Re: [PATCH v2 2/2] mm/memory_hotplug: fix hwpoisoned large folio handling in do_migrate_range

[David Hildenbrand <david@redhat.com>]

On 07.07.25 13:51, Jinjiang Tu wrote:
> 
> 在 2025/7/3 17:06, David Hildenbrand 写道:
>> On 03.07.25 10:24, Jinjiang Tu wrote:
>>>
>>> 在 2025/7/3 15:57, David Hildenbrand 写道:
>>>> On 03.07.25 09:46, Jinjiang Tu wrote:
>>>>>
>>>>> 在 2025/7/1 22:21, Oscar Salvador 写道:
>>>>>> On Fri, Jun 27, 2025 at 08:57:47PM +0800, Jinjiang Tu wrote:
>>>>>>> In do_migrate_range(), the hwpoisoned folio may be large folio,
>>>>>>> which
>>>>>>> can't be handled by unmap_poisoned_folio().
>>>>>>>
>>>>>>> I can reproduce this issue in qemu after adding delay in
>>>>>>> memory_failure()
>>>>>>>
>>>>>>> BUG: kernel NULL pointer dereference, address: 0000000000000000
>>>>>>> Workqueue: kacpi_hotplug acpi_hotplug_work_fn
>>>>>>> RIP: 0010:try_to_unmap_one+0x16a/0xfc0
>>>>>>>      <TASK>
>>>>>>>      rmap_walk_anon+0xda/0x1f0
>>>>>>>      try_to_unmap+0x78/0x80
>>>>>>>      ? __pfx_try_to_unmap_one+0x10/0x10
>>>>>>>      ? __pfx_folio_not_mapped+0x10/0x10
>>>>>>>      ? __pfx_folio_lock_anon_vma_read+0x10/0x10
>>>>>>>      unmap_poisoned_folio+0x60/0x140
>>>>>>>      do_migrate_range+0x4d1/0x600
>>>>>>>      ? slab_memory_callback+0x6a/0x190
>>>>>>>      ? notifier_call_chain+0x56/0xb0
>>>>>>>      offline_pages+0x3e6/0x460
>>>>>>>      memory_subsys_offline+0x130/0x1f0
>>>>>>>      device_offline+0xba/0x110
>>>>>>>      acpi_bus_offline+0xb7/0x130
>>>>>>>      acpi_scan_hot_remove+0x77/0x290
>>>>>>>      acpi_device_hotplug+0x1e0/0x240
>>>>>>>      acpi_hotplug_work_fn+0x1a/0x30
>>>>>>>      process_one_work+0x186/0x340
>>>>>>>
>>>>>>> In this case, just make offline_pages() fail.
>>>>>>>
>>>>>>> Besides, do_migrate_range() may be called between memory_failure set
>>>>>>> hwposion flag and ioslate the folio from lru, so remove WARN_ON().
>>>>>>> In other
>>>>>>> places, unmap_poisoned_folio() is called when the folio is
>>>>>>> isolated, obey
>>>>>>> it in do_migrate_range() too.
>>>>>>>
>>>>>>> Fixes: b15c87263a69 ("hwpoison, memory_hotplug: allow hwpoisoned
>>>>>>> pages to be offlined")
>>>>>>> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
>>>>>> ...
>>>>>>> @@ -2041,11 +2048,9 @@ int offline_pages(unsigned long start_pfn,
>>>>>>> unsigned long nr_pages,
>>>>>>>                     ret = scan_movable_pages(pfn, end_pfn, &pfn);
>>>>>>>                  if (!ret) {
>>>>>>> -                /*
>>>>>>> -                 * TODO: fatal migration failures should bail
>>>>>>> -                 * out
>>>>>>> -                 */
>>>>>>> -                do_migrate_range(pfn, end_pfn);
>>>>>>> +                ret = do_migrate_range(pfn, end_pfn);
>>>>>>> +                if (ret)
>>>>>>> +                    break;
>>>>>> I am not really sure about this one.
>>>>>> I get the reason you're adding it, but note that migrate_pages() can
>>>>>> also return
>>>>>> "fatal" errors and we don't propagate that.
>>>>>>
>>>>>> The moto has always been to migrate as much as possible, and this
>>>>>> changes this
>>>>>> behaviour.
>>>>> If we just skip to next pfn, offline_pages() will deadloop meaningless
>>>>> util received signal.
>>>>
>>>> Yeah, that's also not good,
>>>>
>>>>> It seems there is no document to guarantee memory offline have to
>>>>> migrate as much as possible.
>>>>
>>>> We should try offlining as good as possible. But if there is something
>>>> we just cannot possibly migrate, there is no sense in retrying.
>>>>
>>>> Now, could we run into this case here because we are racing with other
>>>> code, and actually retrying again could make it work?
>>>>
>>>> Remind me again: how exactly do we arrive at this point of having a
>>>> large folio that is hwpoisoned but still mapped?
>>>>
>>>> In memory_failure(), we do on a  large folio
>>>>
>>>> 1) folio_set_has_hwpoisoned
>>>> 2) try_to_split_thp_page
>>>> 3) if splitting fails, kill_procs_now
>>> If 2) is executed when do_migrate_range() increment the refcount of the
>>> folio, the split fails, and retry is meaningless.
>>
>> kill_procs_now will kill all processes, effectively unmapping the
>> folio in that case?
>>
>> So retrying would later just ... get us an unmapped folio and we can
>> make progress?
>>
> kill_procs_now()->collect_procs() collects the tasks to kill. But not
> all tasks that maps the folio
> will be collected,
> collect_procs_anon()->task_early_kill()->find_early_kill_thread() will not
> select the task (not current) if PF_MCE_PROCESS isn't set and
> sysctl_memory_failure_early_kill
> isn't enabled (this is the default behaviour).

I think you're right, that's rather nasty.

We fail to split, but keep the folio mapped into some processes.

And we can't unmap it because unmap_poisoned_folio() does not properly
support large folios yet.

We really should unmap the folio when splitting fail. :(

-- 
Cheers,

David / dhildenb