#ifndef _BCACHE_BTREE_H
  #define _BCACHE_BTREE_H
  
  /*
   * THE BTREE:
   *
   * At a high level, bcache's btree is relatively standard b+ tree. All keys and
   * pointers are in the leaves; interior nodes only have pointers to the child
   * nodes.
   *
   * In the interior nodes, a struct bkey always points to a child btree node, and
   * the key is the highest key in the child node - except that the highest key in
   * an interior node is always MAX_KEY. The size field refers to the size on disk
   * of the child node - this would allow us to have variable sized btree nodes
   * (handy for keeping the depth of the btree 1 by expanding just the root).
   *
   * Btree nodes are themselves log structured, but this is hidden fairly
   * thoroughly. Btree nodes on disk will in practice have extents that overlap
   * (because they were written at different times), but in memory we never have
   * overlapping extents - when we read in a btree node from disk, the first thing
   * we do is resort all the sets of keys with a mergesort, and in the same pass
   * we check for overlapping extents and adjust them appropriately.
   *
   * struct btree_op is a central interface to the btree code. It's used for
   * specifying read vs. write locking, and the embedded closure is used for
   * waiting on IO or reserve memory.
   *
   * BTREE CACHE:
   *
   * Btree nodes are cached in memory; traversing the btree might require reading
   * in btree nodes which is handled mostly transparently.
   *
   * bch_btree_node_get() looks up a btree node in the cache and reads it in from
   * disk if necessary. This function is almost never called directly though - the
   * btree() macro is used to get a btree node, call some function on it, and
   * unlock the node after the function returns.
   *
   * The root is special cased - it's taken out of the cache's lru (thus pinning
   * it in memory), so we can find the root of the btree by just dereferencing a
   * pointer instead of looking it up in the cache. This makes locking a bit
   * tricky, since the root pointer is protected by the lock in the btree node it
   * points to - the btree_root() macro handles this.
   *
   * In various places we must be able to allocate memory for multiple btree nodes
   * in order to make forward progress. To do this we use the btree cache itself
   * as a reserve; if __get_free_pages() fails, we'll find a node in the btree
   * cache we can reuse. We can't allow more than one thread to be doing this at a
   * time, so there's a lock, implemented by a pointer to the btree_op closure -
   * this allows the btree_root() macro to implicitly release this lock.
   *
   * BTREE IO:
   *
   * Btree nodes never have to be explicitly read in; bch_btree_node_get() handles
   * this.
   *
   * For writing, we have two btree_write structs embeddded in struct btree - one
   * write in flight, and one being set up, and we toggle between them.
   *
   * Writing is done with a single function -  bch_btree_write() really serves two
   * different purposes and should be broken up into two different functions. When
   * passing now = false, it merely indicates that the node is now dirty - calling
   * it ensures that the dirty keys will be written at some point in the future.
   *
   * When passing now = true, bch_btree_write() causes a write to happen
   * "immediately" (if there was already a write in flight, it'll cause the write
   * to happen as soon as the previous write completes). It returns immediately
   * though - but it takes a refcount on the closure in struct btree_op you passed
   * to it, so a closure_sync() later can be used to wait for the write to
   * complete.
   *
   * This is handy because btree_split() and garbage collection can issue writes
   * in parallel, reducing the amount of time they have to hold write locks.
   *
   * LOCKING:
   *
   * When traversing the btree, we may need write locks starting at some level -
   * inserting a key into the btree will typically only require a write lock on
   * the leaf node.
   *
   * This is specified with the lock field in struct btree_op; lock = 0 means we
   * take write locks at level <= 0, i.e. only leaf nodes. bch_btree_node_get()
   * checks this field and returns the node with the appropriate lock held.
   *
   * If, after traversing the btree, the insertion code discovers it has to split
   * then it must restart from the root and take new locks - to do this it changes
   * the lock field and returns -EINTR, which causes the btree_root() macro to
   * loop.
   *
   * Handling cache misses require a different mechanism for upgrading to a write
   * lock. We do cache lookups with only a read lock held, but if we get a cache
   * miss and we wish to insert this data into the cache, we have to insert a
   * placeholder key to detect races - otherwise, we could race with a write and
   * overwrite the data that was just written to the cache with stale data from
   * the backing device.
   *
   * For this we use a sequence number that write locks and unlocks increment - to
   * insert the check key it unlocks the btree node and then takes a write lock,
   * and fails if the sequence number doesn't match.
   */
  
  #include "bset.h"
  #include "debug.h"
  
  struct btree_write {
  	atomic_t		*journal;
  
  	/* If btree_split() frees a btree node, it writes a new pointer to that
  	 * btree node indicating it was freed; it takes a refcount on
  	 * c->prio_blocked because we can't write the gens until the new
  	 * pointer is on disk. This allows btree_write_endio() to release the
  	 * refcount that btree_split() took.
  	 */
  	int			prio_blocked;
  };
  
  struct btree {
  	/* Hottest entries first */
  	struct hlist_node	hash;
  
  	/* Key/pointer for this btree node */
  	BKEY_PADDED(key);
  
  	/* Single bit - set when accessed, cleared by shrinker */
  	unsigned long		accessed;
  	unsigned long		seq;
  	struct rw_semaphore	lock;
  	struct cache_set	*c;
  	struct btree		*parent;
  
  	unsigned long		flags;
  	uint16_t		written;	/* would be nice to kill */
  	uint8_t			level;
  
  	struct btree_keys	keys;
  
  	/* For outstanding btree writes, used as a lock - protects write_idx */
  	struct closure		io;
  	struct semaphore	io_mutex;
  
  	struct list_head	list;
  	struct delayed_work	work;
  
  	struct btree_write	writes[2];
  	struct bio		*bio;
  };
  
  #define BTREE_FLAG(flag)						\
  static inline bool btree_node_ ## flag(struct btree *b)			\
  {	return test_bit(BTREE_NODE_ ## flag, &b->flags); }		\
  									\
  static inline void set_btree_node_ ## flag(struct btree *b)		\
  {	set_bit(BTREE_NODE_ ## flag, &b->flags); }			\
  
  enum btree_flags {
  	BTREE_NODE_io_error,
  	BTREE_NODE_dirty,
  	BTREE_NODE_write_idx,
  };
  
  BTREE_FLAG(io_error);
  BTREE_FLAG(dirty);
  BTREE_FLAG(write_idx);
  
  static inline struct btree_write *btree_current_write(struct btree *b)
  {
  	return b->writes + btree_node_write_idx(b);
  }
  
  static inline struct btree_write *btree_prev_write(struct btree *b)
  {
  	return b->writes + (btree_node_write_idx(b) ^ 1);
  }
  
  static inline struct bset *btree_bset_first(struct btree *b)
  {
  	return b->keys.set->data;
  }
  
  static inline struct bset *btree_bset_last(struct btree *b)
  {
  	return bset_tree_last(&b->keys)->data;
  }
  
  static inline unsigned bset_block_offset(struct btree *b, struct bset *i)
  {
  	return bset_sector_offset(&b->keys, i) >> b->c->block_bits;
  }
  
  static inline void set_gc_sectors(struct cache_set *c)
  {
  	atomic_set(&c->sectors_to_gc, c->sb.bucket_size * c->nbuckets / 16);
  }
  
  void bkey_put(struct cache_set *c, struct bkey *k);
  
  /* Looping macros */
  
  #define for_each_cached_btree(b, c, iter)				\
  	for (iter = 0;							\
  	     iter < ARRAY_SIZE((c)->bucket_hash);			\
  	     iter++)							\
  		hlist_for_each_entry_rcu((b), (c)->bucket_hash + iter, hash)
  
  /* Recursing down the btree */
  
  struct btree_op {
  	/* for waiting on btree reserve in btree_split() */
  	wait_queue_t		wait;
  
  	/* Btree level at which we start taking write locks */
  	short			lock;
  
  	unsigned		insert_collision:1;
  };
  
  static inline void bch_btree_op_init(struct btree_op *op, int write_lock_level)
  {
  	memset(op, 0, sizeof(struct btree_op));
  	init_wait(&op->wait);
  	op->lock = write_lock_level;
  }
  
  static inline void rw_lock(bool w, struct btree *b, int level)
  {
  	w ? down_write_nested(&b->lock, level + 1)
  	  : down_read_nested(&b->lock, level + 1);
  	if (w)
  		b->seq++;
  }
  
  static inline void rw_unlock(bool w, struct btree *b)
  {
  	if (w)
  		b->seq++;
  	(w ? up_write : up_read)(&b->lock);
  }
  
  void bch_btree_node_read_done(struct btree *);
  void bch_btree_node_write(struct btree *, struct closure *);
  
  void bch_btree_set_root(struct btree *);
  struct btree *bch_btree_node_alloc(struct cache_set *, int, bool);
  struct btree *bch_btree_node_get(struct cache_set *, struct bkey *, int, bool);
  
  int bch_btree_insert_check_key(struct btree *, struct btree_op *,
  			       struct bkey *);
  int bch_btree_insert(struct cache_set *, struct keylist *,
  		     atomic_t *, struct bkey *);
  
  int bch_gc_thread_start(struct cache_set *);
  size_t bch_btree_gc_finish(struct cache_set *);
  void bch_moving_gc(struct cache_set *);
  int bch_btree_check(struct cache_set *);
  uint8_t __bch_btree_mark_key(struct cache_set *, int, struct bkey *);
  
  static inline void wake_up_gc(struct cache_set *c)
  {
  	if (c->gc_thread)
  		wake_up_process(c->gc_thread);
  }
  
  #define MAP_DONE	0
  #define MAP_CONTINUE	1
  
  #define MAP_ALL_NODES	0
  #define MAP_LEAF_NODES	1
  
  #define MAP_END_KEY	1
  
  typedef int (btree_map_nodes_fn)(struct btree_op *, struct btree *);
  int __bch_btree_map_nodes(struct btree_op *, struct cache_set *,
  			  struct bkey *, btree_map_nodes_fn *, int);
  
  static inline int bch_btree_map_nodes(struct btree_op *op, struct cache_set *c,
  				      struct bkey *from, btree_map_nodes_fn *fn)
  {
  	return __bch_btree_map_nodes(op, c, from, fn, MAP_ALL_NODES);
  }
  
  static inline int bch_btree_map_leaf_nodes(struct btree_op *op,
  					   struct cache_set *c,
  					   struct bkey *from,
  					   btree_map_nodes_fn *fn)
  {
  	return __bch_btree_map_nodes(op, c, from, fn, MAP_LEAF_NODES);
  }
  
  typedef int (btree_map_keys_fn)(struct btree_op *, struct btree *,
  				struct bkey *);
  int bch_btree_map_keys(struct btree_op *, struct cache_set *,
  		       struct bkey *, btree_map_keys_fn *, int);
  
  typedef bool (keybuf_pred_fn)(struct keybuf *, struct bkey *);
  
  void bch_keybuf_init(struct keybuf *);
  void bch_refill_keybuf(struct cache_set *, struct keybuf *,
  		       struct bkey *, keybuf_pred_fn *);
  bool bch_keybuf_check_overlapping(struct keybuf *, struct bkey *,
  				  struct bkey *);
  void bch_keybuf_del(struct keybuf *, struct keybuf_key *);
  struct keybuf_key *bch_keybuf_next(struct keybuf *);
  struct keybuf_key *bch_keybuf_next_rescan(struct cache_set *, struct keybuf *,
  					  struct bkey *, keybuf_pred_fn *);
  
  #endif