Re: [V2][PATCH] cgroup: fix memory leak caused by missing cgroup_bpf_offline

Quanyang Wang
2021-10-19T10:41:14Z

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:
>> Hi Ming,
>>
>> On 10/18/21 5:02 PM, Ming Lei wrote:
>>> On Mon, Oct 18, 2021 at 03:56:23PM +0800, quanyang.wang-CWA4WttNNZF54TAoqtyWWQ@public.gmane.org wrote:
>>>> From: Quanyang Wang <quanyang.wang-CWA4WttNNZF54TAoqtyWWQ@public.gmane.org>
>>>>
>>>> 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
>