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 */ 231