ext4: initialize multi-block allocator before checking block descriptors

With EXT4FS_DEBUG ext4_count_free_clusters() will call
ext4_read_block_bitmap() without s_group_info initialized, so we need to
initialize multi-block allocator before.

And dependencies that must be solved, to allow this:
- multi-block allocator needs in group descriptors
- need to install s_op before initializing multi-block allocator,
  because in ext4_mb_init_backend() new inode is created.
- initialize number of group desc blocks (s_gdb_count) otherwise
  number of clusters returned by ext4_free_clusters_after_init() is not correct.
  (see ext4_bg_num_gdb_nometa())

Here is the stack backtrace:

(gdb) bt
 #0  ext4_get_group_info (group=0, sb=0xffff880079a10000) at ext4.h:2430
 #1  ext4_validate_block_bitmap (sb=sb@entry=0xffff880079a10000,
     desc=desc@entry=0xffff880056510000, block_group=block_group@entry=0,
     bh=bh@entry=0xffff88007bf2b2d8) at balloc.c:358
 #2  0xffffffff81232202 in ext4_wait_block_bitmap (sb=sb@entry=0xffff880079a10000,
     block_group=block_group@entry=0,
     bh=bh@entry=0xffff88007bf2b2d8) at balloc.c:476
 #3  0xffffffff81232eaf in ext4_read_block_bitmap (sb=sb@entry=0xffff880079a10000,
     block_group=block_group@entry=0) at balloc.c:489
 #4  0xffffffff81232fc0 in ext4_count_free_clusters (sb=sb@entry=0xffff880079a10000) at balloc.c:665
 #5  0xffffffff81259ffa in ext4_check_descriptors (first_not_zeroed=<synthetic pointer>,
     sb=0xffff880079a10000) at super.c:2143
 #6  ext4_fill_super (sb=sb@entry=0xffff880079a10000, data=<optimized out>,
     data@entry=0x0 <irq_stack_union>, silent=silent@entry=0) at super.c:3851
     ...
Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -3869,19 +3869,38 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 			goto failed_mount2;
 		}
 	}
+
+	/*
+	 * set up enough so that it can read an inode,
+	 * and create new inode for buddy allocator
+	 */
+	sbi->s_gdb_count = db_count;
+	if (!test_opt(sb, NOLOAD) &&
+	    EXT4_HAS_COMPAT_FEATURE(sb, EXT4_FEATURE_COMPAT_HAS_JOURNAL))
+		sb->s_op = &ext4_sops;
+	else
+		sb->s_op = &ext4_nojournal_sops;
+
+	ext4_ext_init(sb);
+	err = ext4_mb_init(sb);
+	if (err) {
+		ext4_msg(sb, KERN_ERR, "failed to initialize mballoc (%d)",
+			 err);
+		goto failed_mount2;
+	}
+
 	if (!ext4_check_descriptors(sb, &first_not_zeroed)) {
 		ext4_msg(sb, KERN_ERR, "group descriptors corrupted!");
-		goto failed_mount2;
+		goto failed_mount2a;
 	}
 	if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG))
 		if (!ext4_fill_flex_info(sb)) {
 			ext4_msg(sb, KERN_ERR,
 			       "unable to initialize "
 			       "flex_bg meta info!");
-			goto failed_mount2;
+			goto failed_mount2a;
 		}
 
-	sbi->s_gdb_count = db_count;
 	get_random_bytes(&sbi->s_next_generation, sizeof(u32));
 	spin_lock_init(&sbi->s_next_gen_lock);
 
@@ -3916,14 +3935,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_stripe = ext4_get_stripe_size(sbi);
 	sbi->s_extent_max_zeroout_kb = 32;
 
-	/*
-	 * set up enough so that it can read an inode
-	 */
-	if (!test_opt(sb, NOLOAD) &&
-	    EXT4_HAS_COMPAT_FEATURE(sb, EXT4_FEATURE_COMPAT_HAS_JOURNAL))
-		sb->s_op = &ext4_sops;
-	else
-		sb->s_op = &ext4_nojournal_sops;
 	sb->s_export_op = &ext4_export_ops;
 	sb->s_xattr = ext4_xattr_handlers;
 #ifdef CONFIG_QUOTA
@@ -4113,21 +4124,13 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	if (err) {
 		ext4_msg(sb, KERN_ERR, "failed to reserve %llu clusters for "
 			 "reserved pool", ext4_calculate_resv_clusters(sb));
-		goto failed_mount4a;
+		goto failed_mount5;
 	}
 
 	err = ext4_setup_system_zone(sb);
 	if (err) {
 		ext4_msg(sb, KERN_ERR, "failed to initialize system "
 			 "zone (%d)", err);
-		goto failed_mount4a;
-	}
-
-	ext4_ext_init(sb);
-	err = ext4_mb_init(sb);
-	if (err) {
-		ext4_msg(sb, KERN_ERR, "failed to initialize mballoc (%d)",
-			 err);
 		goto failed_mount5;
 	}
 
@@ -4204,11 +4207,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 failed_mount7:
 	ext4_unregister_li_request(sb);
 failed_mount6:
-	ext4_mb_release(sb);
-failed_mount5:
-	ext4_ext_release(sb);
 	ext4_release_system_zone(sb);
-failed_mount4a:
+failed_mount5:
 	dput(sb->s_root);
 	sb->s_root = NULL;
 failed_mount4:
@@ -4232,11 +4232,14 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	percpu_counter_destroy(&sbi->s_extent_cache_cnt);
 	if (sbi->s_mmp_tsk)
 		kthread_stop(sbi->s_mmp_tsk);
+failed_mount2a:
+	ext4_mb_release(sb);
 failed_mount2:
 	for (i = 0; i < db_count; i++)
 		brelse(sbi->s_group_desc[i]);
 	ext4_kvfree(sbi->s_group_desc);
 failed_mount:
+	ext4_ext_release(sb);
 	if (sbi->s_chksum_driver)
 		crypto_free_shash(sbi->s_chksum_driver);
 	if (sbi->s_proc) {