btrfs: add a comment describing block reserves

#include "space-info.h"

#include "transaction.h"

/*

 * HOW DO BLOCK RESERVES WORK

 *

 *   Think of block_rsv's as buckets for logically grouped metadata

 *   reservations.  Each block_rsv has a ->size and a ->reserved.  ->size is

 *   how large we want our block rsv to be, ->reserved is how much space is

 *   currently reserved for this block reserve.

 *

 *   ->failfast exists for the truncate case, and is described below.

 *

 * NORMAL OPERATION

 *

 *   -> Reserve

 *     Entrance: btrfs_block_rsv_add, btrfs_block_rsv_refill

 *

 *     We call into btrfs_reserve_metadata_bytes() with our bytes, which is

 *     accounted for in space_info->bytes_may_use, and then add the bytes to

 *     ->reserved, and ->size in the case of btrfs_block_rsv_add.

 *

 *     ->size is an over-estimation of how much we may use for a particular

 *     operation.

 *

 *   -> Use

 *     Entrance: btrfs_use_block_rsv

 *

 *     When we do a btrfs_alloc_tree_block() we call into btrfs_use_block_rsv()

 *     to determine the appropriate block_rsv to use, and then verify that

 *     ->reserved has enough space for our tree block allocation.  Once

 *     successful we subtract fs_info->nodesize from ->reserved.

 *

 *   -> Finish

 *     Entrance: btrfs_block_rsv_release

 *

 *     We are finished with our operation, subtract our individual reservation

 *     from ->size, and then subtract ->size from ->reserved and free up the

 *     excess if there is any.

 *

 *     There is some logic here to refill the delayed refs rsv or the global rsv

 *     as needed, otherwise the excess is subtracted from

 *     space_info->bytes_may_use.

 *

 * TYPES OF BLOCK RESERVES

 *

 * BLOCK_RSV_TRANS, BLOCK_RSV_DELOPS, BLOCK_RSV_CHUNK

 *   These behave normally, as described above, just within the confines of the

 *   lifetime of their particular operation (transaction for the whole trans

 *   handle lifetime, for example).

 *

 * BLOCK_RSV_GLOBAL

 *   It is impossible to properly account for all the space that may be required

 *   to make our extent tree updates.  This block reserve acts as an overflow

 *   buffer in case our delayed refs reserve does not reserve enough space to

 *   update the extent tree.

 *

 *   We can steal from this in some cases as well, notably on evict() or

 *   truncate() in order to help users recover from ENOSPC conditions.

 *

 * BLOCK_RSV_DELALLOC

 *   The individual item sizes are determined by the per-inode size

 *   calculations, which are described with the delalloc code.  This is pretty

 *   straightforward, it's just the calculation of ->size encodes a lot of

 *   different items, and thus it gets used when updating inodes, inserting file

 *   extents, and inserting checksums.

 *

 * BLOCK_RSV_DELREFS

 *   We keep a running tally of how many delayed refs we have on the system.

 *   We assume each one of these delayed refs are going to use a full

 *   reservation.  We use the transaction items and pre-reserve space for every

 *   operation, and use this reservation to refill any gap between ->size and

 *   ->reserved that may exist.

 *

 *   From there it's straightforward, removing a delayed ref means we remove its

 *   count from ->size and free up reservations as necessary.  Since this is

 *   the most dynamic block reserve in the system, we will try to refill this

 *   block reserve first with any excess returned by any other block reserve.

 *

 * BLOCK_RSV_EMPTY

 *   This is the fallback block reserve to make us try to reserve space if we

 *   don't have a specific bucket for this allocation.  It is mostly used for

 *   updating the device tree and such, since that is a separate pool we're

 *   content to just reserve space from the space_info on demand.

 *

 * BLOCK_RSV_TEMP

 *   This is used by things like truncate and iput.  We will temporarily

 *   allocate a block reserve, set it to some size, and then truncate bytes

 *   until we have no space left.  With ->failfast set we'll simply return

 *   ENOSPC from btrfs_use_block_rsv() to signal that we need to unwind and try

 *   to make a new reservation.  This is because these operations are

 *   unbounded, so we want to do as much work as we can, and then back off and

 *   re-reserve.

 */

static u64 block_rsv_release_bytes(struct btrfs_fs_info *fs_info,

				    struct btrfs_block_rsv *block_rsv,

				    struct btrfs_block_rsv *dest, u64 num_bytes,