[PATCH 5/5] arm64: mm: implement the architecture-specific test_and_clear_young_ptes()

[Baolin Wang]

Implement the Arm64 architecture-specific test_and_clear_young_ptes() to enable
batched checking of young flags, improving performance during large folio
reclamation when MGLRU is enabled.

While we're at it, simplify ptep_test_and_clear_young() by calling
test_and_clear_young_ptes(). Since callers guarantee that PTEs are present
before calling these functions, we can use pte_cont() to check the CONT_PTE
flag instead of pte_valid_cont().

Performance testing:
Enable MGLRU, then 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 60%+ performance improvement on my Arm64 32-core server (and about
15% improvement on my X86 machine).

W/o patchset:
real	0m0.470s
user	0m0.000s
sys	0m0.470s

W/ patchset:
real	0m0.180s
user	0m0.001s
sys	0m0.179s

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
 arch/arm64/include/asm/pgtable.h | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index aa4b13da6371..ab451d20e4c5 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1812,16 +1812,22 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 	return __ptep_get_and_clear(mm, addr, ptep);
 }
 
+#define test_and_clear_young_ptes test_and_clear_young_ptes
+static inline int test_and_clear_young_ptes(struct vm_area_struct *vma,
+					    unsigned long addr, pte_t *ptep,
+					    unsigned int nr)
+{
+	if (likely(nr == 1 && !pte_cont(__ptep_get(ptep))))
+		return __ptep_test_and_clear_young(vma, addr, ptep);
+
+	return contpte_test_and_clear_young_ptes(vma, addr, ptep, nr);
+}
+
 #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
 static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
 				unsigned long addr, pte_t *ptep)
 {
-	pte_t orig_pte = __ptep_get(ptep);
-
-	if (likely(!pte_valid_cont(orig_pte)))
-		return __ptep_test_and_clear_young(vma, addr, ptep);
-
-	return contpte_test_and_clear_young_ptes(vma, addr, ptep, 1);
+	return test_and_clear_young_ptes(vma, addr, ptep, 1);
 }
 
 #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
-- 
2.47.3