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

[Davidlohr Bueso <dave@stgolabs.net>]

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
>
>