drivers/md/dm-cache-policy.h

   1 /*
   2  * Copyright (C) 2012 Red Hat. All rights reserved.
   3  *
   4  * This file is released under the GPL.
   5  */
   6
   7 #ifndef DM_CACHE_POLICY_H
   8 #define DM_CACHE_POLICY_H
   9
  10 #include "dm-cache-block-types.h"
  11
  12 #include <linux/device-mapper.h>
  13
  14 /*----------------------------------------------------------------*/
  15
  16 /* FIXME: make it clear which methods are optional.  Get debug policy to
  17  * double check this at start.
  18  */
  19
  20 /*
  21  * The cache policy makes the important decisions about which blocks get to
  22  * live on the faster cache device.
  23  *
  24  * When the core target has to remap a bio it calls the 'map' method of the
  25  * policy.  This returns an instruction telling the core target what to do.
  26  *
  27  * POLICY_HIT:
  28  *   That block is in the cache.  Remap to the cache and carry on.
  29  *
  30  * POLICY_MISS:
  31  *   This block is on the origin device.  Remap and carry on.
  32  *
  33  * POLICY_NEW:
  34  *   This block is currently on the origin device, but the policy wants to
  35  *   move it.  The core should:
  36  *
  37  *   - hold any further io to this origin block
  38  *   - copy the origin to the given cache block
  39  *   - release all the held blocks
  40  *   - remap the original block to the cache
  41  *
  42  * POLICY_REPLACE:
  43  *   This block is currently on the origin device.  The policy wants to
  44  *   move it to the cache, with the added complication that the destination
  45  *   cache block needs a writeback first.  The core should:
  46  *
  47  *   - hold any further io to this origin block
  48  *   - hold any further io to the origin block that's being written back
  49  *   - writeback
  50  *   - copy new block to cache
  51  *   - release held blocks
  52  *   - remap bio to cache and reissue.
  53  *
  54  * Should the core run into trouble while processing a POLICY_NEW or
  55  * POLICY_REPLACE instruction it will roll back the policies mapping using
  56  * remove_mapping() or force_mapping().  These methods must not fail.  This
  57  * approach avoids having transactional semantics in the policy (ie, the
  58  * core informing the policy when a migration is complete), and hence makes
  59  * it easier to write new policies.
  60  *
  61  * In general policy methods should never block, except in the case of the
  62  * map function when can_migrate is set.  So be careful to implement using
  63  * bounded, preallocated memory.
  64  */
  65 enum policy_operation {
  66         POLICY_HIT,
  67         POLICY_MISS,
  68         POLICY_NEW,
  69         POLICY_REPLACE
  70 };
  71
  72 /*
  73  * This is the instruction passed back to the core target.
  74  */
  75 struct policy_result {
  76         enum policy_operation op;
  77         dm_oblock_t old_oblock; /* POLICY_REPLACE */
  78         dm_cblock_t cblock;     /* POLICY_HIT, POLICY_NEW, POLICY_REPLACE */
  79 };
  80
  81 typedef int (*policy_walk_fn)(void *context, dm_cblock_t cblock,
  82                               dm_oblock_t oblock, uint32_t hint);
  83
  84 /*
  85  * The cache policy object.  Just a bunch of methods.  It is envisaged that
  86  * this structure will be embedded in a bigger, policy specific structure
  87  * (ie. use container_of()).
  88  */
  89 struct dm_cache_policy {
  90
  91         /*
  92          * FIXME: make it clear which methods are optional, and which may
  93          * block.
  94          */
  95
  96         /*
  97          * Destroys this object.
  98          */
  99         void (*destroy)(struct dm_cache_policy *p);
 100
 101         /*
 102          * See large comment above.
 103          *
 104          * oblock      - the origin block we're interested in.
 105          *
 106          * can_block - indicates whether the current thread is allowed to
 107          *             block.  -EWOULDBLOCK returned if it can't and would.
 108          *
 109          * can_migrate - gives permission for POLICY_NEW or POLICY_REPLACE
 110          *               instructions.  If denied and the policy would have
 111          *               returned one of these instructions it should
 112          *               return -EWOULDBLOCK.
 113          *
 114          * discarded_oblock - indicates whether the whole origin block is
 115          *               in a discarded state (FIXME: better to tell the
 116          *               policy about this sooner, so it can recycle that
 117          *               cache block if it wants.)
 118          * bio         - the bio that triggered this call.
 119          * result      - gets filled in with the instruction.
 120          *
 121          * May only return 0, or -EWOULDBLOCK (if !can_migrate)
 122          */
 123         int (*map)(struct dm_cache_policy *p, dm_oblock_t oblock,
 124                    bool can_block, bool can_migrate, bool discarded_oblock,
 125                    struct bio *bio, struct policy_result *result);
 126
 127         /*
 128          * Sometimes we want to see if a block is in the cache, without
 129          * triggering any update of stats.  (ie. it's not a real hit).
 130          *
 131          * Must not block.
 132          *
 133          * Returns 0 if in cache, -ENOENT if not, < 0 for other errors
 134          * (-EWOULDBLOCK would be typical).
 135          */
 136         int (*lookup)(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock);
 137
 138         /*
 139          * oblock must be a mapped block.  Must not block.
 140          */
 141         void (*set_dirty)(struct dm_cache_policy *p, dm_oblock_t oblock);
 142         void (*clear_dirty)(struct dm_cache_policy *p, dm_oblock_t oblock);
 143
 144         /*
 145          * Called when a cache target is first created.  Used to load a
 146          * mapping from the metadata device into the policy.
 147          */
 148         int (*load_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock,
 149                             dm_cblock_t cblock, uint32_t hint, bool hint_valid);
 150
 151         int (*walk_mappings)(struct dm_cache_policy *p, policy_walk_fn fn,
 152                              void *context);
 153
 154         /*
 155          * Override functions used on the error paths of the core target.
 156          * They must succeed.
 157          */
 158         void (*remove_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock);
 159         void (*force_mapping)(struct dm_cache_policy *p, dm_oblock_t current_oblock,
 160                               dm_oblock_t new_oblock);
 161
 162         int (*writeback_work)(struct dm_cache_policy *p, dm_oblock_t *oblock, dm_cblock_t *cblock);
 163
 164
 165         /*
 166          * How full is the cache?
 167          */
 168         dm_cblock_t (*residency)(struct dm_cache_policy *p);
 169
 170         /*
 171          * Because of where we sit in the block layer, we can be asked to
 172          * map a lot of little bios that are all in the same block (no
 173          * queue merging has occurred).  To stop the policy being fooled by
 174          * these the core target sends regular tick() calls to the policy.
 175          * The policy should only count an entry as hit once per tick.
 176          */
 177         void (*tick)(struct dm_cache_policy *p);
 178
 179         /*
 180          * Configuration.
 181          */
 182         int (*emit_config_values)(struct dm_cache_policy *p,
 183                                   char *result, unsigned maxlen);
 184         int (*set_config_value)(struct dm_cache_policy *p,
 185                                 const char *key, const char *value);
 186
 187         /*
 188          * Book keeping ptr for the policy register, not for general use.
 189          */
 190         void *private;
 191 };
 192
 193 /*----------------------------------------------------------------*/
 194
 195 /*
 196  * We maintain a little register of the different policy types.
 197  */
 198 #define CACHE_POLICY_NAME_SIZE 16
 199 #define CACHE_POLICY_VERSION_SIZE 3
 200
 201 struct dm_cache_policy_type {
 202         /* For use by the register code only. */
 203         struct list_head list;
 204
 205         /*
 206          * Policy writers should fill in these fields.  The name field is
 207          * what gets passed on the target line to select your policy.
 208          */
 209         char name[CACHE_POLICY_NAME_SIZE];
 210         unsigned version[CACHE_POLICY_VERSION_SIZE];
 211
 212         /*
 213          * Policies may store a hint for each each cache block.
 214          * Currently the size of this hint must be 0 or 4 bytes but we
 215          * expect to relax this in future.
 216          */
 217         size_t hint_size;
 218
 219         struct module *owner;
 220         struct dm_cache_policy *(*create)(dm_cblock_t cache_size,
 221                                           sector_t origin_size,
 222                                           sector_t block_size);
 223 };
 224
 225 int dm_cache_policy_register(struct dm_cache_policy_type *type);
 226 void dm_cache_policy_unregister(struct dm_cache_policy_type *type);
 227
 228 /*----------------------------------------------------------------*/
 229
 230 #endif  /* DM_CACHE_POLICY_H */