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

[Dev Jain <dev.jain@arm.com>]

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.

> [--- snip ---]
>