Btrfs: use the global reserve as a backup for deleting inodes

Xfstests 83 really stresses our ENOSPC since it uses a 100mb fs which ends up
with the mixed block group stuff.  Because of this we can run into a situation
where we don't have enough space to delete inodes, or even worse we can't free
the inodes when we next mount the fs which causes the orphan code to lose its
mind.  So if we fail to make our reservation, steal from the global reserve.
The global reserve will end up taking up the entire rest of the free space on
the fs in this worst case so there really is no other option.  With this patch
test 83 doesn't freak out.  Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/inode.c

@@ -3526,7 +3526,7 @@ void btrfs_evict_inode(struct inode *inode)
 {
 	struct btrfs_trans_handle *trans;
 	struct btrfs_root *root = BTRFS_I(inode)->root;
-	struct btrfs_block_rsv *rsv;
+	struct btrfs_block_rsv *rsv, *global_rsv;
 	u64 min_size = btrfs_calc_trunc_metadata_size(root, 1);
 	unsigned long nr;
 	int ret;
@@ -3561,6 +3561,7 @@ void btrfs_evict_inode(struct inode *inode)
 		goto no_delete;
 	}
 	rsv->size = min_size;
+	global_rsv = &root->fs_info->global_block_rsv;
 
 	btrfs_i_size_write(inode, 0);
 
@@ -3577,6 +3578,15 @@ void btrfs_evict_inode(struct inode *inode)
 	 */
 	while (1) {
 		ret = btrfs_block_rsv_check(root, rsv, min_size, 0, 1);
+
+		/*
+		 * Try and steal from the global reserve since we will
+		 * likely not use this space anyway, we want to try as
+		 * hard as possible to get this to work.
+		 */
+		if (ret)
+			ret = btrfs_block_rsv_migrate(global_rsv, rsv, min_size);
+
 		if (ret) {
 			printk(KERN_WARNING "Could not get space for a "
 			       "delete, will truncate on mount %d\n", ret);