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 guillaume@morinfr.org Suggested-by: Guillaume Morin guillaume@morinfr.org Signed-off-by: Mike Kravetz mike.kravetz@oracle.com Cc: stable@vger.kernel.org
I verified that the patch does fix the underflow. I appreciate the help!
Feel free to add: Tested-by: Guillaume Morin guillaume@morinfr.org