mm,vmacache: count number of system-wide flushes

These flushes deal with sequence number overflows, such as for long lived
threads. These are rare, but interesting from a debugging PoV. As such,
display the number of flushes when vmacache debugging is enabled.

Signed-off-by: Davidlohr Bueso <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/vm_event_item.h

···
 #ifdef CONFIG_DEBUG_VM_VMACACHE
 		VMACACHE_FIND_CALLS,
 		VMACACHE_FIND_HITS,
+		VMACACHE_FULL_FLUSHES,
 #endif
 		NR_VM_EVENT_ITEMS
 };

mm/vmacache.c

···
 {
 	struct task_struct *g, *p;
 
+	count_vm_vmacache_event(VMACACHE_FULL_FLUSHES);
+
 	/*
 	 * Single threaded tasks need not iterate the entire
 	 * list of process. We can avoid the flushing as well

mm/vmstat.c

···
 #ifdef CONFIG_DEBUG_VM_VMACACHE
 	"vmacache_find_calls",
 	"vmacache_find_hits",
+	"vmacache_full_flushes",
 #endif
 #endif /* CONFIG_VM_EVENTS_COUNTERS */
 };