Re: [PATCH] mm: move rmap of mTHP upon CoW reuse

[David Hildenbrand <david@redhat.com>]

On 25.09.25 12:33, Dev Jain wrote:
> 
> On 25/09/25 2:46 pm, David Hildenbrand wrote:
>> On 25.09.25 10:54, Dev Jain wrote:
>>> At wp-fault time, when we find that a folio is exclusively mapped, we
>>> move
>>> folio->mapping to the faulting VMA's anon_vma, so that rmap overhead
>>> reduces. This is currently done for small folios (base pages) and
>>> PMD-mapped THPs. Do this for mTHP too.
>>
>> I deliberately didn't add this back then because I was not able to
>> convince myself easily that it is ok in all corner cases. So this
>> needs some thought.
> 
> Thanks for your detailed reply.
> 
> 
>>
>>
>> We know that the folio is exclusively mapped to a single MM and that
>> there are no unexpected references from others (GUP pins, whatsoever).
>>
>> But a large folio might be
>>
>> (a) mapped into multiple VMAs (e.g., partial mprotect()) in the same MM
> 
> I think we have the same problem then for PMD-THPs? I see that
> vma_adjust_trans_huge() only does a PMD split and not folio split.

Sure, we can end up in this reuse function here for any large anon 
folio, including PMD ones after a PMD->PTE remapping.

-- 
Cheers

David / dhildenb