Re: [PATCH v3 2/2] mm/mprotect: special-case small folios when applying write permissions

Davidlohr Bueso
2026-04-07T00:50:36Z

On Thu, 02 Apr 2026, Pedro Falcato wrote:

>@@ -334,34 +371,20 @@ static long change_pte_range(struct mmu_gather *tlb,
>
> nr_ptes = mprotect_folio_pte_batch(folio, pte, oldpte, max_nr_ptes, flags);
>
>- oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes);
>- ptent = pte_modify(oldpte, newprot);
>-
>- if (uffd_wp)
>- ptent = pte_mkuffd_wp(ptent);
>- else if (uffd_wp_resolve)
>- ptent = pte_clear_uffd_wp(ptent);
>-
> /*
>- * In some writable, shared mappings, we might want
>- * to catch actual write access -- see
>- * vma_wants_writenotify().
>- *
>- * In all writable, private mappings, we have to
>- * properly handle COW.
>- *
>- * In both cases, we can sometimes still change PTEs
>- * writable and avoid the write-fault handler, for
>- * example, if a PTE is already dirty and no other
>- * COW or special handling is required.
>+ * Optimize for the small-folio common case by
>+ * special-casing it here. Compiler constant propagation
>+ * plus copious amounts of __always_inline does wonders.
> */
>- if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>- !pte_write(ptent))
>- set_write_prot_commit_flush_ptes(vma, folio, page,
>- addr, pte, oldpte, ptent, nr_ptes, tlb);
>- else
>- prot_commit_flush_ptes(vma, addr, pte, oldpte, ptent,
>- nr_ptes, /* idx = */ 0, /* set_write = */ false, tlb);
>+ if (likely(nr_ptes == 1)) {

Are there any numbers for this optimization? While I am all for optimizing the common
case, it seems unfair to penalize the uncommon one here. Why is nr_ptes > 1 such an
exotic use case (specially today)? ie: How does this change affect the program in
b9bf6c2872c ("mm: refactor MM_CP_PROT_NUMA skipping case into new function"),
which is a series for optimizing large folio cases.

Thanks,
Davidlohr

>+ change_present_ptes(tlb, vma, addr, pte, 1,
>+ end, newprot, folio, page, cp_flags);
>+ } else {
>+ change_present_ptes(tlb, vma, addr, pte,
>+ nr_ptes, end, newprot, folio, page,
>+ cp_flags);
>+ }
>+
> pages += nr_ptes;
> } else if (pte_none(oldpte)) {
> /*
>--
>2.53.0
>
>