Re: [PATCH v2 3/6] mm/memory_hotplug: Process all zones when removing memory

[David Hildenbrand]

On 29.08.19 18:27, Michal Hocko wrote:
> On Thu 29-08-19 17:54:35, David Hildenbrand wrote:
>> On 29.08.19 17:39, Michal Hocko wrote:
>>> On Mon 26-08-19 12:10:09, David Hildenbrand wrote:
>>>> It is easier than I though to trigger a kernel bug by removing memory that
>>>> was never onlined. With CONFIG_DEBUG_VM the memmap is initialized with
>>>> garbage, resulting in the detection of a broken zone when removing memory.
>>>> Without CONFIG_DEBUG_VM it is less likely - but we could still have
>>>> garbage in the memmap.
>>>>
>>>> :/# [   23.912993] BUG: unable to handle page fault for address: 000000000000353d
>>>> [   23.914219] #PF: supervisor write access in kernel mode
>>>> [   23.915199] #PF: error_code(0x0002) - not-present page
>>>> [   23.916160] PGD 0 P4D 0
>>>> [   23.916627] Oops: 0002 [#1] SMP PTI
>>>> [   23.917256] CPU: 1 PID: 7 Comm: kworker/u8:0 Not tainted 5.3.0-rc5-next-20190820+ #317
>>>> [   23.918900] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.4
>>>> [   23.921194] Workqueue: kacpi_hotplug acpi_hotplug_work_fn
>>>> [   23.922249] RIP: 0010:clear_zone_contiguous+0x5/0x10
>>>> [   23.923173] Code: 48 89 c6 48 89 c3 e8 2a fe ff ff 48 85 c0 75 cf 5b 5d c3 c6 85 fd 05 00 00 01 5b 5d c3 0f 1f 840
>>>> [   23.926876] RSP: 0018:ffffad2400043c98 EFLAGS: 00010246
>>>> [   23.927928] RAX: 0000000000000000 RBX: 0000000200000000 RCX: 0000000000000000
>>>> [   23.929458] RDX: 0000000000200000 RSI: 0000000000140000 RDI: 0000000000002f40
>>>> [   23.930899] RBP: 0000000140000000 R08: 0000000000000000 R09: 0000000000000001
>>>> [   23.932362] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000140000
>>>> [   23.933603] R13: 0000000000140000 R14: 0000000000002f40 R15: ffff9e3e7aff3680
>>>> [   23.934913] FS:  0000000000000000(0000) GS:ffff9e3e7bb00000(0000) knlGS:0000000000000000
>>>> [   23.936294] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [   23.937481] CR2: 000000000000353d CR3: 0000000058610000 CR4: 00000000000006e0
>>>> [   23.938687] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>> [   23.939889] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>> [   23.941168] Call Trace:
>>>> [   23.941580]  __remove_pages+0x4b/0x640
>>>> [   23.942303]  ? mark_held_locks+0x49/0x70
>>>> [   23.943149]  arch_remove_memory+0x63/0x8d
>>>> [   23.943921]  try_remove_memory+0xdb/0x130
>>>> [   23.944766]  ? walk_memory_blocks+0x7f/0x9e
>>>> [   23.945616]  __remove_memory+0xa/0x11
>>>> [   23.946274]  acpi_memory_device_remove+0x70/0x100
>>>> [   23.947308]  acpi_bus_trim+0x55/0x90
>>>> [   23.947914]  acpi_device_hotplug+0x227/0x3a0
>>>> [   23.948714]  acpi_hotplug_work_fn+0x1a/0x30
>>>> [   23.949433]  process_one_work+0x221/0x550
>>>> [   23.950190]  worker_thread+0x50/0x3b0
>>>> [   23.950993]  kthread+0x105/0x140
>>>> [   23.951644]  ? process_one_work+0x550/0x550
>>>> [   23.952508]  ? kthread_park+0x80/0x80
>>>> [   23.953367]  ret_from_fork+0x3a/0x50
>>>> [   23.954025] Modules linked in:
>>>> [   23.954613] CR2: 000000000000353d
>>>> [   23.955248] ---[ end trace 93d982b1fb3e1a69 ]---
>>>
>>> Yes, this is indeed nasty. I didin't think of this when separating
>>> memmap initialization from the hotremove. This means that the zone
>>> pointer is a garbage in arch_remove_memory already. The proper fix is to
>>> remove it from that level down. Moreover the zone is only needed for the
>>> shrinking code and zone continuous thingy. The later belongs to offlining
>>> code unless I am missing something. I can see that you are removing zone
>>> parameter in a later patch but wouldn't it be just better to remove the
>>> whole zone thing in a single patch and have this as a bug fix for a rare
>>> bug with a fixes tag?
>>>
>>
>> If I remember correctly, this patch already fixed the issue for me,
> 
> That might be the case because nothing else does access zone on the way.
> But the pointer is simply bogus. Removing it is the proper way to fix
> it. And I argue that zone shouldn't even be necessary. Re-evaluating
> continuous status of the zone is really something for offlining phase.
> Check how we use pfn_to_online_page there.

Yeah I'm with you, I think you spotted patch 6/6 of this series and v3
already that does exactly that. It's just a matter of rearranging things.

> 
>> without the other cleanup (removing the zone parameter). But I might be
>> wrong.
>>
>> Anyhow, I'll send a v4 shortly (either this evening or tomorrow), so you
>> can safe yourself some review time and wait for that one :)
> 
> No rush, really... It seems this is quite unlikely event as most hotplug
> usecases simply online memory before removing it later on.
> 

I can trigger it reliably right now while working/testing virtio-mem, so
I finally want to clean up this mess :) (has been on my list for a long
time). I'll try to hunt for the right commit id's that broke it.

-- 

Thanks,

David / dhildenb