mm/vmstat: add event for ksm swapping in copy

When faults in from swap what used to be a KSM page and that page
had been swapped in before, system has to make a copy, and leaves
remerging the pages to a later pass of ksmd.

That is not good for performace, we'd better to reduce this kind
of copy. There are some ways to reduce it, for example lessen
swappiness or madvise(, , MADV_MERGEABLE) range. So add this
event to support doing this tuning. Just like this patch:
"mm, THP, swap: add THP swapping out fallback counting".

Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>

include/linux/vm_event_item.h

@@ -126,6 +126,9 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 #ifdef CONFIG_SWAP
 		SWAP_RA,
 		SWAP_RA_HIT,
+#ifdef CONFIG_KSM
+		KSM_SWPIN_COPY,
+#endif
 #endif
 #ifdef CONFIG_X86
 		DIRECT_MAP_LEVEL2_SPLIT,

mm/ksm.c

@@ -2594,6 +2594,7 @@ struct page *ksm_might_need_to_copy(struct page *page,
 		SetPageDirty(new_page);
 		__SetPageUptodate(new_page);
 		__SetPageLocked(new_page);
+		count_vm_event(KSM_SWPIN_COPY);
 	}
 
 	return new_page;

mm/vmstat.c

@@ -1382,6 +1382,9 @@ const char * const vmstat_text[] = {
 #ifdef CONFIG_SWAP
 	"swap_ra",
 	"swap_ra_hit",
+#ifdef CONFIG_KSM
+	"ksm_swpin_copy",
+#endif
 #endif
 #ifdef CONFIG_X86
 	"direct_map_level2_splits",