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

[Zi Yan <ziy@nvidia.com>]

On 14 Jul 2025, at 11:25, Zi Yan wrote:

> On 14 Jul 2025, at 11:14, David Hildenbrand wrote:
>
>> On 14.07.25 17:09, Pankaj Raghav (Samsung) wrote:
>>>>>>> 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?
>>>>
>>> Hmm, maybe I misunderstood the question?
>>>
>>>> E.g., just mmap() a single page of such a file? Who would make that fail?
>>>>
>>>
>>> My point was, even if you try to mmap a single page of a file, page
>>> cache will read the whole block (that corresponds to min order folio).
>>>
>>> Technically we can mmap a single page of file, but FS will always read
>>> and write **at least** in min folio order chunks.
>>
>> Okay, so it can be partially mapped into page tables :) What happens in the background (page cache management) is a different story
>
> David, thanks for getting to the bottom of this.
>
> OK. So we will see deadlock looping in madvise cold or pageout case.
> I wonder how to proceed with this. Since the folio is seen as a whole
> by fs, it should be marked cold/paged out as a whole. Maybe we should
> skip the partially mapped region?

Actually, it is skipped, since split_folio() bumps new_order to the min
order, and if the folio order is already at min order, split code return
-EINVAL. This makes the madvise cold or pageout code move to the next
address.

Best Regards,
Yan, Zi