thp, vmstat: implement HZP_ALLOC and HZP_ALLOC_FAILED events

hzp_alloc is incremented every time a huge zero page is successfully
	allocated. It includes allocations which where dropped due
	race with other allocation. Note, it doesn't count every map
	of the huge zero page, only its allocation.

hzp_alloc_failed is incremented if kernel fails to allocate huge zero
	page and falls back to using small pages.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/transhuge.txt

@@ -197,6 +197,14 @@ thp_split is incremented every time a huge page is split into base
 	pages. This can happen for a variety of reasons but a common
 	reason is that a huge page is old and is being reclaimed.
 
+thp_zero_page_alloc is incremented every time a huge zero page is
+	successfully allocated. It includes allocations which where
+	dropped due race with other allocation. Note, it doesn't count
+	every map of the huge zero page, only its allocation.
+
+thp_zero_page_alloc_failed is incremented if kernel fails to allocate
+	huge zero page and falls back to using small pages.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in /proc/vmstat to help

include/linux/vm_event_item.h

@@ -58,6 +58,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_COLLAPSE_ALLOC,
 		THP_COLLAPSE_ALLOC_FAILED,
 		THP_SPLIT,
+		THP_ZERO_PAGE_ALLOC,
+		THP_ZERO_PAGE_ALLOC_FAILED,
 #endif
 		NR_VM_EVENT_ITEMS
 };

mm/huge_memory.c

@@ -184,8 +184,11 @@ static unsigned long get_huge_zero_page(void)
 
 	zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
 			HPAGE_PMD_ORDER);
-	if (!zero_page)
+	if (!zero_page) {
+		count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);
 		return 0;
+	}
+	count_vm_event(THP_ZERO_PAGE_ALLOC);
 	preempt_disable();
 	if (cmpxchg(&huge_zero_pfn, 0, page_to_pfn(zero_page))) {
 		preempt_enable();

mm/vmstat.c

@@ -801,6 +801,8 @@ const char * const vmstat_text[] = {
 	"thp_collapse_alloc",
 	"thp_collapse_alloc_failed",
 	"thp_split",
+	"thp_zero_page_alloc",
+	"thp_zero_page_alloc_failed",
 #endif
 
 #endif /* CONFIG_VM_EVENTS_COUNTERS */