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

[Jinjiang Tu <tujinjiang@huawei.com>]

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).