Re: [PATCH v2 2/2] mm/mprotect: special-case small folios when applying write permissions

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 30 Mar 2026 17:16:51 +0200 "David Hildenbrand (Arm)" <david@kernel.org> wrote:

> > That could also work, but then set_write_prot_commit_flush_ptes (holy cow
> > what a long name) would definitely need inlining. And might be a little uglier
> > overall.
> 
> Right. The idea is that you __always__inline any code that has PTE
> loops, such that all loops for nr_pages == 1 gets optimized out.
> 
> We do that for zap and fork logic.
> 
> > 
> > This is the part where having data points other than my giga-fast-giga-powerful
> > zen5 could prove handy :/
> I just recently lost access to my reliably, well tunes, system ...
> 
> Is it just the following benchmark?
> 
> 	https://gist.github.com/heatd/1450d273005aba91fa5744f44dfcd933
> 
> ?
> 
> 
> I can easily extending
> 
> 	https://gitlab.com/davidhildenbrand/scratchspace/-/blob/main/pte-mapped-folio-benchmarks.c
> 
> to have an "mprotect" mode. I had that in the past bit discarded it.
> 
> Then, we can easily measure the effect on various folio sizes when
> mprotect'ing a larger memory area.
> 
> With order-0 we can then benchmark small folios exclusively.

It sounds like this is all possible future work?

We have Lorenzo's R-b on this [2/2].  I'm reading this discussion as
"upstream both"?

--- a/mm/mprotect.c~mm-mprotect-special-case-small-folios-when-applying-write-permissions
+++ a/mm/mprotect.c
@@ -103,7 +103,7 @@ bool can_change_pte_writable(struct vm_a
 	return can_change_shared_pte_writable(vma, pte);
 }
 
-static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
+static __always_inline int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
 				    pte_t pte, int max_nr_ptes, fpb_t flags)
 {
 	/* No underlying folio, so cannot batch */
@@ -117,9 +117,9 @@ static int mprotect_folio_pte_batch(stru
 }
 
 /* Set nr_ptes number of ptes, starting from idx */
-static void prot_commit_flush_ptes(struct vm_area_struct *vma, unsigned long addr,
-		pte_t *ptep, pte_t oldpte, pte_t ptent, int nr_ptes,
-		int idx, bool set_write, struct mmu_gather *tlb)
+static __always_inline void prot_commit_flush_ptes(struct vm_area_struct *vma,
+		unsigned long addr, pte_t *ptep, pte_t oldpte, pte_t ptent,
+		int nr_ptes, int idx, bool set_write, struct mmu_gather *tlb)
 {
 	/*
 	 * Advance the position in the batch by idx; note that if idx > 0,
@@ -169,7 +169,7 @@ static int page_anon_exclusive_sub_batch
  * pte of the batch. Therefore, we must individually check all pages and
  * retrieve sub-batches.
  */
-static void commit_anon_folio_batch(struct vm_area_struct *vma,
+static __always_inline void commit_anon_folio_batch(struct vm_area_struct *vma,
 		struct folio *folio, struct page *first_page, unsigned long addr, pte_t *ptep,
 		pte_t oldpte, pte_t ptent, int nr_ptes, struct mmu_gather *tlb)
 {
@@ -177,6 +177,13 @@ static void commit_anon_folio_batch(stru
 	int sub_batch_idx = 0;
 	int len;
 
+	/* Optimize for the common order-0 case. */
+	if (likely(nr_ptes == 1)) {
+		prot_commit_flush_ptes(vma, addr, ptep, oldpte, ptent, 1,
+				       0, PageAnonExclusive(first_page), tlb);
+		return;
+	}
+
 	while (nr_ptes) {
 		expected_anon_exclusive = PageAnonExclusive(first_page + sub_batch_idx);
 		len = page_anon_exclusive_sub_batch(sub_batch_idx, nr_ptes,
_