[PATCH v2] mm/vmstat: add events for ksm cow

[cgel.zte@gmail.com]

From: Yang Yang <yang.yang29@zte.com.cn>

Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want
to save memory, it's a tradeoff by suffering delay on ksm cow. Users can
get to know how much memory ksm saved by reading
/sys/kernel/mm/ksm/pages_sharing, but they don't know what's the costs
of ksm cow, and this is important of some delay sensitive tasks.

So add ksm cow events to help users evaluate whether or how to use ksm.

Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
---
v2:
- fix compile error when CONFIG_KSM is not set
---
 include/linux/vm_event_item.h |  2 ++
 mm/memory.c                   | 20 +++++++++++++++++---
 mm/vmstat.c                   |  2 ++
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 16a0a4fd000b..6f32be04212f 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -131,6 +131,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		SWAP_RA_HIT,
 #ifdef CONFIG_KSM
 		KSM_SWPIN_COPY,
+		KSM_COW_SUCCESS,
+		KSM_COW_FAIL,
 #endif
 #endif
 #ifdef CONFIG_X86
diff --git a/mm/memory.c b/mm/memory.c
index 4111f97c91a0..c24d5f04fab5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3257,6 +3257,8 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 	__releases(vmf->ptl)
 {
 	struct vm_area_struct *vma = vmf->vma;
+	vm_fault_t ret = 0;
+	bool ksm = 0;
 
 	if (userfaultfd_pte_wp(vma, *vmf->pte)) {
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -3294,6 +3296,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 	 */
 	if (PageAnon(vmf->page)) {
 		struct page *page = vmf->page;
+		ksm = PageKsm(page);
 
 		/*
 		 * We have to verify under page lock: these early checks are
@@ -3302,7 +3305,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		 *
 		 * PageKsm() doesn't necessarily raise the page refcount.
 		 */
-		if (PageKsm(page) || page_count(page) > 3)
+		if (ksm || page_count(page) > 3)
 			goto copy;
 		if (!PageLRU(page))
 			/*
@@ -3316,7 +3319,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 			goto copy;
 		if (PageSwapCache(page))
 			try_to_free_swap(page);
-		if (PageKsm(page) || page_count(page) != 1) {
+		if (ksm || page_count(page) != 1) {
 			unlock_page(page);
 			goto copy;
 		}
@@ -3339,7 +3342,18 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 	get_page(vmf->page);
 
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
-	return wp_page_copy(vmf);
+	ret = wp_page_copy(vmf);
+
+#ifdef CONFIG_KSM
+	if (ksm) {
+		if (unlikely(ret & VM_FAULT_ERROR))
+			count_vm_event(KSM_COW_FAIL);
+		else
+			count_vm_event(KSM_COW_SUCCESS);
+	}
+#endif
+
+	return ret;
 }
 
 static void unmap_mapping_range_vma(struct vm_area_struct *vma,
diff --git a/mm/vmstat.c b/mm/vmstat.c
index d5cc8d739fac..a2c29a5206ec 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1390,6 +1390,8 @@ const char * const vmstat_text[] = {
 	"swap_ra_hit",
 #ifdef CONFIG_KSM
 	"ksm_swpin_copy",
+	"ksm_cow_success",
+	"ksm_cow_fail",
 #endif
 #endif
 #ifdef CONFIG_X86
-- 
2.25.1