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 17:28, Zi Yan wrote:
> 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.

But what if the folio order is 2x min_order etc?

-- 
Cheers,

David / dhildenb