mem: Treat /proc/pid/pagemap2 file as optional

It will appear not earlier than in 3.10.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

kerndat.c

@@ -4,6 +4,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/mman.h>
+#include <errno.h>
 
 #include "log.h"
 #include "kerndat.h"
@@ -71,6 +72,12 @@ int kerndat_get_dirty_track(void)
 
 	pm2 = open("/proc/self/pagemap2", O_RDONLY);
 	if (pm2 < 0) {
+		munmap(map, PAGE_SIZE);
+		if (errno == ENOENT) {
+			pr_info("No pagemap2 file\n");
+			return 0;
+		}
+
 		pr_perror("Can't open pagemap2 file");
 		return ret;
 	}

mem.c

@@ -315,8 +315,14 @@ static int __parasite_dump_pages_seized(struct parasite_ctl *ctl,
 		goto out_snap;
 
 	ret = pagemap = open_proc(ctl->pid.real, "pagemap2");
-	if (ret < 0)
-		goto out_free;
+	if (ret < 0) {
+		if (errno != ENOENT)
+			goto out_free;
+
+		ret = pagemap = open_proc(ctl->pid.real, "pagemap");
+		if (ret < 0)
+			goto out_free;
+	}
 
 	ret = -1;
 	pp = create_page_pipe(vma_area_list->priv_size / 2, args->iovs);