f2fs: fix to force keeping write barrier for strict fsync mode

[1] https://www.mail-archive.com/linux-f2fs-devel@lists.sourceforge.net/msg15126.html

As [1] reported, if lower device doesn't support write barrier, in below
case:

- write page #0; persist
- overwrite page #0
- fsync
 - write data page #0 OPU into device's cache
 - write inode page into device's cache
 - issue flush

If SPO is triggered during flush command, inode page can be persisted
before data page #0, so that after recovery, inode page can be recovered
with new physical block address of data page #0, however there may
contains dummy data in new physical block address.

Then what user will see is: after overwrite & fsync + SPO, old data in
file was corrupted, if any user do care about such case, we can suggest
user to use STRICT fsync mode, in this mode, we will force to use atomic
write sematics to keep write order in between data/node and last node,
so that it avoids potential data corruption during fsync().
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -301,6 +301,18 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
 				f2fs_exist_written_data(sbi, ino, UPDATE_INO))
 			goto flush_out;
 		goto out;
+	} else {
+		/*
+		 * for OPU case, during fsync(), node can be persisted before
+		 * data when lower device doesn't support write barrier, result
+		 * in data corruption after SPO.
+		 * So for strict fsync mode, force to use atomic write sematics
+		 * to keep write order in between data/node and last node to
+		 * avoid potential data corruption.
+		 */
+		if (F2FS_OPTION(sbi).fsync_mode ==
+				FSYNC_MODE_STRICT && !atomic)
+			atomic = true;
 	}
 go_write:
 	/*