btrfs: relocation: Add introduction of how relocation works

#include "delalloc-space.h"

#include "block-group.h"

/*

 * Relocation overview

 *

 * [What does relocation do]

 *

 * The objective of relocation is to relocate all extents of the target block

 * group to other block groups.

 * This is utilized by resize (shrink only), profile converting, compacting

 * space, or balance routine to spread chunks over devices.

 *

 * 		Before		|		After

 * ------------------------------------------------------------------

 *  BG A: 10 data extents	| BG A: deleted

 *  BG B:  2 data extents	| BG B: 10 data extents (2 old + 8 relocated)

 *  BG C:  1 extents		| BG C:  3 data extents (1 old + 2 relocated)

 *

 * [How does relocation work]

 *

 * 1.   Mark the target block group read-only

 *      New extents won't be allocated from the target block group.

 *

 * 2.1  Record each extent in the target block group

 *      To build a proper map of extents to be relocated.

 *

 * 2.2  Build data reloc tree and reloc trees

 *      Data reloc tree will contain an inode, recording all newly relocated

 *      data extents.

 *      There will be only one data reloc tree for one data block group.

 *

 *      Reloc tree will be a special snapshot of its source tree, containing

 *      relocated tree blocks.

 *      Each tree referring to a tree block in target block group will get its

 *      reloc tree built.

 *

 * 2.3  Swap source tree with its corresponding reloc tree

 *      Each involved tree only refers to new extents after swap.

 *

 * 3.   Cleanup reloc trees and data reloc tree.

 *      As old extents in the target block group are still referenced by reloc

 *      trees, we need to clean them up before really freeing the target block

 *      group.

 *

 * The main complexity is in steps 2.2 and 2.3.

 *

 * The entry point of relocation is relocate_block_group() function.

 */

/*

 * backref_node, mapping_node and tree_block start with this

 */