Re: [PATCH] cgroup: fix memory leak caused by missing cgroup_bpf_offline

[Michal Koutný]

Hello.

On Thu, Oct 07, 2021 at 08:16:03PM +0800, quanyang.wang@xxxxxxxxxxxxx wrote:
> This is because that root_cgrp->bpf.refcnt.data is allocated by the
> function percpu_ref_init in cgroup_bpf_inherit which is called by
> cgroup_setup_root when mounting, but not freed along with root_cgrp
> when umounting.

Good catch!

> Adding cgroup_bpf_offline which calls percpu_ref_kill to
> cgroup_kill_sb can free root_cgrp->bpf.refcnt.data in umount path.

That is sensible.

> Fixes: 2b0d3d3e4fcfb ("percpu_ref: reduce memory footprint of percpu_ref in fast path")

Why this Fixes:? Is the leak absent before the percpu_ref refactoring?
I guess the embedded data are free'd together with cgroup. Makes me
wonder why struct cgroup_bpf has a separate percpu_ref counter from
struct cgroup...

> +++ b/kernel/cgroup/cgroup.c
> @@ -2147,8 +2147,10 @@ static void cgroup_kill_sb(struct super_block *sb)
>  	 * And don't kill the default root.
>  	 */
>  	if (list_empty(&root->cgrp.self.children) && root != &cgrp_dfl_root &&
> -	    !percpu_ref_is_dying(&root->cgrp.self.refcnt))
> +			!percpu_ref_is_dying(&root->cgrp.self.refcnt)) {
> +		cgroup_bpf_offline(&root->cgrp);

(You made some unnecessary whitespace here breaking indention :-)

>  		percpu_ref_kill(&root->cgrp.self.refcnt);
> +	}
>  	cgroup_put(&root->cgrp);
>  	kernfs_kill_sb(sb);
>  }