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Subject: Re: Memory hotplug softlock issue
To:     Michal Hocko <mhocko@kernel.org>, Baoquan He <bhe@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, aarcange@redhat.com
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <a1d4aa33-5c6d-1a6d-1426-46b2cebbf57e@redhat.com>
Date:   Thu, 15 Nov 2018 10:42:46 +0100
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On 15.11.18 09:30, Michal Hocko wrote:
> On Thu 15-11-18 15:53:56, Baoquan He wrote:
>> On 11/15/18 at 08:30am, Michal Hocko wrote:
>>> On Thu 15-11-18 13:10:34, Baoquan He wrote:
>>>> On 11/14/18 at 04:00pm, Michal Hocko wrote:
>>>>> On Wed 14-11-18 22:52:50, Baoquan He wrote:
>>>>>> On 11/14/18 at 10:01am, Michal Hocko wrote:
>>>>>>> I have seen an issue when the migration cannot make a forward progress
>>>>>>> because of a glibc page with a reference count bumping up and down. Most
>>>>>>> probable explanation is the faultaround code. I am working on this and
>>>>>>> will post a patch soon. In any case the migration should converge and if
>>>>>>> it doesn't do then there is a bug lurking somewhere.
>>>>>>>
>>>>>>> Failing on ENOMEM is a questionable thing. I haven't seen that happening
>>>>>>> wildly but if it is a case then I wouldn't be opposed.
>>>>>>
>>>>>> Applied your debugging patches, it helps a lot to printing message.
>>>>>>
>>>>>> Below is the dmesg log about the migrating failure. It can't pass
>>>>>> migrate_pages() and loop forever.
>>>>>>
>>>>>> [  +0.083841] migrating pfn 10fff7d0 failed 
>>>>>> [  +0.000005] page:ffffea043ffdf400 count:208 mapcount:201 mapping:ffff888dff4bdda8 index:0x2
>>>>>> [  +0.012689] xfs_address_space_operations [xfs] 
>>>>>> [  +0.000030] name:"stress" 
>>>>>> [  +0.004556] flags: 0x5fffffc0000004(uptodate)
>>>>>> [  +0.007339] raw: 005fffffc0000004 ffffc900000e3d80 ffffc900000e3d80 ffff888dff4bdda8
>>>>>> [  +0.009488] raw: 0000000000000002 0000000000000000 000000cb000000c8 ffff888e7353d000
>>>>>> [  +0.007726] page->mem_cgroup:ffff888e7353d000
>>>>>> [  +0.084538] migrating pfn 10fff7d0 failed 
>>>>>> [  +0.000006] page:ffffea043ffdf400 count:210 mapcount:201 mapping:ffff888dff4bdda8 index:0x2
>>>>>> [  +0.012798] xfs_address_space_operations [xfs] 
>>>>>> [  +0.000034] name:"stress" 
>>>>>> [  +0.004524] flags: 0x5fffffc0000004(uptodate)
>>>>>> [  +0.007068] raw: 005fffffc0000004 ffffc900000e3d80 ffffc900000e3d80 ffff888dff4bdda8
>>>>>> [  +0.009359] raw: 0000000000000002 0000000000000000 000000cb000000c8 ffff888e7353d000
>>>>>> [  +0.007728] page->mem_cgroup:ffff888e7353d000
>>>>>
>>>>> I wouldn't be surprised if this was a similar/same issue I've been
>>>>> chasing recently. Could you try to disable faultaround to see if that
>>>>> helps. It seems that it helped in my particular case but I am still
>>>>> waiting for the final good-to-go to post the patch as I do not own the
>>>>> workload which triggered that issue.
>>>>
>>>> Tried, still stuck in last block sometime. Usually after several times
>>>> of hotplug/unplug. If stop stress program, the last block will be
>>>> offlined immediately.
>>>
>>> Is the pattern still the same? I mean failing over few pages with
>>> reference count jumping up and down between attempts?
>>
>> ->count jumping up and down, mapcount stays the same value.
>>
>>>
>>>> [root@ ~]# cat /sys/kernel/debug/fault_around_bytes 
>>>> 4096
>>>
>>> Can you make it 0?
>>
>> I executed 'echo 0 > fault_around_bytes', value less than one page size
>> will round up to one page.
> 
> OK, I have missed that. So then there must be a different source of the
> page count volatility. Is it always the same file?
> 
> I think we can rule out memory reclaim because that depends on the page
> lock. Is the stress test hitting on memory compaction? In other words,
> are
> grep compact /proc/vmstat
> counters changing during the offline test heavily? I am asking because I
> do not see compaction pfn walkers skipping over MIGRATE_ISOLATE
> pageblocks. But I might be missing something easily.
> 
> It would be also good to find out whether this is fs specific. E.g. does
> it make any difference if you use a different one for your stress
> testing?
> 

I am wondering why it is always the last memory block of that device
(and even that node). Coincidence?

-- 

Thanks,

David / dhildenb