percpu: update header to contain bitmap allocator explanation.

 * Copyright (C) 2009		SUSE Linux Products GmbH

 * Copyright (C) 2009		Tejun Heo <tj@kernel.org>

 *

 * Copyright (C) 2017		Facebook Inc.

 * Copyright (C) 2017		Dennis Zhou <dennisszhou@gmail.com>

 *

 * This file is released under the GPLv2 license.

 *

 * The percpu allocator handles both static and dynamic areas.  Percpu

 *

 * There is special consideration for the first chunk which must handle

 * the static percpu variables in the kernel image as allocation services

 * are not online yet.  In short, the first chunk is structure like so:

 * are not online yet.  In short, the first chunk is structured like so:

 *

 *                  <Static | [Reserved] | Dynamic>

 *

 * percpu variables from kernel modules.  Finally, the dynamic section

 * takes care of normal allocations.

 *

 * Allocation state in each chunk is kept using an array of integers

 * on chunk->map.  A positive value in the map represents a free

 * region and negative allocated.  Allocation inside a chunk is done

 * by scanning this map sequentially and serving the first matching

 * entry.  This is mostly copied from the percpu_modalloc() allocator.

 * Chunks can be determined from the address using the index field

 * in the page struct. The index field contains a pointer to the chunk.

 *

 * These chunks are organized into lists according to free_size and

 * tries to allocate from the fullest chunk first. Each chunk maintains

 * a maximum contiguous area size hint which is guaranteed to be equal

 * to or larger than the maximum contiguous area in the chunk. This

 * helps prevent the allocator from iterating over chunks unnecessarily.

 * The allocator organizes chunks into lists according to free size and

 * tries to allocate from the fullest chunk first.  Each chunk is managed

 * by a bitmap with metadata blocks.  The allocation map is updated on

 * every allocation and free to reflect the current state while the boundary

 * map is only updated on allocation.  Each metadata block contains

 * information to help mitigate the need to iterate over large portions

 * of the bitmap.  The reverse mapping from page to chunk is stored in

 * the page's index.  Lastly, units are lazily backed and grow in unison.

 *

 * There is a unique conversion that goes on here between bytes and bits.

 * Each bit represents a fragment of size PCPU_MIN_ALLOC_SIZE.  The chunk

 * tracks the number of pages it is responsible for in nr_pages.  Helper

 * functions are used to convert from between the bytes, bits, and blocks.

 * All hints are managed in bits unless explicitly stated.

 *

 * To use this allocator, arch code should do the following:

 *