From: Roman Gushchin guro@fb.com Subject: mm: memcg/slab: fix panic in __free_slab() caused by premature memcg pointer release

Karsten reported the following panic in __free_slab() happening on a s390x machine:

349.361168 Unable to handle kernel pointer dereference in virtual kernel address space 349.361210 Failing address: 0000000000000000 TEID: 0000000000000483 349.361223 Fault in home space mode while using kernel ASCE. 349.361240 AS:00000000017d4007 R3:000000007fbd0007 S:000000007fbff000 P:000000000000003d 349.361340 Oops: 0004 ilc:3 Ý#1¨ PREEMPT SMP 349.361349 Modules linked in: tcp_diag inet_diag xt_tcpudp ip6t_rpfilter ip6t_REJECT \ nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_nat ip6table_mangle \ ip6table_raw ip6table_security iptable_at nf_nat 349.361436 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-05872-g6133e3e4bada-dirty #14 349.361445 Hardware name: IBM 2964 NC9 702 (z/VM 6.4.0) 349.361450 Krnl PSW : 0704d00180000000 00000000003cadb6 (__free_slab+0x686/0x6b0) 349.361464 R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3 349.361470 Krnl GPRS: 00000000f3a32928 0000000000000000 000000007fbf5d00 000000000117c4b8 349.361475 0000000000000000 000000009e3291c1 0000000000000000 0000000000000000 349.361481 0000000000000003 0000000000000008 000000002b478b00 000003d080a97600 349.361481 0000000000000003 0000000000000008 000000002b478b00 000003d080a97600 349.361486 000000000117ba00 000003e000057db0 00000000003cabcc 000003e000057c78 349.361500 Krnl Code: 00000000003cada6: e310a1400004 lg %r1,320(%r10) 349.361500 00000000003cadac: c0e50046c286 brasl %r14,ca32b8 349.361500 #00000000003cadb2: a7f4fe36 brc 15,3caa1e 349.361500 >00000000003cadb6: e32060800024 stg %r2,128(%r6) 349.361500 00000000003cadbc: a7f4fd9e brc 15,3ca8f8 349.361500 00000000003cadc0: c0e50046790c brasl %r14,c99fd8 349.361500 00000000003cadc6: a7f4fe2c brc 15,3caa 349.361500 00000000003cadc6: a7f4fe2c brc 15,3caa1e 349.361500 00000000003cadca: ecb1ffff00d9 aghik %r11,%r1,-1 349.361619 Call Trace: 349.361627 (<00000000003cabcc> __free_slab+0x49c/0x6b0) 349.361634 <00000000001f5886> rcu_core+0x5a6/0x7e0 349.361643 <0000000000ca2dea> __do_softirq+0xf2/0x5c0 349.361652 <0000000000152644> irq_exit+0x104/0x130 349.361659 <000000000010d222> do_IRQ+0x9a/0xf0 349.361667 <0000000000ca2344> ext_int_handler+0x130/0x134 349.361674 <0000000000103648> enabled_wait+0x58/0x128 349.361681 (<0000000000103634> enabled_wait+0x44/0x128) 349.361688 <0000000000103b00> arch_cpu_idle+0x40/0x58 349.361695 <0000000000ca0544> default_idle_call+0x3c/0x68 349.361704 <000000000018eaa4> do_idle+0xec/0x1c0 349.361748 <000000000018ee0e> cpu_startup_entry+0x36/0x40 349.361756 <000000000122df34> arch_call_rest_init+0x5c/0x88 349.361761 <0000000000000000> 0x0 349.361765 INFO: lockdep is turned off. 349.361769 Last Breaking-Event-Address: 349.361774 <00000000003ca8f4> __free_slab+0x1c4/0x6b0 349.361781 Kernel panic - not syncing: Fatal exception in interrupt

The kernel panics on an attempt to dereference the NULL memcg pointer. When shutdown_cache() is called from the kmem_cache_destroy() context, a memcg kmem_cache might have empty slab pages in a partial list, which are still charged to the memory cgroup. These pages are released by free_partial() at the beginning of shutdown_cache(): either directly or by scheduling a RCU-delayed work (if the kmem_cache has the SLAB_TYPESAFE_BY_RCU flag). The latter case is when the reported panic can happen: memcg_unlink_cache() is called immediately after shrinking partial lists, without waiting for scheduled RCU works. It sets the kmem_cache->memcg_params.memcg pointer to NULL, and the following attempt to dereference it by __free_slab() from the RCU work context causes the panic.

To fix the issue, let's postpone the release of the memcg pointer to destroy_memcg_params(). It's called from a separate work context by slab_caches_to_rcu_destroy_workfn(), which contains a full RCU barrier. This guarantees that all scheduled page release RCU works will complete before the memcg pointer will be zeroed.

Big thanks for Karsten for the perfect report containing all necessary information, his help with the analysis of the problem and testing of the fix.

Link: http://lkml.kernel.org/r/20191010160549.1584316-1-guro@fb.com Fixes: fb2f2b0adb98 ("mm: memcg/slab: reparent memcg kmem_caches on cgroup removal") Signed-off-by: Roman Gushchin guro@fb.com Reported-by: Karsten Graul kgraul@linux.ibm.com Tested-by: Karsten Graul kgraul@linux.ibm.com Acked-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Shakeel Butt shakeelb@google.com Cc: Karsten Graul kgraul@linux.ibm.com Cc: Vladimir Davydov vdavydov.dev@gmail.com Cc: David Rientjes rientjes@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/slab_common.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/mm/slab_common.c~mm-memcg-slab-fix-panic-in-__free_slab-caused-by-premature-memcg-pointer-release +++ a/mm/slab_common.c @@ -178,10 +178,13 @@ static int init_memcg_params(struct kmem

static void destroy_memcg_params(struct kmem_cache *s) { - if (is_root_cache(s)) + if (is_root_cache(s)) { kvfree(rcu_access_pointer(s->memcg_params.memcg_caches)); - else + } else { + mem_cgroup_put(s->memcg_params.memcg); + WRITE_ONCE(s->memcg_params.memcg, NULL); percpu_ref_exit(&s->memcg_params.refcnt); + } }

static void free_memcg_params(struct rcu_head *rcu) @@ -253,8 +256,6 @@ static void memcg_unlink_cache(struct km } else { list_del(&s->memcg_params.children_node); list_del(&s->memcg_params.kmem_caches_node); - mem_cgroup_put(s->memcg_params.memcg); - WRITE_ONCE(s->memcg_params.memcg, NULL); } } #else _