[PATCH v5 22/25] mm: Add pte_batch_hint() to reduce scanning in folio_pte_batch()

[Ryan Roberts <ryan.roberts@arm.com>]

Some architectures (e.g. arm64) can tell from looking at a pte, if some
follow-on ptes also map contiguous physical memory with the same pgprot.
(for arm64, these are contpte mappings).

Take advantage of this knowledge to optimize folio_pte_batch() so that
it can skip these ptes when scanning to create a batch. By default, if
an arch does not opt-in, folio_pte_batch() returns a compile-time 1, so
the changes are optimized out and the behaviour is as before.

arm64 will opt-in to providing this hint in the next patch, which will
greatly reduce the cost of ptep_get() when scanning a range of contptes.

Tested-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 include/linux/pgtable.h | 18 ++++++++++++++++++
 mm/memory.c             | 20 +++++++++++++-------
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 50f32cccbd92..cba31f177d27 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -212,6 +212,24 @@ static inline int pmd_dirty(pmd_t pmd)
 #define arch_flush_lazy_mmu_mode()	do {} while (0)
 #endif
 
+#ifndef pte_batch_hint
+/**
+ * pte_batch_hint - Number of pages that can be added to batch without scanning.
+ * @ptep: Page table pointer for the entry.
+ * @pte: Page table entry.
+ *
+ * Some architectures know that a set of contiguous ptes all map the same
+ * contiguous memory with the same permissions. In this case, it can provide a
+ * hint to aid pte batching without the core code needing to scan every pte.
+ *
+ * May be overridden by the architecture, else pte_batch_hint is always 1.
+ */
+static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
+{
+	return 1;
+}
+#endif
+
 #ifndef pte_advance_pfn
 static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
 {
diff --git a/mm/memory.c b/mm/memory.c
index 65fbe4f886c1..902665b27702 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -988,16 +988,21 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
 {
 	unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
 	const pte_t *end_ptep = start_ptep + max_nr;
-	pte_t expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, 1), flags);
-	pte_t *ptep = start_ptep + 1;
+	pte_t expected_pte = __pte_batch_clear_ignored(pte, flags);
+	pte_t *ptep = start_ptep;
 	bool writable;
+	int nr;
 
 	if (any_writable)
 		*any_writable = false;
 
 	VM_WARN_ON_FOLIO(!pte_present(pte), folio);
 
-	while (ptep != end_ptep) {
+	nr = pte_batch_hint(ptep, pte);
+	expected_pte = pte_advance_pfn(expected_pte, nr);
+	ptep += nr;
+
+	while (ptep < end_ptep) {
 		pte = ptep_get(ptep);
 		if (any_writable)
 			writable = !!pte_write(pte);
@@ -1011,17 +1016,18 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
 		 * corner cases the next PFN might fall into a different
 		 * folio.
 		 */
-		if (pte_pfn(pte) == folio_end_pfn)
+		if (pte_pfn(pte) >= folio_end_pfn)
 			break;
 
 		if (any_writable)
 			*any_writable |= writable;
 
-		expected_pte = pte_advance_pfn(expected_pte, 1);
-		ptep++;
+		nr = pte_batch_hint(ptep, pte);
+		expected_pte = pte_advance_pfn(expected_pte, nr);
+		ptep += nr;
 	}
 
-	return ptep - start_ptep;
+	return min(ptep - start_ptep, max_nr);
 }
 
 /*
-- 
2.25.1