tools/vm/page-types.c: avoid memset() in walk_pfn() when count == 1

I found that page-types is very slow and my testing shows many timeout
errors. Here's an example with a simple program allocating 1000 thps.

$ time ./page-types -p $(pgrep -f test_alloc)
...
real 0m17.201s
user 0m16.889s
sys 0m0.312s

Most of time is spent in memset(). Currently memset() clears over whole
buffer for every walk_pfn() call, which is inefficient when walk_pfn()
is called from walk_vma(), because in that case walk_pfn() is called for
each pfn. So this patch limits the zero initialization only for the
first element.

$ time ./page-types.patched -p $(pgrep -f test_alloc)
...
real 0m0.182s
user 0m0.046s
sys 0m0.135s

Fixes: 954e95584579 ("tools/vm/page-types.c: add memory cgroup dumping and filtering")
Signed-off-by: Naoya Horiguchi <[e-mail]>
Suggested-by: Konstantin Khlebnikov <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/vm/page-types.c

···
 	unsigned long pages;
 	unsigned long i;
 
-	memset(cgi, 0, sizeof cgi);
+	/*
+	 * kpagecgroup_read() reads only if kpagecgroup were opened, but
+	 * /proc/kpagecgroup might even not exist, so it's better to fill
+	 * them with zeros here.
+	 */
+	if (count == 1)
+		cgi[0] = 0;
+	else
+		memset(cgi, 0, sizeof cgi);
 
 	while (count) {
 		batch = min_t(unsigned long, count, KPAGEFLAGS_BATCH);