Re: [PATCH v4] nvmet: add cgroup_path to charge namespace I/O to a cgroup

[peng yu]

On Wed, Sep 23, 2026 at 11:10 PM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Wed, Sep 23, 2026 at 10:31:21PM -0700, Peng Yu wrote:
> ...
> > +#ifdef CONFIG_BLK_CGROUP
> > +static int nvmet_blkcg_ns_enable(struct nvmet_ns *ns)
> > +{
> ...
> > +     cgrp = cgroup_get_from_id(ns->cgroup_id);
> > +     if (IS_ERR(cgrp)) {
> > +             pr_err("failed to resolve cgroup id %llu: %ld\n",
> > +                    ns->cgroup_id, PTR_ERR(cgrp));
> > +             return PTR_ERR(cgrp);
> > +     }
> > +
> > +     css = cgroup_get_e_css(cgrp, &io_cgrp_subsys);
> > +     if (!css || css->cgroup != cgrp) {
> > +             pr_err("the io controller is not enabled in cgroup %llu\n",
> > +                    ns->cgroup_id);
> > +             if (css)
> > +                     css_put(css);
> > +             cgroup_put(cgrp);
> > +             return -EINVAL;
> > +     }
> > +     ns->blkcg_css = css;
>
> This is really odd from interface POV. The user specified "associate IOs to
> this cgroup" and that can fail if blkcg is not enabled on the cgroup. Even
> if it succeeds, if blkcg gets disabled and re-enabled on the cgroup, the
> device is still going to be issuing IOs as the old draining blkcg.
>
> What the user can express is the target cgroup and the IOs should follow
> whatever IO control that specified cgroup is under at the time of each IO
> issue. Currently, cgroup_e_css() has to walk up the tree to find the
> effective css for a given subsystem but updating cgroup code to add e_css[]
> array on each cgroup to avoid the overhead shouldn't be too difficult.
>
> Thanks.
>
> --
> tejun

How about this:
1. Add a one line change in the cgroup code: export the cgroup_e_css
2. The nvmet invokes the cgroup_e_css for each bio and under
    the rcu protection.

I think we don't need to add an e_css[] array on each cgroup. At least in
my case I don't expect to have a very deep cgroup hierarchy, so we won't
spend too much time on the cgroup_e_css.