drivers/md/bcache/btree.h

   1 #ifndef _BCACHE_BTREE_H
   2 #define _BCACHE_BTREE_H
   3
   4 /*
   5  * THE BTREE:
   6  *
   7  * At a high level, bcache's btree is relatively standard b+ tree. All keys and
   8  * pointers are in the leaves; interior nodes only have pointers to the child
   9  * nodes.
  10  *
  11  * In the interior nodes, a struct bkey always points to a child btree node, and
  12  * the key is the highest key in the child node - except that the highest key in
  13  * an interior node is always MAX_KEY. The size field refers to the size on disk
  14  * of the child node - this would allow us to have variable sized btree nodes
  15  * (handy for keeping the depth of the btree 1 by expanding just the root).
  16  *
  17  * Btree nodes are themselves log structured, but this is hidden fairly
  18  * thoroughly. Btree nodes on disk will in practice have extents that overlap
  19  * (because they were written at different times), but in memory we never have
  20  * overlapping extents - when we read in a btree node from disk, the first thing
  21  * we do is resort all the sets of keys with a mergesort, and in the same pass
  22  * we check for overlapping extents and adjust them appropriately.
  23  *
  24  * struct btree_op is a central interface to the btree code. It's used for
  25  * specifying read vs. write locking, and the embedded closure is used for
  26  * waiting on IO or reserve memory.
  27  *
  28  * BTREE CACHE:
  29  *
  30  * Btree nodes are cached in memory; traversing the btree might require reading
  31  * in btree nodes which is handled mostly transparently.
  32  *
  33  * bch_btree_node_get() looks up a btree node in the cache and reads it in from
  34  * disk if necessary. This function is almost never called directly though - the
  35  * btree() macro is used to get a btree node, call some function on it, and
  36  * unlock the node after the function returns.
  37  *
  38  * The root is special cased - it's taken out of the cache's lru (thus pinning
  39  * it in memory), so we can find the root of the btree by just dereferencing a
  40  * pointer instead of looking it up in the cache. This makes locking a bit
  41  * tricky, since the root pointer is protected by the lock in the btree node it
  42  * points to - the btree_root() macro handles this.
  43  *
  44  * In various places we must be able to allocate memory for multiple btree nodes
  45  * in order to make forward progress. To do this we use the btree cache itself
  46  * as a reserve; if __get_free_pages() fails, we'll find a node in the btree
  47  * cache we can reuse. We can't allow more than one thread to be doing this at a
  48  * time, so there's a lock, implemented by a pointer to the btree_op closure -
  49  * this allows the btree_root() macro to implicitly release this lock.
  50  *
  51  * BTREE IO:
  52  *
  53  * Btree nodes never have to be explicitly read in; bch_btree_node_get() handles
  54  * this.
  55  *
  56  * For writing, we have two btree_write structs embeddded in struct btree - one
  57  * write in flight, and one being set up, and we toggle between them.
  58  *
  59  * Writing is done with a single function -  bch_btree_write() really serves two
  60  * different purposes and should be broken up into two different functions. When
  61  * passing now = false, it merely indicates that the node is now dirty - calling
  62  * it ensures that the dirty keys will be written at some point in the future.
  63  *
  64  * When passing now = true, bch_btree_write() causes a write to happen
  65  * "immediately" (if there was already a write in flight, it'll cause the write
  66  * to happen as soon as the previous write completes). It returns immediately
  67  * though - but it takes a refcount on the closure in struct btree_op you passed
  68  * to it, so a closure_sync() later can be used to wait for the write to
  69  * complete.
  70  *
  71  * This is handy because btree_split() and garbage collection can issue writes
  72  * in parallel, reducing the amount of time they have to hold write locks.
  73  *
  74  * LOCKING:
  75  *
  76  * When traversing the btree, we may need write locks starting at some level -
  77  * inserting a key into the btree will typically only require a write lock on
  78  * the leaf node.
  79  *
  80  * This is specified with the lock field in struct btree_op; lock = 0 means we
  81  * take write locks at level <= 0, i.e. only leaf nodes. bch_btree_node_get()
  82  * checks this field and returns the node with the appropriate lock held.
  83  *
  84  * If, after traversing the btree, the insertion code discovers it has to split
  85  * then it must restart from the root and take new locks - to do this it changes
  86  * the lock field and returns -EINTR, which causes the btree_root() macro to
  87  * loop.
  88  *
  89  * Handling cache misses require a different mechanism for upgrading to a write
  90  * lock. We do cache lookups with only a read lock held, but if we get a cache
  91  * miss and we wish to insert this data into the cache, we have to insert a
  92  * placeholder key to detect races - otherwise, we could race with a write and
  93  * overwrite the data that was just written to the cache with stale data from
  94  * the backing device.
  95  *
  96  * For this we use a sequence number that write locks and unlocks increment - to
  97  * insert the check key it unlocks the btree node and then takes a write lock,
  98  * and fails if the sequence number doesn't match.
  99  */
 100
 101 #include "bset.h"
 102 #include "debug.h"
 103
 104 struct btree_write {
 105         atomic_t                *journal;
 106
 107         /* If btree_split() frees a btree node, it writes a new pointer to that
 108          * btree node indicating it was freed; it takes a refcount on
 109          * c->prio_blocked because we can't write the gens until the new
 110          * pointer is on disk. This allows btree_write_endio() to release the
 111          * refcount that btree_split() took.
 112          */
 113         int                     prio_blocked;
 114 };
 115
 116 struct btree {
 117         /* Hottest entries first */
 118         struct hlist_node       hash;
 119
 120         /* Key/pointer for this btree node */
 121         BKEY_PADDED(key);
 122
 123         /* Single bit - set when accessed, cleared by shrinker */
 124         unsigned long           accessed;
 125         unsigned long           seq;
 126         struct rw_semaphore     lock;
 127         struct cache_set        *c;
 128         struct btree            *parent;
 129
 130         struct mutex            write_lock;
 131
 132         unsigned long           flags;
 133         uint16_t                written;        /* would be nice to kill */
 134         uint8_t                 level;
 135
 136         struct btree_keys       keys;
 137
 138         /* For outstanding btree writes, used as a lock - protects write_idx */
 139         struct closure          io;
 140         struct semaphore        io_mutex;
 141
 142         struct list_head        list;
 143         struct delayed_work     work;
 144
 145         struct btree_write      writes[2];
 146         struct bio              *bio;
 147 };
 148
 149 #define BTREE_FLAG(flag)                                                \
 150 static inline bool btree_node_ ## flag(struct btree *b)                 \
 151 {       return test_bit(BTREE_NODE_ ## flag, &b->flags); }              \
 152                                                                         \
 153 static inline void set_btree_node_ ## flag(struct btree *b)             \
 154 {       set_bit(BTREE_NODE_ ## flag, &b->flags); }                      \
 155
 156 enum btree_flags {
 157         BTREE_NODE_io_error,
 158         BTREE_NODE_dirty,
 159         BTREE_NODE_write_idx,
 160 };
 161
 162 BTREE_FLAG(io_error);
 163 BTREE_FLAG(dirty);
 164 BTREE_FLAG(write_idx);
 165
 166 static inline struct btree_write *btree_current_write(struct btree *b)
 167 {
 168         return b->writes + btree_node_write_idx(b);
 169 }
 170
 171 static inline struct btree_write *btree_prev_write(struct btree *b)
 172 {
 173         return b->writes + (btree_node_write_idx(b) ^ 1);
 174 }
 175
 176 static inline struct bset *btree_bset_first(struct btree *b)
 177 {
 178         return b->keys.set->data;
 179 }
 180
 181 static inline struct bset *btree_bset_last(struct btree *b)
 182 {
 183         return bset_tree_last(&b->keys)->data;
 184 }
 185
 186 static inline unsigned bset_block_offset(struct btree *b, struct bset *i)
 187 {
 188         return bset_sector_offset(&b->keys, i) >> b->c->block_bits;
 189 }
 190
 191 static inline void set_gc_sectors(struct cache_set *c)
 192 {
 193         atomic_set(&c->sectors_to_gc, c->sb.bucket_size * c->nbuckets / 16);
 194 }
 195
 196 void bkey_put(struct cache_set *c, struct bkey *k);
 197
 198 /* Looping macros */
 199
 200 #define for_each_cached_btree(b, c, iter)                               \
 201         for (iter = 0;                                                  \
 202              iter < ARRAY_SIZE((c)->bucket_hash);                       \
 203              iter++)                                                    \
 204                 hlist_for_each_entry_rcu((b), (c)->bucket_hash + iter, hash)
 205
 206 /* Recursing down the btree */
 207
 208 struct btree_op {
 209         /* for waiting on btree reserve in btree_split() */
 210         wait_queue_t            wait;
 211
 212         /* Btree level at which we start taking write locks */
 213         short                   lock;
 214
 215         unsigned                insert_collision:1;
 216 };
 217
 218 static inline void bch_btree_op_init(struct btree_op *op, int write_lock_level)
 219 {
 220         memset(op, 0, sizeof(struct btree_op));
 221         init_wait(&op->wait);
 222         op->lock = write_lock_level;
 223 }
 224
 225 static inline void rw_lock(bool w, struct btree *b, int level)
 226 {
 227         w ? down_write_nested(&b->lock, level + 1)
 228           : down_read_nested(&b->lock, level + 1);
 229         if (w)
 230                 b->seq++;
 231 }
 232
 233 static inline void rw_unlock(bool w, struct btree *b)
 234 {
 235         if (w)
 236                 b->seq++;
 237         (w ? up_write : up_read)(&b->lock);
 238 }
 239
 240 void bch_btree_node_read_done(struct btree *);
 241 void __bch_btree_node_write(struct btree *, struct closure *);
 242 void bch_btree_node_write(struct btree *, struct closure *);
 243
 244 void bch_btree_set_root(struct btree *);
 245 struct btree *__bch_btree_node_alloc(struct cache_set *, struct btree_op *,
 246                                      int, bool, struct btree *);
 247 struct btree *bch_btree_node_get(struct cache_set *, struct btree_op *,
 248                                  struct bkey *, int, bool, struct btree *);
 249
 250 int bch_btree_insert_check_key(struct btree *, struct btree_op *,
 251                                struct bkey *);
 252 int bch_btree_insert(struct cache_set *, struct keylist *,
 253                      atomic_t *, struct bkey *);
 254
 255 int bch_gc_thread_start(struct cache_set *);
 256 void bch_initial_gc_finish(struct cache_set *);
 257 void bch_moving_gc(struct cache_set *);
 258 int bch_btree_check(struct cache_set *);
 259 void bch_initial_mark_key(struct cache_set *, int, struct bkey *);
 260
 261 static inline void wake_up_gc(struct cache_set *c)
 262 {
 263         if (c->gc_thread)
 264                 wake_up_process(c->gc_thread);
 265 }
 266
 267 #define MAP_DONE        0
 268 #define MAP_CONTINUE    1
 269
 270 #define MAP_ALL_NODES   0
 271 #define MAP_LEAF_NODES  1
 272
 273 #define MAP_END_KEY     1
 274
 275 typedef int (btree_map_nodes_fn)(struct btree_op *, struct btree *);
 276 int __bch_btree_map_nodes(struct btree_op *, struct cache_set *,
 277                           struct bkey *, btree_map_nodes_fn *, int);
 278
 279 static inline int bch_btree_map_nodes(struct btree_op *op, struct cache_set *c,
 280                                       struct bkey *from, btree_map_nodes_fn *fn)
 281 {
 282         return __bch_btree_map_nodes(op, c, from, fn, MAP_ALL_NODES);
 283 }
 284
 285 static inline int bch_btree_map_leaf_nodes(struct btree_op *op,
 286                                            struct cache_set *c,
 287                                            struct bkey *from,
 288                                            btree_map_nodes_fn *fn)
 289 {
 290         return __bch_btree_map_nodes(op, c, from, fn, MAP_LEAF_NODES);
 291 }
 292
 293 typedef int (btree_map_keys_fn)(struct btree_op *, struct btree *,
 294                                 struct bkey *);
 295 int bch_btree_map_keys(struct btree_op *, struct cache_set *,
 296                        struct bkey *, btree_map_keys_fn *, int);
 297
 298 typedef bool (keybuf_pred_fn)(struct keybuf *, struct bkey *);
 299
 300 void bch_keybuf_init(struct keybuf *);
 301 void bch_refill_keybuf(struct cache_set *, struct keybuf *,
 302                        struct bkey *, keybuf_pred_fn *);
 303 bool bch_keybuf_check_overlapping(struct keybuf *, struct bkey *,
 304                                   struct bkey *);
 305 void bch_keybuf_del(struct keybuf *, struct keybuf_key *);
 306 struct keybuf_key *bch_keybuf_next(struct keybuf *);
 307 struct keybuf_key *bch_keybuf_next_rescan(struct cache_set *, struct keybuf *,
 308                                           struct bkey *, keybuf_pred_fn *);
 309
 310 #endif