Re: WARNING bisected (was Re: [PATCH v7 08/10] mm: rework non-root kmem_cache lifecycle management)

[Roman Gushchin]

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?

I'll take a look at this code path.

Thanks!