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

Hi Michal & Roman,

Thank you for your review.

On 10/12/21 12:21 AM, Michal Koutný wrote:

Hello.

On Thu, Oct 07, 2021 at 08:16:03PM +0800, quanyang.wang@windriver.com wrote:

quoted

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!

quoted

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.

quoted

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?

Before this commit, percpu_ref is embedded in cgroup, it can be freed 
along with cgroup, so there is no memory leak. Since this commit, it 
causes the memory leak.
Should I change it to "Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime 
of cgroup_bpf from cgroup itself")"?

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...

quoted

+++ 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 :-)

Thanks for pointing it out. I will send a V2 to fix this.

Thanks,
Quanyang

quoted

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