On 30 Aug 14:50, Mike Kravetz wrote:

Guillaume Morin reported hitting the following WARNING followed
by GPF or NULL pointer deference either in cgroups_destroy or in
the kill_css path.:

percpu ref (css_release) <= 0 (-1) after switching to atomic
WARNING: CPU: 23 PID: 130 at lib/percpu-refcount.c:196 percpu_ref_switch_to_atomic_rcu+0x127/0x130
CPU: 23 PID: 130 Comm: ksoftirqd/23 Kdump: loaded Tainted: G           O      5.10.60 #1
RIP: 0010:percpu_ref_switch_to_atomic_rcu+0x127/0x130
Call Trace:
 rcu_core+0x30f/0x530
 rcu_core_si+0xe/0x10
 __do_softirq+0x103/0x2a2
 ? sort_range+0x30/0x30
 run_ksoftirqd+0x2b/0x40
 smpboot_thread_fn+0x11a/0x170
 kthread+0x10a/0x140
 ? kthread_create_worker_on_cpu+0x70/0x70
 ret_from_fork+0x22/0x30

Upon further examination, it was discovered that the css structure
was associated with hugetlb reservations.

For private hugetlb mappings the vma points to a reserve map that
contains a pointer to the css.  At mmap time, reservations are set up
and a reference to the css is taken.  This reference is dropped in the
vma close operation; hugetlb_vm_op_close.  However, if a vma is split
no additional reference to the css is taken yet hugetlb_vm_op_close will
be called twice for the split vma resulting in an underflow.

Fix by taking another reference in hugetlb_vm_op_open.  Note that the
reference is only taken for the owner of the reserve map.  In the more
common fork case, the pointer to the reserve map is cleared for
non-owning vmas.

Fixes: e9fe92ae0cd2 ("hugetlb_cgroup: add reservation accounting for
private mappings")
Reported-by: Guillaume Morin <redacted>
Suggested-by: Guillaume Morin <redacted>
Signed-off-by: Mike Kravetz <redacted>
Cc: <redacted>

I verified that the patch does fix the underflow. I appreciate the help!

Feel free to add:
Tested-by: Guillaume Morin <redacted>

-- 
Guillaume Morin [off-list ref]