Hi Ming,

On 10/18/21 8:59 PM, Ming Lei wrote:

On Mon, Oct 18, 2021 at 06:06:28PM +0800, Quanyang Wang wrote:

quoted

Hi Ming,

On 10/18/21 5:02 PM, Ming Lei wrote:

quoted

On Mon, Oct 18, 2021 at 03:56:23PM +0800, quanyang.wang-CWA4WttNNZF54TAoqtyWWQ@public.gmane.org wrote:

quoted

From: Quanyang Wang <redacted>

When enabling CONFIG_CGROUP_BPF, kmemleak can be observed by running
the command as below:

      $mount -t cgroup -o none,name=foo cgroup cgroup/
      $umount cgroup/

unreferenced object 0xc3585c40 (size 64):
    comm "mount", pid 425, jiffies 4294959825 (age 31.990s)
    hex dump (first 32 bytes):
      01 00 00 80 84 8c 28 c0 00 00 00 00 00 00 00 00  ......(.........
      00 00 00 00 00 00 00 00 6c 43 a0 c3 00 00 00 00  ........lC......
    backtrace:
      [<e95a2f9e>] cgroup_bpf_inherit+0x44/0x24c
      [<1f03679c>] cgroup_setup_root+0x174/0x37c
      [<ed4b0ac5>] cgroup1_get_tree+0x2c0/0x4a0
      [<f85b12fd>] vfs_get_tree+0x24/0x108
      [<f55aec5c>] path_mount+0x384/0x988
      [<e2d5e9cd>] do_mount+0x64/0x9c
      [<208c9cfe>] sys_mount+0xfc/0x1f4
      [<06dd06e0>] ret_fast_syscall+0x0/0x48
      [<a8308cb3>] 0xbeb4daa8

This is because that since the commit 2b0d3d3e4fcf ("percpu_ref: reduce
memory footprint of percpu_ref in fast path") 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. Adding cgroup_bpf_offline which calls
percpu_ref_kill to cgroup_kill_sb can free root_cgrp->bpf.refcnt.data in
umount path.

This patch also fixes the commit 4bfc0bb2c60e ("bpf: decouple the lifetime
of cgroup_bpf from cgroup itself"). A cgroup_bpf_offline is needed to do a
cleanup that frees the resources which are allocated by cgroup_bpf_inherit
in cgroup_setup_root.

And inside cgroup_bpf_offline, cgroup_get() is at the beginning and
cgroup_put is at the end of cgroup_bpf_release which is called by
cgroup_bpf_offline. So cgroup_bpf_offline can keep the balance of
cgroup's refcount.

Fixes: 2b0d3d3e4fcf ("percpu_ref: reduce memory footprint of percpu_ref in fast path")

If I understand correctly, cgroup_bpf_release() won't be called without
your patch. So anything allocated in cgroup_bpf_inherit() will be
leaked?

No, for now cgroup_bpf_release is called to release bpf.refcnt.data of the
cgroup which is not root_cgroup. Only root_cgroup's bpf data is leaked.

You mean that cgroup_bpf_inherit() allocates nothing for root_cgroup?

Yes, cgroup_bpf_inherit allocates something for root_cgroup.

The earlier commit 4bfc0bb2c60e ("bpf: decouple the lifetime of 
cgroup_bpf from cgroup itself") introduces an imbalance that call 
cgroup_bpf_inherit(&root_cgroup) but not call 
cgroup_bpf_offline(&root_cgroup). But there was no memory leak here.

When the commit 2b0d3d3e4fcf ("percpu_ref: reduce memory footprint of 
percpu_ref in fast path") applies, some data is allocated for 
root_cgroup and not released with root_cgroup, and memory leak is observed.

So I add 2 "Fixes tags" here to indicate that 2 commits introduce two 
different issues.

But it seems that 2 "Fixes tags" is misleading now.
So maybe just fix earlier commit 4bfc0bb2c60e which introduces imbalance?

Thanks,
Quanyang

If yes, I agree you can add 'Fixes: 2b0d3d3e4fcf', otherwise please
remove it.


Thanks,
Ming