Re: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction

[Michal Hocko]

On Mon 25-11-19 10:54:53, Roman Gushchin wrote:
> 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.

Could you explain how rare this really is please? I still have to wrap
my head around the overall logic here. It looks quite fragile to me TBH.
I am worried that is relies on implementation detail of the PCP ref
counters too much.

> Reported-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Tested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Fixes: f0a3a24b532d ("mm: memcg/slab: rework non-root kmem_cache lifecycle management")
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/slab_common.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 8afa188f6e20..f0ab6d4ceb4c 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -904,6 +904,18 @@ static void flush_memcg_workqueue(struct kmem_cache *s)
>  	 * 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)
> -- 
> 2.23.0

-- 
Michal Hocko
SUSE Labs