Btrfs: setup blank root and fs_info for mount time

There is a problem with how we use sget, it searches through the list of supers
attached to the fs_type looking for a super with the same fs_devices as what
we're trying to mount. This depends on sb->s_fs_info being filled, but we don't
fill that in until we get to btrfs_fill_super, so we could hit supers on the
fs_type super list that have a null s_fs_info. In order to fix that we need to
go ahead and setup a blank root with a blank fs_info to hold fs_devices, that
way our test will work out right and then we can set s_fs_info in
btrfs_set_super, and then open_ctree will simply use our pre-allocated root and
fs_info when setting everything up. Thanks,

Signed-off-by: Josef Bacik <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/disk-io.c

···
 						 GFP_NOFS);
 	struct btrfs_root *csum_root = kzalloc(sizeof(struct btrfs_root),
 						 GFP_NOFS);
-	struct btrfs_root *tree_root = kzalloc(sizeof(struct btrfs_root),
-					       GFP_NOFS);
-	struct btrfs_fs_info *fs_info = kzalloc(sizeof(*fs_info),
-						GFP_NOFS);
+	struct btrfs_root *tree_root = btrfs_sb(sb);
+	struct btrfs_fs_info *fs_info = tree_root->fs_info;
 	struct btrfs_root *chunk_root = kzalloc(sizeof(struct btrfs_root),
 						GFP_NOFS);
 	struct btrfs_root *dev_root = kzalloc(sizeof(struct btrfs_root),

fs/btrfs/super.c

···
 
 static int btrfs_test_super(struct super_block *s, void *data)
 {
-	struct btrfs_fs_devices *test_fs_devices = data;
+	struct btrfs_root *test_root = data;
 	struct btrfs_root *root = btrfs_sb(s);
 
 	/*
···
 	 */
 	if (!atomic_read(&s->s_active))
 		return 0;
-	return root->fs_info->fs_devices == test_fs_devices;
+	return root->fs_info->fs_devices == test_root->fs_info->fs_devices;
 }
+
+static int btrfs_set_super(struct super_block *s, void *data)
+{
+	s->s_fs_info = data;
+
+	return set_anon_super(s, data);
+}
+
 
 /*
  * Find a superblock for the given device / mount point.
···
 	struct super_block *s;
 	struct dentry *root;
 	struct btrfs_fs_devices *fs_devices = NULL;
+	struct btrfs_root *tree_root = NULL;
+	struct btrfs_fs_info *fs_info = NULL;
 	fmode_t mode = FMODE_READ;
 	char *subvol_name = NULL;
 	u64 subvol_objectid = 0;
···
 		goto error_close_devices;
 	}
 
+	/*
+	 * Setup a dummy root and fs_info for test/set super.  This is because
+	 * we don't actually fill this stuff out until open_ctree, but we need
+	 * it for searching for existing supers, so this lets us do that and
+	 * then open_ctree will properly initialize everything later.
+	 */
+	fs_info = kzalloc(sizeof(struct btrfs_fs_info), GFP_NOFS);
+	tree_root = kzalloc(sizeof(struct btrfs_root), GFP_NOFS);
+	if (!fs_info || !tree_root) {
+		error = -ENOMEM;
+		goto error_close_devices;
+	}
+	fs_info->tree_root = tree_root;
+	fs_info->fs_devices = fs_devices;
+	tree_root->fs_info = fs_info;
+
 	bdev = fs_devices->latest_bdev;
-	s = sget(fs_type, btrfs_test_super, set_anon_super, fs_devices);
+	s = sget(fs_type, btrfs_test_super, btrfs_set_super, tree_root);
 	if (IS_ERR(s))
 		goto error_s;
 
···
 	error = PTR_ERR(s);
 error_close_devices:
 	btrfs_close_devices(fs_devices);
+	kfree(fs_info);
+	kfree(tree_root);
 error_free_subvol_name:
 	kfree(subvol_name);
 	return error;