mm,vmacache: add debug data

Introduce a CONFIG_DEBUG_VM_VMACACHE option to enable counting the cache
hit rate -- exported in /proc/vmstat.

Any updates to the caching scheme needs this kind of data, thus it can
save some work re-implementing the counting all the time.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmstat.c

@@ -866,6 +866,10 @@ const char * const vmstat_text[] = {
 	"nr_tlb_local_flush_one",
 #endif /* CONFIG_DEBUG_TLBFLUSH */
 
+#ifdef CONFIG_DEBUG_VM_VMACACHE
+	"vmacache_find_calls",
+	"vmacache_find_hits",
+#endif
 #endif /* CONFIG_VM_EVENTS_COUNTERS */
 };
 #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA */