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

[David Hildenbrand <david@redhat.com>]

On 25.09.25 12:50, Dev Jain wrote:
> 
> On 25/09/25 4:07 pm, David Hildenbrand wrote:
>> 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.
> 
> Ah alright, I was thinking that something may go wrong through
> folio_move_anon_rmap() in do_huge_pmd_wp_page, but

Right, there we have sine VMA and single PTL.

-- 
Cheers

David / dhildenb