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From:   Roman Gushchin <guro@fb.com>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
CC:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
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        Janosch Frank <frankja@linux.ibm.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: WARNING bisected (was Re: [PATCH v7 08/10] mm: rework non-root
 kmem_cache lifecycle management)
Thread-Topic: WARNING bisected (was Re: [PATCH v7 08/10] mm: rework non-root
 kmem_cache lifecycle management)
Thread-Index: AQHVoF1eUJGIXl/jo02oA2+fhmBRU6eV2PaA
Date:   Thu, 21 Nov 2019 16:58:11 +0000
Message-ID: <20191121165807.GA201621@localhost.localdomain>
References: <20190611231813.3148843-9-guro@fb.com>
 <20191121111739.3054-1-borntraeger@de.ibm.com>
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On Thu, Nov 21, 2019 at 12:17:39PM +0100, Christian Borntraeger wrote:
> Folks,
>=20
> 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=3D1 (enable huge =
page
> support for guests on s390) start a guest with libvirt and hugepages, shu=
t the
> guest down and unload the kvm module.=20
>=20
> WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0x5=
0/0x58
> Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp ip6=
t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrac=
k 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 00360081000=
00000
>            0000001f00000000 0035008100000000 0000001fb3573ab8 00000000000=
00000
>            0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb35=
73b00
>            0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e0092=
63cd0
> Krnl Code: 0000001529746842: f0a0000407fe        srp        4(11,%r0),204=
6,0
>            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=20
>  [<0000001529b04882>] kobject_put+0xaa/0xe8=20
>  [<000000152918cf28>] process_one_work+0x1e8/0x428=20
>  [<000000152918d1b0>] worker_thread+0x48/0x460=20
>  [<00000015291942c6>] kthread+0x126/0x160=20
>  [<0000001529b22344>] ret_from_fork+0x28/0x30=20
>  [<0000001529b2234c>] kernel_thread_starter+0x0/0x10=20
> Last Breaking-Event-Address:
>  [<000000152974684c>] percpu_ref_exit+0x4c/0x58
> ---[ end trace b035e7da5788eb09 ]---
>=20
> I have bisected this to
> # first bad commit: [f0a3a24b532d9a7e56a33c5112b2a212ed6ec580] mm: memcg/=
slab: rework non-root kmem_cache lifecycle management
>=20
> unmounting /sys/fs/cgroup/memory/ before the test makes the problem go aw=
ay so
> it really seems to be related to the new percpu-refs from this patch.
> =20
> Any ideas?

Hello, Christian!

It seems to be a race between releasing of the root kmem_cache (caused by r=
mmod)
and a memcg kmem_cache. Does delaying rmmod for say, a minute, "resolve" th=
e
issue?

I'll take a look at this code path.

Thanks!