mm: add pte_batch_hint() to reduce scanning in folio_pte_batch()

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.

Link: https://lkml.kernel.org/r/20240215103205.2607016-16-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pgtable.h

@@ -212,6 +212,27 @@ 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.
+ *
+ * An architecture implementation may ignore the PTE accessed state. Further,
+ * the dirty state must apply atomically to all the PTEs described by the hint.
+ *
+ * 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)
 {

mm/memory.c

@@ -988,16 +988,20 @@ 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_next_pfn(pte), flags);
-	pte_t *ptep = start_ptep + 1;
+	pte_t expected_pte, *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(start_ptep, pte);
+	expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
+	ptep = start_ptep + nr;
+
+	while (ptep < end_ptep) {
 		pte = ptep_get(ptep);
 		if (any_writable)
 			writable = !!pte_write(pte);
@@ -1011,17 +1015,18 @@ 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_next_pfn(expected_pte);
-		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);
 }
 
 /*