f2fs: disallow switching io_bits option during remount

If IO alignment feature is turned on after remount, we didn't
initialize mempool of it, it turns out we will encounter panic
during IO submission due to access NULL mempool pointer.

This feature should be set only at mount time, so simply deny
configuring during remount.

This fixes bug reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=204135Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1522,6 +1522,7 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 	bool need_stop_gc = false;
 	bool no_extent_cache = !test_opt(sbi, EXTENT_CACHE);
 	bool disable_checkpoint = test_opt(sbi, DISABLE_CHECKPOINT);
+	bool no_io_align = !F2FS_IO_ALIGNED(sbi);
 	bool checkpoint_changed;
 #ifdef CONFIG_QUOTA
 	int i, j;
@@ -1601,6 +1602,12 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 		goto restore_opts;
 	}
 
+	if (no_io_align == !!F2FS_IO_ALIGNED(sbi)) {
+		err = -EINVAL;
+		f2fs_warn(sbi, "switch io_bits option is not allowed");
+		goto restore_opts;
+	}
+
 	if ((*flags & SB_RDONLY) && test_opt(sbi, DISABLE_CHECKPOINT)) {
 		err = -EINVAL;
 		f2fs_warn(sbi, "disabling checkpoint not compatible with read-only");