Hi Ming,

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

On Mon, Oct 18, 2021 at 03:56:23PM +0800, quanyang.wang@windriver.com 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.

For non-root cgroup:
cgroup_mkdir
-> cgroup_create
-->cgroup_bpf_inherit(cgrp_A)
cgroup_rmdir
->cgroup_destroy_locked()
-->cgroup_bpf_offline(cgrp_A)
So for non-root cgroup, there is no memory leak.


For root cgroup:
cgroup_setup_root
->cgroup_bpf_inherit(root_cgrp)
cgroup_kill_sb:
-> (Here should be call cgroup_bpf_offline, or else leak occurs)

Thanks,
Quanyang

If that is true, 'Fixes: 2b0d3d3e4fcf' looks misleading, cause people has to
backport your patch if 4bfc0bb2c60e is applied. Meantime, this fix isn't
needed if 4bfc0bb2c60e isn't merged.


Thanks,
Ming