ext4: add debug_want_extra_isize mount option

In order to test the inode extra isize expansion code, it is useful to
be able to easily create file systems that have inodes with extra
isize values smaller than the current desired value.
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -1284,7 +1284,7 @@ enum {
 	Opt_noquota, Opt_barrier, Opt_nobarrier, Opt_err,
 	Opt_usrquota, Opt_grpquota, Opt_prjquota, Opt_i_version, Opt_dax,
 	Opt_stripe, Opt_delalloc, Opt_nodelalloc, Opt_mblk_io_submit,
-	Opt_lazytime, Opt_nolazytime,
+	Opt_lazytime, Opt_nolazytime, Opt_debug_want_extra_isize,
 	Opt_nomblk_io_submit, Opt_block_validity, Opt_noblock_validity,
 	Opt_inode_readahead_blks, Opt_journal_ioprio,
 	Opt_dioread_nolock, Opt_dioread_lock,
@@ -1352,6 +1352,7 @@ static const match_table_t tokens = {
 	{Opt_delalloc, "delalloc"},
 	{Opt_lazytime, "lazytime"},
 	{Opt_nolazytime, "nolazytime"},
+	{Opt_debug_want_extra_isize, "debug_want_extra_isize=%u"},
 	{Opt_nodelalloc, "nodelalloc"},
 	{Opt_removed, "mblk_io_submit"},
 	{Opt_removed, "nomblk_io_submit"},
@@ -1557,6 +1558,7 @@ static const struct mount_opts {
 #endif
 	{Opt_nouid32, EXT4_MOUNT_NO_UID32, MOPT_SET},
 	{Opt_debug, EXT4_MOUNT_DEBUG, MOPT_SET},
+	{Opt_debug_want_extra_isize, 0, MOPT_GTE0},
 	{Opt_quota, EXT4_MOUNT_QUOTA | EXT4_MOUNT_USRQUOTA, MOPT_SET | MOPT_Q},
 	{Opt_usrquota, EXT4_MOUNT_QUOTA | EXT4_MOUNT_USRQUOTA,
 							MOPT_SET | MOPT_Q},
@@ -1670,6 +1672,8 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 		if (arg == 0)
 			arg = JBD2_DEFAULT_MAX_COMMIT_AGE;
 		sbi->s_commit_interval = HZ * arg;
+	} else if (token == Opt_debug_want_extra_isize) {
+		sbi->s_want_extra_isize = arg;
 	} else if (token == Opt_max_batch_time) {
 		sbi->s_max_batch_time = arg;
 	} else if (token == Opt_min_batch_time) {
@@ -4081,7 +4085,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		sb->s_flags |= MS_RDONLY;
 
 	/* determine the minimum size of new large inodes, if present */
-	if (sbi->s_inode_size > EXT4_GOOD_OLD_INODE_SIZE) {
+	if (sbi->s_inode_size > EXT4_GOOD_OLD_INODE_SIZE &&
+	    sbi->s_want_extra_isize == 0) {
 		sbi->s_want_extra_isize = sizeof(struct ext4_inode) -
 						     EXT4_GOOD_OLD_INODE_SIZE;
 		if (ext4_has_feature_extra_isize(sb)) {