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SubjectRe: [RESEND][PATCH] cgroup: fix memory leak caused by missing cgroup_bpf_offline
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On 10/8/21 2:46 AM, quanyang.wang@windriver.com wrote:
> From: Quanyang Wang <quanyang.wang@windriver.com>
>
> 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 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.
>
> Fixes: 2b0d3d3e4fcfb ("percpu_ref: reduce memory footprint of percpu_ref in fast path")
> Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
> ---
> One of the recipients' email is wrong, so I resend this patch.
> Sorry for any confusion caused.
> ---
> kernel/cgroup/cgroup.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 570b0c97392a9..183736ad72f2b 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -2187,8 +2187,10 @@ 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);
> percpu_ref_kill(&root->cgrp.self.refcnt);
> + }
> cgroup_put(&root->cgrp);

Doesn't cgroup_bpf_offline() internally bump the root cgroup's refcount via cgroup_get()?
How does this relate to the single cgroup_put() in the above line? Would have been nice to
see the commit msg elaborate more on this.

> kernfs_kill_sb(sb);
> }
>

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