mm/ksm: add pages scanned metric

ksm currently maintains several statistics, which let you determine how
successful KSM is at sharing pages.  However it does not contain a metric
to determine how much work it does.

This commit adds the pages scanned metric.  This allows the administrator
to determine how many pages have been scanned over a period of time.

Link: https://lkml.kernel.org/r/20230811193655.2518943-1-shr@devkernel.ioSigned-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/ksm.rst

@@ -159,6 +159,8 @@ The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
 general_profit
         how effective is KSM. The calculation is explained below.
+pages_scanned
+        how many pages are being scanned for ksm
 pages_shared
         how many shared pages are being used
 pages_sharing

mm/ksm.c

@@ -242,6 +242,9 @@ static struct kmem_cache *rmap_item_cache;
 static struct kmem_cache *stable_node_cache;
 static struct kmem_cache *mm_slot_cache;
 
+/* The number of pages scanned */
+static unsigned long ksm_pages_scanned;
+
 /* The number of nodes in the stable tree */
 static unsigned long ksm_pages_shared;
 
@@ -2476,8 +2479,9 @@ static void ksm_do_scan(unsigned int scan_npages)
 {
 	struct ksm_rmap_item *rmap_item;
 	struct page *page;
+	unsigned int npages = scan_npages;
 
-	while (scan_npages-- && likely(!freezing(current))) {
+	while (npages-- && likely(!freezing(current))) {
 		cond_resched();
 		rmap_item = scan_get_next_rmap_item(&page);
 		if (!rmap_item)
@@ -2485,6 +2489,8 @@ static void ksm_do_scan(unsigned int scan_npages)
 		cmp_and_merge_page(page, rmap_item);
 		put_page(page);
 	}
+
+	ksm_pages_scanned += scan_npages - npages;
 }
 
 static int ksmd_should_run(void)
@@ -3323,6 +3329,13 @@ static ssize_t max_page_sharing_store(struct kobject *kobj,
 }
 KSM_ATTR(max_page_sharing);
 
+static ssize_t pages_scanned_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", ksm_pages_scanned);
+}
+KSM_ATTR_RO(pages_scanned);
+
 static ssize_t pages_shared_show(struct kobject *kobj,
 				 struct kobj_attribute *attr, char *buf)
 {
@@ -3431,6 +3444,7 @@ static struct attribute *ksm_attrs[] = {
 	&sleep_millisecs_attr.attr,
 	&pages_to_scan_attr.attr,
 	&run_attr.attr,
+	&pages_scanned_attr.attr,
 	&pages_shared_attr.attr,
 	&pages_sharing_attr.attr,
 	&pages_unshared_attr.attr,