Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch

[Lorenzo Stoakes]

On Fri, Aug 08, 2025 at 09:45:39AM +0200, David Hildenbrand wrote:
> I am probably to blame here, because I think I rejected early to have
> arm64-only optimization, assuming other arch could benefit here as well with
> batching. But as it seems, batching in mremap() code really only serves the
> cont-pte managing code, and the folio_pte_batch() is really entirely
> unnecessary.
>
> In case of mprotect(), I think really only (a) NUMA and (b) anon-folio
> write-upgrade required the folio. So it's a bit more tricky than mremap()
> here where ... the folio is entirely irrelevant.
>
> One could detect the "anon write-upgrade possible" case early as well, and
> only lookup the folio in that case, otherwise use the straight pte hint.
>
> So I think there is some room for further improvement.

These are all great ideas and given we handle A/D correctly in mremap case due
to get_and_clear_ptes() + additionally Ryan has hinted that it shouldn't be a
big deal to lose the 'what if there are other batches/physically contiguous
entries' logic, then your suggestion for simplification, that is - litearlly
having mremap-folio_pte_batch() figure out the result from the pte batch hint -
is probably the best way.

We could RFC it and Dev could go check arm64 perf perhaps?

Cheers, Lorenzo