[PATCH 1/3] btrfs: add a comment describing block-rsvs

[Josef Bacik <josef@toxicpanda.com>]

This is a giant comment at the top of block-rsv.c describing generally
how block rsvs work.  It is purely about the block rsv's themselves, and
nothing to do with how the actual reservation system works.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/block-rsv.c | 81 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index d07bd41a7c1e..54380f477f80 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -6,6 +6,87 @@
 #include "space-info.h"
 #include "transaction.h"
 
+/*
+ * HOW DO BLOCK RSVS WORK
+ *
+ *   Think of block_rsv's as bucktes for logically grouped 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
+ *   We determine we need N items of reservation, we use the appropriate
+ *   btrfs_calc*() helper to determine the number of bytes.  We call into
+ *   reserve_metadata_bytes() and get our bytes, we then add this space to our
+ *   ->size and our ->reserved.
+ *
+ *   We go to modify the tree for our operation, we allocate a tree block, which
+ *   calls btrfs_use_block_rsv(), and subtracts nodesize from
+ *   block_rsv->reserved.
+ *
+ *   We finish our operation, we subtract our original reservation from ->size,
+ *   and then we subtract ->size from ->reserved if there is an excess and free
+ *   the excess back to the space info, by reducing space_info->bytes_may_use by
+ *   the excess amount.
+ *
+ *   In some cases we may return this excess to the global block reserve or
+ *   delayed refs reserve if either of their ->size is greater than their
+ *   ->reserved.
+ *
+ * BLOCK_RSV_TRANS, BLOCK_RSV_DELOPS, BLOCK_RSV_CHUNK
+ *   These behave normally, as described above, just within the confines of the
+ *   lifetime of ther particular operation (transaction for the whole trans
+ *   handle lifetime, for example).
+ *
+ * BLOCK_RSV_GLOBAL
+ *   This has existed forever, with diminishing degrees of importance.
+ *   Currently it exists to save us from ourselves.  We definitely over-reserve
+ *   space most of the time, but the nature of COW is that we do not know how
+ *   much space we may need to use for any given operation.  This is
+ *   particularly true about the extent tree.  Modifying one extent could
+ *   balloon into 1000 modifications of the extent tree, which we have no way of
+ *   properly predicting.  To cover this case we have the global reserve act as
+ *   the "root" space to allow us to not abort the transaciton when things are
+ *   very tight.  As such we tend to treat this space as sacred, and only use it
+ *   if we are desparate.  Generally we should no longer be depending on its
+ *   space, and if new use cases arise we need to address them elsewhere.
+ *
+ * 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 talley 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 rsv in the system, we will try to refill this block rsv
+ *   first with any excess returned by any other block reserve.
+ *
+ * BLOCK_RSV_EMPTY
+ *   This is the fallback block rsv 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 rsv, 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,
-- 
2.24.1