Date:   Wed, 14 Nov 2018 10:48:09 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Baoquan He <bhe@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        aarcange@...hat.com
Subject: Re: Memory hotplug softlock issue

On 14.11.18 10:41, Michal Hocko wrote:
> On Wed 14-11-18 10:25:57, David Hildenbrand wrote:
>> On 14.11.18 10:00, Baoquan He wrote:
>>> Hi David,
>>>
>>> On 11/14/18 at 09:18am, David Hildenbrand wrote:
>>>> Code seems to be waiting for the mem_hotplug_lock in read.
>>>> We hold mem_hotplug_lock in write whenever we online/offline/add/remove
>>>> memory. There are two ways to trigger offlining of memory:
>>>>
>>>> 1. Offlining via "cat offline > /sys/devices/system/memory/memory0/state"
>>>>
>>>> This always properly took the mem_hotplug_lock. Nothing changed
>>>>
>>>> 2. Offlining via "cat 0 > /sys/devices/system/memory/memory0/online"
>>>>
>>>> This didn't take the mem_hotplug_lock and I fixed that for this release.
>>>>
>>>> So if you were testing with 1., you should have seen the same error
>>>> before this release (unless there is something else now broken in this
>>>> release).
>>>
>>> Thanks a lot for looking into this.
>>>
>>> I triggered sysrq+t to check threads' state. You can see that we use
>>> firmware to trigger ACPI event to go to acpi_bus_offline(), it truly
>>> didn't take mem_hotplug_lock() and has taken it with your fix in
>>> commit 381eab4a6ee mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock
>>>
>>> [  +0.007062] Workqueue: kacpi_hotplug acpi_hotplug_work_fn
>>> [  +0.005398] Call Trace:
>>> [  +0.002476]  ? page_vma_mapped_walk+0x307/0x710
>>> [  +0.004538]  ? page_remove_rmap+0xa2/0x340
>>> [  +0.004104]  ? ptep_clear_flush+0x54/0x60
>>> [  +0.004027]  ? enqueue_entity+0x11c/0x620
>>> [  +0.005904]  ? schedule+0x28/0x80
>>> [  +0.003336]  ? rmap_walk_file+0xf9/0x270
>>> [  +0.003940]  ? try_to_unmap+0x9c/0xf0
>>> [  +0.003695]  ? migrate_pages+0x2b0/0xb90
>>> [  +0.003959]  ? try_offline_node+0x160/0x160
>>> [  +0.004214]  ? __offline_pages+0x6ce/0x8e0
>>> [  +0.004134]  ? memory_subsys_offline+0x40/0x60
>>> [  +0.004474]  ? device_offline+0x81/0xb0
>>> [  +0.003867]  ? acpi_bus_offline+0xdb/0x140
>>> [  +0.004117]  ? acpi_device_hotplug+0x21c/0x460
>>> [  +0.004458]  ? acpi_hotplug_work_fn+0x1a/0x30
>>> [  +0.004372]  ? process_one_work+0x1a1/0x3a0
>>> [  +0.004195]  ? worker_thread+0x30/0x380
>>> [  +0.003851]  ? drain_workqueue+0x120/0x120
>>> [  +0.004117]  ? kthread+0x112/0x130
>>> [  +0.003411]  ? kthread_park+0x80/0x80
>>> [  +0.005325]  ? ret_from_fork+0x35/0x40
>>>
>>
>> Yes, this is indeed another code path that was fixed (and I didn't
>> actually realize it ;) ). Thanks for the callchain. Before my fix
>> hotplug still would have never succeeded (offline_pages would have
>> silently looped forever) as far as I can tell.
> 
> I haven't studied your patch yet so I am not really sure why you have
> added the lock into this path. The memory hotplug locking is certainly
> far from great but I believe we should really rething the scope of the
> lock. There shouldn't be any fundamental reason to use the global lock
> for the full offlining. So rather than moving the lock from one place to
> another we need a range locking I believe.
See the patches for details, the lock was removed on this path by
mistake not by design.

Replacing the lock by some range lock can now be done. The tricky part
will be get_online_mems(), we'll have to indicate a range somehow. For
online/offline/add/remove, we have the range.

-- 

Thanks,

David / dhildenb

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