[PATCH v2] arm64/mm: Elide TLB flush in certain pte protection transitions

[Dev Jain]

Currently arm64 does an unconditional TLB flush in mprotect(). This is not
required for some cases, for example, when changing from PROT_NONE to
PROT_READ | PROT_WRITE (a real usecase - glibc malloc does this to emulate
growing into the non-main heaps), and unsetting uffd-wp in a range.

Therefore, implement pte_needs_flush() for arm64, which is already
implemented by some other arches as well.

Running a userspace program changing permissions back and forth between
PROT_NONE and PROT_READ | PROT_WRITE, and measuring the average time taken
for the none->rw transition, I get a reduction from 3.2 microseconds to
2.85 microseconds, giving a 12.3% improvement.

Reviewed-by: Kefeng Wang <wangkefeng.wang at huawei.com>
Signed-off-by: Dev Jain <dev.jain at arm.com>
---
mm-selftests pass.

v1->v2:
 - Drop PTE_PRESENT_INVALID and PTE_AF checks, use ptdesc_t instead of
   pteval_t, return !!diff (Ryan)

 arch/arm64/include/asm/tlbflush.h | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
index 18a5dc0c9a54..40df783ba09a 100644
--- a/arch/arm64/include/asm/tlbflush.h
+++ b/arch/arm64/include/asm/tlbflush.h
@@ -524,6 +524,33 @@ static inline void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *b
 {
 	__flush_tlb_range_nosync(mm, start, end, PAGE_SIZE, true, 3);
 }
+
+static inline bool __pte_flags_need_flush(ptdesc_t oldval, ptdesc_t newval)
+{
+	ptdesc_t diff = oldval ^ newval;
+
+	/* invalid to valid transition requires no flush */
+	if (!(oldval & PTE_VALID))
+		return false;
+
+	/* Transition in the SW bits requires no flush */
+	diff &= ~PTE_SWBITS_MASK;
+
+	return !!diff;
+}
+
+static inline bool pte_needs_flush(pte_t oldpte, pte_t newpte)
+{
+	return __pte_flags_need_flush(pte_val(oldpte), pte_val(newpte));
+}
+#define pte_needs_flush pte_needs_flush
+
+static inline bool huge_pmd_needs_flush(pmd_t oldpmd, pmd_t newpmd)
+{
+	return __pte_flags_need_flush(pmd_val(oldpmd), pmd_val(newpmd));
+}
+#define huge_pmd_needs_flush huge_pmd_needs_flush
+
 #endif
 
 #endif
-- 
2.30.2