Re: [PATCH v2] mm: prohibit the last subpage from reusing the entire large folio

[Ryan Roberts]

On 08/03/2024 13:24, David Hildenbrand wrote:
>>>> This patch migrates the last subpage to a small folio and immediately
>>>> returns the large folio to the system. It benefits both memory availability
>>>> and anti-fragmentation.
>>>
>>> It might be controversial optimization, and as Ryan said, there, are likely
>>> other cases where we'd want to migrate off-of a thp if possible earlier.
>>
>> Personally, I think there might also be cases where you want to copy/reuse the
>> entire large folio. If you're application is using 16K THPs perhaps it's a
>> bigger win to just treat it like a base page? I expect the cost/benefit will
>> change as the THP size increases?
> 
> Yes, I think for small folios (i.e., 16KiB) it will be rather easy to make a
> decision. The larger the folio, the larger the page fault latency due to
> scanning, copying, modifying, which can easily turn undesirable.
> 
> At least when it comes to page reuse, I have some simple backup plans for small
> folios if I won't be able to make progress with my other approach. 

Do you mean "small large folios" here? i.e. order >= 1? If so, great!


For larger
> folios, it won't really work/be desirable, though.
>