btrfs: add flags to give a hint to the chunk allocator

Add the following flags to give a hint about which chunk should be
allocated on which a disk.

The following flags are created:

- BTRFS_DEV_ALLOCATION_PREFERRED_DATA
  preferred data chunk, but metadata chunk allowed
- BTRFS_DEV_ALLOCATION_PREFERRED_METADATA
  preferred metadata chunk, but data chunk allowed
- BTRFS_DEV_ALLOCATION_METADATA_ONLY
  only metadata chunk allowed
- BTRFS_DEV_ALLOCATION_DATA_ONLY
  only data chunk allowed

Co-authored-by: Goffredo Baroncelli <kreijack@inwid.it>
Signed-off-by: Kai Krakow <kai@kaishome.de>

Author: kakra

include/uapi/linux/btrfs_tree.h

@@ -578,6 +578,23 @@ struct btrfs_node {
 	struct btrfs_key_ptr ptrs[];
 } __attribute__ ((__packed__));
 
+#ifdef CONFIG_BTRFS_ALLOCATOR_HINTS
+/* dev_item.type */
+
+/* btrfs chunk allocation hints */
+#define BTRFS_DEV_ALLOCATION_MASK_BIT_COUNT	3
+/* preferred data chunk, but metadata chunk allowed */
+#define BTRFS_DEV_ALLOCATION_PREFERRED_DATA	(0ULL)
+/* preferred metadata chunk, but data chunk allowed */
+#define BTRFS_DEV_ALLOCATION_PREFERRED_METADATA	(1ULL)
+/* only metadata chunk allowed */
+#define BTRFS_DEV_ALLOCATION_METADATA_ONLY	(2ULL)
+/* only data chunk allowed */
+#define BTRFS_DEV_ALLOCATION_DATA_ONLY		(3ULL)
+/* 5..7 are unused values */
+
+#endif
+
 struct btrfs_dev_item {
 	/* the internal btrfs device id */
 	__le64 devid;