Re: [PATCH 0/3] Batch unmap of uffd-wp file folios

[Dev Jain]

On 16/07/26 12:28 am, Andrew Morton wrote:
> On Wed, 15 Jul 2026 11:18:33 +0000 Dev Jain <dev.jain@xxxxxxx> wrote:
> 
>> Currently, batched unmapping is supported if:
>>
>> 1) folio is a file folio, not belonging to uffd-wp VMA
>> 2) folio is anonymous and not swapbacked (lazyfree), not belonging to
>>    uffd-wp VMA
>>
>> So the cases which are not supported are
>>
>> 1) folio belonging to uffd-wp VMA
>> 2) folio is anonymous and swapbacked
>>
>> It is easy to see that this adds a lot of cognitive load while reading
>> try_to_unmap_one - we need to remember throughout whether nr_pages == 1
>> or > 1.
>>
>> The uffd-wp handling in try_to_unmap_one is regarding preserving the
>> uffd-wp state for file folios via pte_install_uffd_wp_if_needed (for anon
>> folio, we handle that while constructing the swap pte).
>>
>> Stop special casing on uffd-wp VMAs by simply adding batching support
>> to pte_install_uffd_wp_if_needed.
> 
> Thanks, I'll await reviewer input on this.
> 
> Sashiko might have found a pre-existing issue:
> 	https://sashiko.dev/#/patchset/20260715111839.1667914-1-dev.jain@xxxxxxx

That was unexpected from Sashiko.

"When a large folio registered with uffd-wp is unmapped via
try_to_unmap_one(), folio_unmap_pte_batch() batches and clears multiple PTEs."

At patch 2, there is no batching support yet for uffd-wp folios.