+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <time.h>
+#include <unistd.h>
+
+#define PAGE(a) ((a)*0x1000)
+#define STRLEN 256
+
+void err_exit(char *op)
+{
+ fprintf(stderr, "%s: %s\n", op, strerror(errno));
+ exit(1);
+}
+
+void chattr_cmd(char *chattr, char *cmd, char *file)
+{
+ int ret;
+ char command[STRLEN];
+
+ ret = snprintf(command, STRLEN, "%s %s %s 2>/dev/null", chattr, cmd, file);
+ if (ret < 0)
+ err_exit("snprintf");
+
+ ret = system(command);
+ if (ret) /* Success - the kernel fix is to have this chattr fail */
+ exit(77);
+}
+
+int main(int argc, char *argv[])
+{
+ int fd, err, len = PAGE(1);
+ char *data, *dax_data, *chattr, *file;
+ char string[STRLEN];
+
+ if (argc < 3) {
+ printf("Usage: %s <chattr program> <file>\n", basename(argv[0]));
+ exit(0);
+ }
+
+ chattr = argv[1];
+ file = argv[2];
+
+ srand(time(NULL));
+ snprintf(string, STRLEN, "random number %d\n", rand());
+
+ fd = open(file, O_RDWR|O_CREAT, S_IRUSR|S_IWUSR);
+ if (fd < 0)
+ err_exit("fd");
+
+ /* begin with journaling off and DAX on */
+ chattr_cmd(chattr, "-j", file);
+
+ ftruncate(fd, 0);
+ fallocate(fd, 0, 0, len);
+
+ dax_data = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
+ if (!dax_data)
+ err_exit("mmap dax_data");
+
+ /*
+ * This turns on journaling. It also has the side-effect that it
+ * turns off DAX for the given inode since journaling and DAX aren't
+ * allowed to be on at the same time. This happens in
+ * ext4_change_inode_journal_flag() in kernel v4.14 and before.
+ *
+ * Note that this turns off the runtime DAX flag (S_DAX) in the
+ * in-memory inode, and has nothing to do with per-inode on-media DAX
+ * inode flags.
+ */
+ chattr_cmd(chattr, "+j", file);
+
+ data = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
+ if (!data)
+ err_exit("mmap data");
+
+ /*
+ * Write the data using the non-DAX mapping, and try and read it back
+ * using the DAX mapping.
+ */
+ strcpy(data, string);
+ if (strcmp(dax_data, string) != 0)
+ printf("Data miscompare\n");
+
+ err = munmap(data, len);
+ if (err < 0)
+ err_exit("munmap data");
+
+ err = munmap(dax_data, len);
+ if (err < 0)
+ err_exit("munmap dax_data");
+
+ err = close(fd);
+ if (err < 0)
+ err_exit("close");
+ return 0;
+}