcg(), and remove the
>> > > check of css->ss in above functoins.
>> >
>> > RCU_PROTECTED is probably better than get/put().
>>
>> Yes we can make the blkcg* RCU_PROTECTED but then the bpf_blkcg_flush_stats()
>> needs to take the @cgroup* since it is SLEEPABLE. Does this work for you?
>
>Do we really need that function? css_rstat_flush() is already exposed and
>css can already be obtained, right? blkcg_fill_root_iostat() is just
>accessing system-wide bdev stats, which you probably don't need to begin
>with or if necessary just access through bdevs.

Yes the css_rstat_flush() is exposed but currently it is marked as SLEEPABLE, so
it doesn't accept a RCU_PROTECTED css we proposed. This is not a problem for cpu
ones since &cgrp->self is a trusted pointer. 

Best,
Ziyang

>
>Thanks.
>
>-- 
>tejun