Fix the handling of memory allocation for tmpfs.

If the total memory on the system is less then the max memory in cgroup
then do not set memory limit on tmpfs, allow the kernel to set the default.

src/systemdhook.c

@@ -7,6 +7,7 @@
 #include <sys/mount.h>
 #include <syslog.h>
 #include <sys/stat.h>
+#include <sys/sysinfo.h>
 #include <sys/types.h>
 #include <fcntl.h>
 #include <sched.h>
@@ -21,6 +22,14 @@
 
 #include <libmount/libmount.h>
 
+static unsigned long get_mem_total() {
+	struct sysinfo info;
+	int ret = sysinfo(&info);
+	if (ret < 0) {
+		return ret;
+	}
+	return info.totalram;
+}
 
 #define _cleanup_(x) __attribute__((cleanup(x)))
 
@@ -487,15 +496,23 @@ static int prestart(const char *rootfs,
 
 	pr_pdebug("LIMIT: %s\n", memory_limit_str);
 
+	char memory_str[PATH_MAX];
+	uint64_t total_memory = 0;
 	uint64_t memory_limit_in_bytes = 0;
 	char *ptr = NULL;
 
 	memory_limit_in_bytes = strtoull(memory_limit_str, &ptr, 10);
 
 	pr_pdebug("Limit in bytes: ""%" PRIu64 "\n", memory_limit_in_bytes);
 
-	/* Set it to half of limit in kb */
-	uint64_t memory_limit_in_kb = memory_limit_in_bytes / 2048;
+	total_memory = get_mem_total();
+	if (memory_limit_in_bytes < total_memory) {
+		/* Set it to half of limit in kb */
+		uint64_t memory_limit_in_kb = memory_limit_in_bytes / 2048;
+		snprintf(memory_str, sizeof(memory_str)-1 , "size=%" PRIu64 "k", memory_limit_in_kb);
+	} else {
+		strcpy(memory_str, "");
+	}
 
 	char tmp_dir[PATH_MAX];
 	snprintf(tmp_dir, PATH_MAX, "%s/tmp", rootfs);
@@ -510,9 +527,9 @@ static int prestart(const char *rootfs,
 		}
 
 		if (!strcmp("", mount_label)) {
-			rc = asprintf(&options, "mode=1777,size=%" PRIu64 "k", memory_limit_in_kb);
+			rc = asprintf(&options, "mode=1777%s", memory_str);
 		} else {
-			rc = asprintf(&options, "mode=1777,size=%" PRIu64 "k,context=\"%s\"", memory_limit_in_kb, mount_label);
+			rc = asprintf(&options, "mode=1777%s,context=\"%s\"", memory_str, mount_label);
 		}
 		if (rc < 0) {
 			pr_perror("Failed to allocate memory for context");