Re: [PATCH v2 2/3] mm: rmap: support batched checks of the references for large folios

Currently, folio_referenced_one() always checks the young flag for each PTE
sequentially, which is inefficient for large folios. This inefficiency is
especially noticeable when reclaiming clean file-backed large folios, where
folio_referenced() is observed as a significant performance hotspot.

Moreover, on Arm architecture, which supports contiguous PTEs, there is already
an optimization to clear the young flags for PTEs within a contiguous range.
However, this is not sufficient. We can extend this to perform batched operations
for the entire large folio (which might exceed the contiguous range: CONT_PTE_SIZE).

Introduce a new API: clear_flush_young_ptes() to facilitate batched checking
of the young flags and flushing TLB entries, thereby improving performance
during large folio reclamation.

Performance testing:
Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to
reclaim 8G file-backed folios via the memory.reclaim interface. I can observe
33% performance improvement on my Arm64 32-core server (and 10%+ improvement
on my X86 machine). Meanwhile, the hotspot folio_check_references() dropped
from approximately 35% to around 5%.

W/o patchset:
real	0m1.518s
user	0m0.000s
sys	0m1.518s

W/ patchset:
real	0m1.018s
user	0m0.000s
sys	0m1.018s

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 arch/arm64/include/asm/pgtable.h | 11 +++++++++++
 include/linux/mmu_notifier.h     |  9 +++++----
 include/linux/pgtable.h          | 19 +++++++++++++++++++
 mm/rmap.c                        | 22 ++++++++++++++++++++--
 4 files changed, 55 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index e03034683156..a865bd8c46a3 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -1869,6 +1869,17 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
 	return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES);
 }
 
+#define clear_flush_young_ptes clear_flush_young_ptes
+static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
+					unsigned long addr, pte_t *ptep,
+					unsigned int nr)
+{
+	if (likely(nr == 1))
+		return __ptep_clear_flush_young(vma, addr, ptep);

+
+	return contpte_clear_flush_young_ptes(vma, addr, ptep, nr);
+}
+
 #define wrprotect_ptes wrprotect_ptes
 static __always_inline void wrprotect_ptes(struct mm_struct *mm,
 				unsigned long addr, pte_t *ptep, unsigned int nr)

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index d1094c2d5fb6..be594b274729 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h

@@ -515,16 +515,17 @@ static inline void mmu_notifier_range_init_owner(
 	range->owner = owner;
 }
 
-#define ptep_clear_flush_young_notify(__vma, __address, __ptep)		\
+#define ptep_clear_flush_young_notify(__vma, __address, __ptep, __nr)	\

 
 #define mmu_notifier_range_update_to_read_only(r) false
 
-#define ptep_clear_flush_young_notify ptep_clear_flush_young
+#define ptep_clear_flush_young_notify clear_flush_young_ptes
 #define pmdp_clear_flush_young_notify pmdp_clear_flush_young
 #define ptep_clear_young_notify ptep_test_and_clear_young
 #define pmdp_clear_young_notify pmdp_test_and_clear_young

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index b13b6f42be3c..c7d0fd228cb7 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h

@@ -947,6 +947,25 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
 }
 #endif
 
+#ifndef clear_flush_young_ptes

+		young |= ptep_clear_flush_young(vma, addr, ptep);
+		if (--nr == 0)
+			break;
+		ptep++;
+		addr += PAGE_SIZE;
+	}
+
+	return young;
+}
+#endif
+
 /*
  * On some architectures hardware does not set page access bit when accessing
  * memory page, it is responsibility of software setting this bit. It brings

diff --git a/mm/rmap.c b/mm/rmap.c
index d6799afe1114..ec232165c47d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c

@@ -827,9 +827,11 @@ static bool folio_referenced_one(struct folio *folio,
 	struct folio_referenced_arg *pra = arg;
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
 	int ptes = 0, referenced = 0;
+	unsigned int nr;
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		address = pvmw.address;
+		nr = 1;
 
 		if (vma->vm_flags & VM_LOCKED) {
 			ptes++;

@@ -874,9 +876,21 @@ static bool folio_referenced_one(struct folio *folio,
 			if (lru_gen_look_around(&pvmw))
 				referenced++;
 		} else if (pvmw.pte) {
+			if (folio_test_large(folio)) {
+				unsigned long end_addr = pmd_addr_end(address, vma->vm_end);
+				unsigned int max_nr = (end_addr - address) >> PAGE_SHIFT;
+				pte_t pteval = ptep_get(pvmw.pte);
+
+				nr = folio_pte_batch(folio, pvmw.pte, pteval, max_nr);
+			}
+
+			ptes += nr;
 			if (ptep_clear_flush_young_notify(vma, address,
-						pvmw.pte))
+						pvmw.pte, nr))
 				referenced++;
+			/* Skip the batched PTEs */
+			pvmw.pte += nr - 1;
+			pvmw.address += (nr - 1) * PAGE_SIZE;

 		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 			if (pmdp_clear_flush_young_notify(vma, address,
 						pvmw.pmd))

@@ -886,7 +900,11 @@ static bool folio_referenced_one(struct folio *folio,
 			WARN_ON_ONCE(1);
 		}
 
-		pra->mapcount--;
+		pra->mapcount -= nr;
+		if (ptes == pvmw.nr_pages) {
+			page_vma_mapped_walk_done(&pvmw);
+			break;