tools/vm/page_owner_sort.c: filter out unneeded line

To see a sorted result from page_owner, there must be a tiresome
preprocessing step before running page_owner_sort.  This patch simply
filters out lines which start with "PFN" while reading the page owner
report.
Signed-off-by: Changhee Han <ch0.han@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: http://lkml.kernel.org/r/20200429052940.16968-1-ch0.han@lge.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/page_owner.rst

@@ -83,8 +83,7 @@ Usage
 4) Analyze information from page owner::
 
 	cat /sys/kernel/debug/page_owner > page_owner_full.txt
-	grep -v ^PFN page_owner_full.txt > page_owner.txt
-	./page_owner_sort page_owner.txt sorted_page_owner.txt
+	./page_owner_sort page_owner_full.txt sorted_page_owner.txt
 
    See the result about who allocated each page
    in the ``sorted_page_owner.txt``.

tools/vm/page_owner_sort.c

@@ -4,8 +4,7 @@
  *
  * Example use:
  * cat /sys/kernel/debug/page_owner > page_owner_full.txt
- * grep -v ^PFN page_owner_full.txt > page_owner.txt
- * ./page_owner_sort page_owner.txt sorted_page_owner.txt
+ * ./page_owner_sort page_owner_full.txt sorted_page_owner.txt
  *
  * See Documentation/vm/page_owner.rst
 */
@@ -38,6 +37,8 @@ int read_block(char *buf, int buf_size, FILE *fin)
 	while (buf_end - curr > 1 && fgets(curr, buf_end - curr, fin)) {
 		if (*curr == '\n') /* empty line */
 			return curr - buf;
+		if (!strncmp(curr, "PFN", 3))
+			continue;
 		curr += strlen(curr);
 	}