kernel: add z_num_pagefaults_get()

Simple counter of number of successfully handled page faults by
the core kernel.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

kernel/include/mmu.h

@@ -355,6 +355,16 @@ void z_backing_store_init(void);
  * Core kernel demand paging APIs
  */
 
+/**
+ * Number of page faults since system startup
+ *
+ * Counts only those page faults that were handled successfully by the demand
+ * paging mechanism and were not errors.
+ *
+ * @return Number of successful page faults
+ */
+unsigned long z_num_pagefaults_get(void);
+
 /**
  * Free a page frame physical address by evicting its contents
  *

kernel/mmu.c

@@ -564,6 +564,8 @@ void z_mem_manage_init(void)
 }
 
 #ifdef CONFIG_DEMAND_PAGING
+static unsigned long z_num_pagefaults;
+
 /* Current implementation relies on interrupt locking to any prevent page table
  * access, which falls over if other CPUs are active. Addressing this is not
  * as simple as using spinlocks as regular memory reads/writes constitute
@@ -944,7 +946,30 @@ void z_mem_pin(void *addr, size_t size)
 
 bool z_page_fault(void *addr)
 {
-	return do_page_fault(addr, false);
+	bool ret;
+
+	ret = do_page_fault(addr, false);
+	if (ret) {
+		/* Wasn't an error, increment page fault count */
+		int key;
+
+		key = irq_lock();
+		z_num_pagefaults++;
+		irq_unlock(key);
+	}
+	return ret;
+}
+
+unsigned long z_num_pagefaults_get(void)
+{
+	unsigned long ret;
+	int key;
+
+	key = irq_lock();
+	ret = z_num_pagefaults;
+	irq_unlock(key);
+
+	return ret;
 }
 
 static void do_mem_unpin(void *addr)

tests/kernel/mem_protect/demand_paging/src/main.c

@@ -40,8 +40,11 @@ void test_map_anon_pages(void)
 
 void test_touch_anon_pages(void)
 {
+	unsigned long faults;
 	static const char *nums = "0123456789";
 
+	faults = z_num_pagefaults_get();
+
 	printk("checking zeroes\n");
 	/* The mapped area should have started out zeroed. Check this. */
 	for (size_t i = 0; i < arena_size; i++) {
@@ -63,6 +66,12 @@ void test_touch_anon_pages(void)
 			      "arena corrupted at index %d (%p): got 0x%hhx expected 0x%hhx",
 			      i, &arena[i], arena[i], nums[i % 10]);
 	}
+
+	faults = z_num_pagefaults_get() - faults;
+
+	/* Specific number depends on how much RAM we have but shouldn't be 0 */
+	zassert_not_equal(faults, 0UL, "no page faults handled?");
+	printk("Kernel handled %lu page faults\n", faults);
 }
 
 /* ztest main entry*/