Btrfs: lower the bar for chunk allocation

The chunk allocation code has tried to keep a pretty tight lid on creating new
metadata chunks.  This is partially because in the past the reservation
code didn't give us an accurate idea of how much space was being used.

The new code is much more accurate, so we're able to get rid of some of these
checks.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -3263,27 +3263,12 @@ static int should_alloc_chunk(struct btrfs_root *root,
 		if (num_bytes - num_allocated < thresh)
 			return 1;
 	}
-
-	/*
-	 * we have two similar checks here, one based on percentage
-	 * and once based on a hard number of 256MB.  The idea
-	 * is that if we have a good amount of free
-	 * room, don't allocate a chunk.  A good mount is
-	 * less than 80% utilized of the chunks we have allocated,
-	 * or more than 256MB free
-	 */
-	if (num_allocated + alloc_bytes + 256 * 1024 * 1024 < num_bytes)
-		return 0;
-
-	if (num_allocated + alloc_bytes < div_factor(num_bytes, 8))
-		return 0;
-
 	thresh = btrfs_super_total_bytes(root->fs_info->super_copy);
 
-	/* 256MB or 5% of the FS */
-	thresh = max_t(u64, 256 * 1024 * 1024, div_factor_fine(thresh, 5));
+	/* 256MB or 2% of the FS */
+	thresh = max_t(u64, 256 * 1024 * 1024, div_factor_fine(thresh, 2));
 
-	if (num_bytes > thresh && sinfo->bytes_used < div_factor(num_bytes, 3))
+	if (num_bytes > thresh && sinfo->bytes_used < div_factor(num_bytes, 8))
 		return 0;
 	return 1;
 }