On 03/10/2019 18:11, Roman Gushchin wrote:
> On Thu, Oct 03, 2019 at 10:48:15AM +0200, Karsten Graul wrote:
>> Hello, Roman!
>>
>> On 03/10/2019 05:35, Roman Gushchin wrote:
>>> On Wed, Oct 02, 2019 at 12:41:29PM -0700, Roman Gushchin wrote:
>>>> Hello, Karsten!
>>>>
>>>> Thank you for the report!
>>>>
>>>> On Wed, Oct 02, 2019 at 04:50:53PM +0200, Karsten Graul wrote:
>>>>>
>>>>> net/smc is calling proto_register(&smc_proto, 1) with smc_proto.slab_flags = SLAB_TYPESAFE_BY_RCU.
>>>>> Right after the last SMC socket is destroyed, proto_unregister(&smc_proto) is called, which
>>>>> calls kmem_cache_destroy(prot->slab). This results in a kernel crash in __free_slab().
>>>>> Platform is s390x, reproduced on kernel 5.4-rc1. The problem was introduced by commit
>>>>> fb2f2b0adb98 ("mm: memcg/slab: reparent memcg kmem_caches on cgroup removal")
>>>>>
>>>>> I added a 'call graph', below of that is the crash log and a (simple) patch that works for me,
>>>>> but I don't know if this is the correct way to fix it.
>>>>>
>>>>> (Please keep me on CC of this thread because I do not follow the mm mailing list, thank you)
>>>>>
>>>>>
>>>>> kmem_cache_destroy()
>>>>> -> shutdown_memcg_caches()
>>>>> -> shutdown_cache()
>>>>> -> __kmem_cache_shutdown() (slub.c)
>>>>> -> free_partial()
>>>>> -> discard_slab()
>>>>> -> free_slab() -- call to __free_slab() is delayed
>>>>> -> call_rcu(rcu_free_slab)
>>>>> -> memcg_unlink_cache()
>>>>> -> WRITE_ONCE(s->memcg_params.memcg, NULL); -- !!!
>>>>> -> list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
>>>>> -> schedule_work(&slab_caches_to_rcu_destroy_work); -> work_fn uses rcu_barrier() to wait for rcu_batch,
>>>>> so work_fn is not further involved here...
>>>>> ... rcu grace period ...
>>>>> rcu_batch()
>>>>> ...
>>>>> -> rcu_free_slab() (slub.c)
>>>>> -> __free_slab()
>>>>> -> uncharge_slab_page()
>>>>> -> memcg_uncharge_slab()
>>>>> -> memcg = READ_ONCE(s->memcg_params.memcg); -- !!! memcg == NULL
>>>>> -> mem_cgroup_lruvec(memcg)
>>>>> -> mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id); -- mz == NULL
>>>>> -> lruvec = &mz->lruvec; -- lruvec == NULL
>>>>> -> lruvec->pgdat = pgdat; -- *crash*
>>>>>
>>>>> The crash log:
>>>>
>>>> Hm, I might be wrong, but it seems that the problem is deeper: __free_slab()
>>>> called from the rcu path races with kmem_cache_destroy(), which is supposed
>>>> to be called when there are no outstanding allocations (and corresponding pages).
>>>> Any charged slab page actually holds a reference to the kmem_cache, which prevents
>>>> its destruction (look at s->memcg_params.refcnt), but kmem_cache_destroy() ignores
>>>> it.
>>
>> I don't see a race between kmem_cache_destroy() and __fre_slab(). kmem_cache_destroy()
>> is already done when __free_slab() is called. Maybe the below trace shows you the order of calls on my
>> system: kmem_cache_destroy() unlinks the memcg caches, sets up the rcu callbacks, then
>> it starts the slab_caches_to_rcu_destroy_workfn() worker and then kmem_cache_destroy() is done.
>
> Right, and this is the problem. The question when call_rcu() in free_slab() has
> been called: if it did happen before the kmem_cache_destroy(), it's clearly
> a bug inside the slab allocator. Otherwise it's probably an incorrect API
> invocation.
>
>> You see that the smc code is getting control again after that.
>> The worker starts in between (before the smc_exit trace), but keeps waiting on the rcu_barrier().
>> Ages later (see time difference) the rcu grace period ends and rcu_free_slab() is called, and it
>> crashes.
>> I hope that helps!
>>
>> [ 145.539917] kmem_cache_destroy before shutdown_memcg_caches() for 0000000068106f00
>> [ 145.539929] free_slab call_rcu() for 00000000392c2e00, page is 000003d080e45000
>> [ 145.539961] memcg_unlink_cache clearing memcg for 00000000392c2e00
>> [ 145.539970] shutdown_cache adding to slab_caches_to_rcu_destroy queue for work: 00000000392c2e00
>
> Does it mean that call_rcu() has been called after kmem_cache_destroy()?
> In this case, do you know who called it?
>
Let me show the 'call graph' again, call_rcu() is called by free_slab()
as part of kmem_cache_destroy(), and just before memcg_unlink_cache() clears
the memcg reference.
kmem_cache_destroy()
-> shutdown_memcg_caches()
-> shutdown_cache()
-> __kmem_cache_shutdown() (slub.c)
-> free_partial()
-> discard_slab()
-> free_slab() -- call to __free_slab() is delayed
-> call_rcu(rcu_free_slab)
-> memcg_unlink_cache()
-> WRITE_ONCE(s->memcg_params.memcg, NULL); -- !!!
> I'd add an atomic flag to the root kmem_cache, set it at the beginning of the
> kmem_cache_destroy() and check it in free_slab(). If set, dump the stacktrace.
> Just please make sure you're looking at the root kmem_cache flag, not the memcg
> one.
>
> Thank you!
>
> Roman
>
>>
>> [ 145.540001] free_slab call_rcu() for 00000000392c2900, page is 000003d080e4a200
>> [ 145.540031] memcg_unlink_cache clearing memcg for 00000000392c2900
>> [ 145.540041] shutdown_cache adding to slab_caches_to_rcu_destroy queue for work: 00000000392c2900
>>
>> [ 145.540066] kmem_cache_destroy after shutdown_memcg_caches() for 0000000068106f00
>>
>> [ 145.540075] kmem_cache_destroy before final shutdown_cache() for 0000000068106f00
>> [ 145.540086] free_slab call_rcu() for 0000000068106f00, page is 000003d080e0a800
>> [ 145.540189] shutdown_cache adding to slab_caches_to_rcu_destroy queue for work: 0000000068106f00
>>
>> [ 145.540548] kmem_cache_destroy after final shutdown_cache() for 0000000068106f00
>> kmem_cache_destroy is done
>> [ 145.540573] slab_caches_to_rcu_destroy_workfn before rcu_barrier() in workfunc
>> slab_caches_to_rcu_destroy_workfn started and waits in rcu_barrier() now
>> [ 145.540619] smc.0698ae: smc_exit before smc_pnet_exit
>> smc module exit code gets back control ...
>> [ 145.540699] smc.616283: smc_exit before unregister_pernet_subsys
>> [ 145.619747] rcu_free_slab called for 00000000392c2e00, page is 000003d080e45000
>> much later the rcu callbacks are invoked, and will crash
>>
>>>>
>>>> If my thoughts are correct, the commit you've mentioned didn't introduced this
>>>> issue, it just made it easier to reproduce.
>>>>
>>>> The proposed fix looks dubious to me: the problem isn't in the memcg pointer
>>>> (it's just a luck that it crashes on it), and it seems incorrect to not decrease
>>>> the slab statistics of the original memory cgroup.
>>
>> I was quite sure that my approach is way to simple, it's better when the mm experts
>> work on that.
>>
>>>>
>>>> What we probably need to do instead is to extend flush_memcg_workqueue() to
>>>> wait for all outstanding rcu free callbacks. I have to think a bit what's the best
>>>> way to fix it. How easy is to reproduce the problem?
>>
>> I can reproduce this at will and I am happy to test any fixes you provide.
>>
>>>
>>> After a second thought, flush_memcg_workqueue() already contains
>>> a rcu_barrier() call, so now first suspicion is that the last free() call
>>> occurs after the kmem_cache_destroy() call. Can you, please, check if it's not
>>> a case?
>>>
>>
>> In kmem_cache_destroy(), the flush_memcg_workqueue() call is the first one, and after
>> that shutdown_memcg_caches() is called which setup the rcu callbacks.
>
> These are callbacks to destroy kmem_caches, not pages.
>
>> So flush_memcg_workqueue() can not wait for them. If you follow the 'call graph' above
>> using the RCU path in slub.c you can see when the callbacks are set up and why no warning
>> is printed.
>>
>>
>> Second thought after I wrote all of the above: when flush_memcg_workqueue() already contains
>> an rcu_barrier(), whats the point of delaying the slab freeing in the rcu case? All rcu
>> readers should be done now, so the rcu callbacks and the worker are not needed?
>> What am I missing here (I am sure I miss something, I am completely new in the mm area)?
>>
>>> Thanks!
>>>
>>>>
>>>>>
>>>>> 349.361168¨ Unable to handle kernel pointer dereference in virtual kernel address space
>>>>
>>>> Btw, haven't you noticed anything suspicious in dmesg before this line?
>>
>> There is no error or warning line in dmesg before this line. Actually, I think that
>> all pages are no longer in use so no warning is printed. Anyway, the slab freeing is
>> delayed in any case when RCU is in use, right?
>>
>>
>> Karsten
>>
>>>>
>>>> Thank you!
>>>>
>>>> Roman
>>>>
>>>>> 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 iptable_mangle iptable_raw iptable_security nf_conntrack nf_defrag_ipv6 nf_de
>>>>> 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¨ (Ý<00000000003cabcc>¨ __free_slab+0x49c/0x6b0)
>>>>> 349.361634¨ Ý<00000000001f5886>¨ rcu_core+0x5a6/0x7e0
>>>>> 349.361643¨ Ý<0000000000ca2dea>¨ __do_softirq+0xf2/0x5c0
>>>>> 349.361652¨ Ý<0000000000152644>¨ irq_exit+0x104/0x130
>>>>> 349.361659¨ Ý<000000000010d222>¨ do_IRQ+0x9a/0xf0
>>>>> 349.361667¨ Ý<0000000000ca2344>¨ ext_int_handler+0x130/0x134
>>>>> 349.361674¨ Ý<0000000000103648>¨ enabled_wait+0x58/0x128
>>>>> 349.361681¨ (Ý<0000000000103634>¨ enabled_wait+0x44/0x128)
>>>>> 349.361688¨ Ý<0000000000103b00>¨ arch_cpu_idle+0x40/0x58
>>>>> 349.361695¨ Ý<0000000000ca0544>¨ default_idle_call+0x3c/0x68
>>>>> 349.361704¨ Ý<000000000018eaa4>¨ do_idle+0xec/0x1c0
>>>>> 349.361748¨ Ý<000000000018ee0e>¨ cpu_startup_entry+0x36/0x40
>>>>> 349.361756¨ Ý<000000000122df34>¨ arch_call_rest_init+0x5c/0x88
>>>>> 349.361761¨ Ý<0000000000000000>¨ 0x0
>>>>> 349.361765¨ INFO: lockdep is turned off.
>>>>> 349.361769¨ Last Breaking-Event-Address:
>>>>> 349.361774¨ Ý<00000000003ca8f4>¨ __free_slab+0x1c4/0x6b0
>>>>> 349.361781¨ Kernel panic - not syncing: Fatal exception in interrupt
>>>>>
>>>>>
>>>>> A fix that works for me (RFC):
>>>>>
>>>>> diff --git a/mm/slab.h b/mm/slab.h
>>>>> index a62372d0f271..b19a3f940338 100644
>>>>> --- a/mm/slab.h
>>>>> +++ b/mm/slab.h
>>>>> @@ -328,7 +328,7 @@ static __always_inline void memcg_uncharge_slab(struct page *page, int order,
>>>>>
>>>>> rcu_read_lock();
>>>>> memcg = READ_ONCE(s->memcg_params.memcg);
>>>>> - if (likely(!mem_cgroup_is_root(memcg))) {
>>>>> + if (likely(memcg && !mem_cgroup_is_root(memcg))) {
>>>>> lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
>>>>> mod_lruvec_state(lruvec, cache_vmstat_idx(s), -(1 << order));
>>>>> memcg_kmem_uncharge_memcg(page, order, memcg);
>>>>>
>>>>> --
>>>>> Karsten
>>>>>
>>>>> (I'm a dude)
>>>>>
>>>>>
>>
--
Karsten
(I'm a dude)
