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Joe Thornberc6b4fcb2013-03-01 22:45:51 +00001/*
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 */
65enum 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 */
75struct 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
81typedef 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 */
89struct 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 1 iff in cache, 0 iff not, < 0 on error (-EWOULDBLOCK
134 * 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
Mike Snitzer4e7f5062013-03-20 17:21:27 +0000199#define CACHE_POLICY_VERSION_SIZE 3
Joe Thornberc6b4fcb2013-03-01 22:45:51 +0000200
201struct 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];
Mike Snitzer4e7f5062013-03-20 17:21:27 +0000210 unsigned version[CACHE_POLICY_VERSION_SIZE];
Joe Thornberc6b4fcb2013-03-01 22:45:51 +0000211
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
225int dm_cache_policy_register(struct dm_cache_policy_type *type);
226void dm_cache_policy_unregister(struct dm_cache_policy_type *type);
227
228/*----------------------------------------------------------------*/
229
230#endif /* DM_CACHE_POLICY_H */