[PATCH v4 1/4] mm: Optimize mprotect() for MM_CP_PROT_NUMA by batch-skipping PTEs

[Dev Jain]

In case of prot_numa, there are various cases in which we can skip to the
next iteration. Since the skip condition is based on the folio and not
the PTEs, we can skip a PTE batch. Additionally refactor all of this
into a new function to clean up the existing code.

Signed-off-by: Dev Jain <dev.jain at arm.com>
---
 mm/mprotect.c | 134 ++++++++++++++++++++++++++++++++------------------
 1 file changed, 87 insertions(+), 47 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 88709c01177b..af10a7fbe6b8 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -83,6 +83,83 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
 	return pte_dirty(pte);
 }
 
+static int mprotect_folio_pte_batch(struct folio *folio, unsigned long addr,
+		pte_t *ptep, pte_t pte, int max_nr_ptes)
+{
+	const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+
+	if (!folio || !folio_test_large(folio) || (max_nr_ptes == 1))
+		return 1;
+
+	return folio_pte_batch(folio, addr, ptep, pte, max_nr_ptes, flags,
+			       NULL, NULL, NULL);
+}
+
+static int prot_numa_skip_ptes(struct folio **foliop, struct vm_area_struct *vma,
+		unsigned long addr, pte_t oldpte, pte_t *pte, int target_node,
+		int max_nr_ptes)
+{
+	struct folio *folio = NULL;
+	int nr_ptes = 1;
+	bool toptier;
+	int nid;
+
+	/* Avoid TLB flush if possible */
+	if (pte_protnone(oldpte))
+		goto skip_batch;
+
+	folio = vm_normal_folio(vma, addr, oldpte);
+	if (!folio)
+		goto skip_batch;
+
+	if (folio_is_zone_device(folio) || folio_test_ksm(folio))
+		goto skip_batch;
+
+	/* Also skip shared copy-on-write pages */
+	if (is_cow_mapping(vma->vm_flags) &&
+	    (folio_maybe_dma_pinned(folio) || folio_maybe_mapped_shared(folio)))
+		goto skip_batch;
+
+	/*
+	 * While migration can move some dirty pages,
+	 * it cannot move them all from MIGRATE_ASYNC
+	 * context.
+	 */
+	if (folio_is_file_lru(folio) && folio_test_dirty(folio))
+		goto skip_batch;
+
+	/*
+	 * Don't mess with PTEs if page is already on the node
+	 * a single-threaded process is running on.
+	 */
+	nid = folio_nid(folio);
+	if (target_node == nid)
+		goto skip_batch;
+
+	toptier = node_is_toptier(nid);
+
+	/*
+	 * Skip scanning top tier node if normal numa
+	 * balancing is disabled
+	 */
+	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) && toptier)
+		goto skip_batch;
+
+	if (folio_use_access_time(folio)) {
+		folio_xchg_access_time(folio, jiffies_to_msecs(jiffies));
+
+		/* Do not skip in this case */
+		nr_ptes = 0;
+		goto out;
+	}
+
+skip_batch:
+	nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte, max_nr_ptes);
+out:
+	*foliop = folio;
+	return nr_ptes;
+}
+
 static long change_pte_range(struct mmu_gather *tlb,
 		struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr,
 		unsigned long end, pgprot_t newprot, unsigned long cp_flags)
@@ -94,6 +171,7 @@ static long change_pte_range(struct mmu_gather *tlb,
 	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
 	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
 	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
+	int nr_ptes;
 
 	tlb_change_page_size(tlb, PAGE_SIZE);
 	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
@@ -108,8 +186,11 @@ static long change_pte_range(struct mmu_gather *tlb,
 	flush_tlb_batched_pending(vma->vm_mm);
 	arch_enter_lazy_mmu_mode();
 	do {
+		nr_ptes = 1;
 		oldpte = ptep_get(pte);
 		if (pte_present(oldpte)) {
+			int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
+			struct folio *folio = NULL;
 			pte_t ptent;
 
 			/*
@@ -117,53 +198,12 @@ static long change_pte_range(struct mmu_gather *tlb,
 			 * pages. See similar comment in change_huge_pmd.
 			 */
 			if (prot_numa) {
-				struct folio *folio;
-				int nid;
-				bool toptier;
-
-				/* Avoid TLB flush if possible */
-				if (pte_protnone(oldpte))
-					continue;
-
-				folio = vm_normal_folio(vma, addr, oldpte);
-				if (!folio || folio_is_zone_device(folio) ||
-				    folio_test_ksm(folio))
-					continue;
-
-				/* Also skip shared copy-on-write pages */
-				if (is_cow_mapping(vma->vm_flags) &&
-				    (folio_maybe_dma_pinned(folio) ||
-				     folio_maybe_mapped_shared(folio)))
-					continue;
-
-				/*
-				 * While migration can move some dirty pages,
-				 * it cannot move them all from MIGRATE_ASYNC
-				 * context.
-				 */
-				if (folio_is_file_lru(folio) &&
-				    folio_test_dirty(folio))
-					continue;
-
-				/*
-				 * Don't mess with PTEs if page is already on the node
-				 * a single-threaded process is running on.
-				 */
-				nid = folio_nid(folio);
-				if (target_node == nid)
-					continue;
-				toptier = node_is_toptier(nid);
-
-				/*
-				 * Skip scanning top tier node if normal numa
-				 * balancing is disabled
-				 */
-				if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) &&
-				    toptier)
+				nr_ptes = prot_numa_skip_ptes(&folio, vma,
+							      addr, oldpte, pte,
+							      target_node,
+							      max_nr_ptes);
+				if (nr_ptes)
 					continue;
-				if (folio_use_access_time(folio))
-					folio_xchg_access_time(folio,
-						jiffies_to_msecs(jiffies));
 			}
 
 			oldpte = ptep_modify_prot_start(vma, addr, pte);
@@ -280,7 +320,7 @@ static long change_pte_range(struct mmu_gather *tlb,
 				pages++;
 			}
 		}
-	} while (pte++, addr += PAGE_SIZE, addr != end);
+	} while (pte += nr_ptes, addr += nr_ptes * PAGE_SIZE, addr != end);
 	arch_leave_lazy_mmu_mode();
 	pte_unmap_unlock(pte - 1, ptl);
 
-- 
2.30.2