Hi Roman, On 10/22/21 9:30 AM, Roman Gushchin wrote:
On Mon, Oct 18, 2021 at 03:56:23PM +0800, quanyang.wang@xxxxxxxxxxxxx wrote:From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx> 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") Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself") Signed-off-by: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx> --- V1 ---> V2: 1. As per Daniel's suggestion, add description to commit msg about the balance of cgroup's refcount in cgroup_bpf_offline. 2. As per Michal's suggestion, add tag "Fixes: 4bfc0bb2c60e" and add description about it. 3. Fix indentation on the percpu_ref_is_dying line.Acked-by: Roman Gushchin <guro@xxxxxx> The fix looks correct, two fixes tag are fine too, if only it won't confuse scripts picking up patches for stable backports. In fact, it's a very cold path, which is arguably never hit in the real life. On cgroup v2 it's not an issue. I'm not sure we need a stable backport at all, only if it creates a noise for some automation tests. Quanyang, out of curiosity, how did you find it?
I ran ltp testsuite to find this. ./runltp -f controllers -s cgroup Thanks, Quanyang
Anyway, thanks for catching and fixing it! Roman
