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

[David Hildenbrand <david@redhat.com>]

On 14.07.25 16:20, Zi Yan wrote:
> On 14 Jul 2025, at 9:53, Pankaj Raghav wrote:
> 
>> Hi Zi Yan,
>>
>>>>
>>>> Probably the WARN_ON can indeed trigger now.
>>>>
>>>>
>>>> @Zi Yan, on a related note ...
>>>>
>>>> in memory_failure(), we call try_to_split_thp_page(). If it works,
>>>> we assume that we have a small folio.
>>>>
>>>> But that is not the case if split_huge_page() cannot split it to
>>>> order-0 ... min_order_for_split().
>>>
>>> Right. memory failure needs to learn about this. Either poison every
>>> subpage or write back if necessary and drop the page cache folio.
>>>
>>>>
>>>> I'm afraid we havbe more such code that does not expect that if split_huge_page()
>>>> succeeds that we still have a large folio ...
>>>
>>> I did some search, here are the users of split_huge_page*():
>>>
>>> 1. ksm: since it is anonymous only, so split always goes to order-0;
>>> 2. userfaultfd: it is also anonymous;
>>> 3. madvise cold or pageout: a large pagecache folio will be split if it is partially
>>>     mapped. And it will retry. It might cause a deadlock if the folio has a min order.
>>> 4. shmem: split always goes to order-0;
>>> 5. memory-failure: see above.
>>>
>>> So we will need to take care of madvise cold or pageout case?
>>>
>>> Hi Matthew, Pankaj, and Luis,
>>>
>>> Is it possible to partially map a min-order folio in a fs with LBS? Based on my
>>
>> Typically, FSs match the min order with the blocksize of the filesystem.
>> As a filesystem block is the smallest unit of data that the filesystem uses
>> to store file data on the disk, we cannot partially map them.
>>
>> So if I understand your question correctly, the answer is no.

I'm confused. Shouldn't this be trivially possible?

E.g., just mmap() a single page of such a file? Who would make that fail?

And if it doesn't fail, who would stop us from munmap()'ing everything 
except a single page.

-- 
Cheers,

David / dhildenb