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Subject: Re: WARNING bisected (was Re: [PATCH v7 08/10] mm: rework non-root
 kmem_cache lifecycle management)
To:     Roman Gushchin <guro@fb.com>
Cc:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
        Kernel Team <Kernel-team@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "longman@redhat.com" <longman@redhat.com>,
        "shakeelb@google.com" <shakeelb@google.com>,
        "vdavydov.dev@gmail.com" <vdavydov.dev@gmail.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
References: <20190611231813.3148843-9-guro@fb.com>
 <20191121111739.3054-1-borntraeger@de.ibm.com>
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From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Thu, 21 Nov 2019 17:59:54 +0100
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On 21.11.19 17:58, Roman Gushchin wrote:
> On Thu, Nov 21, 2019 at 12:17:39PM +0100, Christian Borntraeger wrote:
>> Folks,
>>
>> I do get errors like the following when running a new testcase in our KVM CI.
>> The test basically unloads kvm, reloads with with hpage=1 (enable huge page
>> support for guests on s390) start a guest with libvirt and hugepages, shut the
>> guest down and unload the kvm module. 
>>
>> 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 ]---
>>
>> I have bisected this to
>> # first bad commit: [f0a3a24b532d9a7e56a33c5112b2a212ed6ec580] mm: memcg/slab: rework non-root kmem_cache lifecycle management
>>
>> unmounting /sys/fs/cgroup/memory/ before the test makes the problem go away so
>> it really seems to be related to the new percpu-refs from this patch.
>>  
>> Any ideas?
> 
> Hello, Christian!
> 
> It seems to be a race between releasing of the root kmem_cache (caused by rmmod)
> and a memcg kmem_cache. Does delaying rmmod for say, a minute, "resolve" the
> issue?

Yes, rmmod has to be called directly after the guest shutdown to see the issue.
See my 2nd mail.