The patch titled Subject: mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction has been added to the -mm tree. Its filename is mm-memcg-slab-wait-for-root-kmem_cache-refcnt-killing-on-root-kmem_cache-destruction.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-memcg-slab-wait-for-root-kmem_ca... and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-slab-wait-for-root-kmem_ca...

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------------------------------------------------------ From: Roman Gushchin guro@fb.com Subject: mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction

Christian reported a warning like the following obtained during running some KVM-related tests on s390:

WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0x50/0x58 Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_na> CPU: 8 PID: 208 Comm: kworker/8:1 Not tainted 5.2.0+ #66 Hardware name: IBM 2964 NC9 712 (LPAR) Workqueue: events sysfs_slab_remove_workfn Krnl PSW : 0704e00180000000 0000001529746850 (percpu_ref_exit+0x50/0x58) R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3 Krnl GPRS: 00000000ffff8808 0000001529746740 000003f4e30e8e18 0036008100000000 0000001f00000000 0035008100000000 0000001fb3573ab8 0000000000000000 0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb3573b00 0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e009263cd0 Krnl Code: 0000001529746842: f0a0000407fe srp 4(11,%r0),2046,0 0000001529746848: 47000700 bc 0,1792 #000000152974684c: a7f40001 brc 15,152974684e >0000001529746850: a7f4fff2 brc 15,1529746834 0000001529746854: 0707 bcr 0,%r7 0000001529746856: 0707 bcr 0,%r7 0000001529746858: eb8ff0580024 stmg %r8,%r15,88(%r15) 000000152974685e: a738ffff lhi %r3,-1 Call Trace: ([<000003e009263d00>] 0x3e009263d00) [<00000015293252ea>] slab_kmem_cache_release+0x3a/0x70 [<0000001529b04882>] kobject_put+0xaa/0xe8 [<000000152918cf28>] process_one_work+0x1e8/0x428 [<000000152918d1b0>] worker_thread+0x48/0x460 [<00000015291942c6>] kthread+0x126/0x160 [<0000001529b22344>] ret_from_fork+0x28/0x30 [<0000001529b2234c>] kernel_thread_starter+0x0/0x10 Last Breaking-Event-Address: [<000000152974684c>] percpu_ref_exit+0x4c/0x58 ---[ end trace b035e7da5788eb09 ]---

The problem occurs because kmem_cache_destroy() is called immediately after deleting of a memcg, so it races with the memcg kmem_cache deactivation.

flush_memcg_workqueue() at the beginning of kmem_cache_destroy() is supposed to guarantee that all deactivation processes are finished, but failed to do so. It waits for an rcu grace period, after which all children kmem_caches should be deactivated. During the deactivation percpu_ref_kill() is called for non root kmem_cache refcounters, but it requires yet another rcu grace period to finish the transition to the atomic (dead) state.

So in a rare case when not all children kmem_caches are destroyed at the moment when the root kmem_cache is about to be gone, we need to wait another rcu grace period before destroying the root kmem_cache.

This issue can be triggered only with dynamically created kmem_caches which are used with memcg accounting. In this case per-memcg child kmem_caches are created. They are deactivated from the cgroup removing path. If the destruction of the root kmem_cache is racing with the removal of the cgroup (both are quite complicated multi-stage processes), the described issue can occur. The only known way to trigger it in the real life, is to unload some kernel module which creates a dedicated kmem_cache, used from different memory cgroups with GFP_ACCOUNT flag. If the unloading happens immediately after calling rmdir on the corresponding cgroup, there is some chance to trigger the issue.

Link: http://lkml.kernel.org/r/20191129025011.3076017-1-guro@fb.com Fixes: f0a3a24b532d ("mm: memcg/slab: rework non-root kmem_cache lifecycle management") Signed-off-by: Roman Gushchin guro@fb.com Reported-by: Christian Borntraeger borntraeger@de.ibm.com Tested-by: Christian Borntraeger borntraeger@de.ibm.com Reviewed-by: Shakeel Butt shakeelb@google.com Acked-by: Michal Hocko mhocko@suse.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/slab_common.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)

--- a/mm/slab_common.c~mm-memcg-slab-wait-for-root-kmem_cache-refcnt-killing-on-root-kmem_cache-destruction +++ a/mm/slab_common.c @@ -904,6 +904,18 @@ static void flush_memcg_workqueue(struct * previous workitems on workqueue are processed. */ flush_workqueue(memcg_kmem_cache_wq); + + /* + * If we're racing with children kmem_cache deactivation, it might + * take another rcu grace period to complete their destruction. + * At this moment the corresponding percpu_ref_kill() call should be + * done, but it might take another rcu grace period to complete + * switching to the atomic mode. + * Please, note that we check without grabbing the slab_mutex. It's safe + * because at this moment the children list can't grow. + */ + if (!list_empty(&s->memcg_params.children)) + rcu_barrier(); } #else static inline int shutdown_memcg_caches(struct kmem_cache *s) _

Patches currently in -mm which might be from guro@fb.com are

mm-memcg-slab-wait-for-root-kmem_cache-refcnt-killing-on-root-kmem_cache-destruction.patch