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Diffstat (limited to 'drivers/md/dm-cache-policy.h')
| -rw-r--r-- | drivers/md/dm-cache-policy.h | 228 |
1 files changed, 228 insertions, 0 deletions
diff --git a/drivers/md/dm-cache-policy.h b/drivers/md/dm-cache-policy.h new file mode 100644 index 00000000000..f0f51b26054 --- /dev/null +++ b/drivers/md/dm-cache-policy.h @@ -0,0 +1,228 @@ +/* + * Copyright (C) 2012 Red Hat. All rights reserved. + * + * This file is released under the GPL. + */ + +#ifndef DM_CACHE_POLICY_H +#define DM_CACHE_POLICY_H + +#include "dm-cache-block-types.h" + +#include <linux/device-mapper.h> + +/*----------------------------------------------------------------*/ + +/* FIXME: make it clear which methods are optional. Get debug policy to + * double check this at start. + */ + +/* + * The cache policy makes the important decisions about which blocks get to + * live on the faster cache device. + * + * When the core target has to remap a bio it calls the 'map' method of the + * policy. This returns an instruction telling the core target what to do. + * + * POLICY_HIT: + * That block is in the cache. Remap to the cache and carry on. + * + * POLICY_MISS: + * This block is on the origin device. Remap and carry on. + * + * POLICY_NEW: + * This block is currently on the origin device, but the policy wants to + * move it. The core should: + * + * - hold any further io to this origin block + * - copy the origin to the given cache block + * - release all the held blocks + * - remap the original block to the cache + * + * POLICY_REPLACE: + * This block is currently on the origin device. The policy wants to + * move it to the cache, with the added complication that the destination + * cache block needs a writeback first. The core should: + * + * - hold any further io to this origin block + * - hold any further io to the origin block that's being written back + * - writeback + * - copy new block to cache + * - release held blocks + * - remap bio to cache and reissue. + * + * Should the core run into trouble while processing a POLICY_NEW or + * POLICY_REPLACE instruction it will roll back the policies mapping using + * remove_mapping() or force_mapping(). These methods must not fail. This + * approach avoids having transactional semantics in the policy (ie, the + * core informing the policy when a migration is complete), and hence makes + * it easier to write new policies. + * + * In general policy methods should never block, except in the case of the + * map function when can_migrate is set. So be careful to implement using + * bounded, preallocated memory. + */ +enum policy_operation { + POLICY_HIT, + POLICY_MISS, + POLICY_NEW, + POLICY_REPLACE +}; + +/* + * This is the instruction passed back to the core target. + */ +struct policy_result { + enum policy_operation op; + dm_oblock_t old_oblock; /* POLICY_REPLACE */ + dm_cblock_t cblock; /* POLICY_HIT, POLICY_NEW, POLICY_REPLACE */ +}; + +typedef int (*policy_walk_fn)(void *context, dm_cblock_t cblock, + dm_oblock_t oblock, uint32_t hint); + +/* + * The cache policy object. Just a bunch of methods. It is envisaged that + * this structure will be embedded in a bigger, policy specific structure + * (ie. use container_of()). + */ +struct dm_cache_policy { + + /* + * FIXME: make it clear which methods are optional, and which may + * block. + */ + + /* + * Destroys this object. + */ + void (*destroy)(struct dm_cache_policy *p); + + /* + * See large comment above. + * + * oblock - the origin block we're interested in. + * + * can_block - indicates whether the current thread is allowed to + * block. -EWOULDBLOCK returned if it can't and would. + * + * can_migrate - gives permission for POLICY_NEW or POLICY_REPLACE + * instructions. If denied and the policy would have + * returned one of these instructions it should + * return -EWOULDBLOCK. + * + * discarded_oblock - indicates whether the whole origin block is + * in a discarded state (FIXME: better to tell the + * policy about this sooner, so it can recycle that + * cache block if it wants.) + * bio - the bio that triggered this call. + * result - gets filled in with the instruction. + * + * May only return 0, or -EWOULDBLOCK (if !can_migrate) + */ + int (*map)(struct dm_cache_policy *p, dm_oblock_t oblock, + bool can_block, bool can_migrate, bool discarded_oblock, + struct bio *bio, struct policy_result *result); + + /* + * Sometimes we want to see if a block is in the cache, without + * triggering any update of stats. (ie. it's not a real hit). + * + * Must not block. + * + * Returns 1 iff in cache, 0 iff not, < 0 on error (-EWOULDBLOCK + * would be typical). + */ + int (*lookup)(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock); + + /* + * oblock must be a mapped block. Must not block. + */ + void (*set_dirty)(struct dm_cache_policy *p, dm_oblock_t oblock); + void (*clear_dirty)(struct dm_cache_policy *p, dm_oblock_t oblock); + + /* + * Called when a cache target is first created. Used to load a + * mapping from the metadata device into the policy. + */ + int (*load_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock, + dm_cblock_t cblock, uint32_t hint, bool hint_valid); + + int (*walk_mappings)(struct dm_cache_policy *p, policy_walk_fn fn, + void *context); + + /* + * Override functions used on the error paths of the core target. + * They must succeed. + */ + void (*remove_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock); + void (*force_mapping)(struct dm_cache_policy *p, dm_oblock_t current_oblock, + dm_oblock_t new_oblock); + + int (*writeback_work)(struct dm_cache_policy *p, dm_oblock_t *oblock, dm_cblock_t *cblock); + + + /* + * How full is the cache? + */ + dm_cblock_t (*residency)(struct dm_cache_policy *p); + + /* + * Because of where we sit in the block layer, we can be asked to + * map a lot of little bios that are all in the same block (no + * queue merging has occurred). To stop the policy being fooled by + * these the core target sends regular tick() calls to the policy. + * The policy should only count an entry as hit once per tick. + */ + void (*tick)(struct dm_cache_policy *p); + + /* + * Configuration. + */ + int (*emit_config_values)(struct dm_cache_policy *p, + char *result, unsigned maxlen); + int (*set_config_value)(struct dm_cache_policy *p, + const char *key, const char *value); + + /* + * Book keeping ptr for the policy register, not for general use. + */ + void *private; +}; + +/*----------------------------------------------------------------*/ + +/* + * We maintain a little register of the different policy types. + */ +#define CACHE_POLICY_NAME_SIZE 16 + +struct dm_cache_policy_type { + /* For use by the register code only. */ + struct list_head list; + + /* + * Policy writers should fill in these fields. The name field is + * what gets passed on the target line to select your policy. + */ + char name[CACHE_POLICY_NAME_SIZE]; + + /* + * Policies may store a hint for each each cache block. + * Currently the size of this hint must be 0 or 4 bytes but we + * expect to relax this in future. + */ + size_t hint_size; + + struct module *owner; + struct dm_cache_policy *(*create)(dm_cblock_t cache_size, + sector_t origin_size, + sector_t block_size); +}; + +int dm_cache_policy_register(struct dm_cache_policy_type *type); +void dm_cache_policy_unregister(struct dm_cache_policy_type *type); + +/*----------------------------------------------------------------*/ + +#endif /* DM_CACHE_POLICY_H */ |