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/*
* 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
};
/*
* When issuing a POLICY_REPLACE the policy needs to make a callback to
* lock the block being demoted. This doesn't need to occur during a
* writeback operation since the block remains in the cache.
*/
struct policy_locker;
typedef int (*policy_lock_fn)(struct policy_locker *l, dm_oblock_t oblock);
struct policy_locker {
policy_lock_fn fn;
};
/*
* 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_locker *locker,
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 0 if in cache, -ENOENT if not, < 0 for other errors
* (-EWOULDBLOCK would be typical).
*/
int (*lookup)(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock);
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);
/*
* This is called via the invalidate_cblocks message. It is
* possible the particular cblock has already been removed due to a
* write io in passthrough mode. In which case this should return
* -ENODATA.
*/
int (*remove_cblock)(struct dm_cache_policy *p, dm_cblock_t cblock);
/*
* Provide a dirty block to be written back by the core target. If
* critical_only is set then the policy should only provide work if
* it urgently needs it.
*
* Returns:
*
* 0 and @cblock,@oblock: block to write back provided
*
* -ENODATA: no dirty blocks available
*/
int (*writeback_work)(struct dm_cache_policy *p, dm_oblock_t *oblock, dm_cblock_t *cblock,
bool critical_only);
/*
* 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, bool can_block);
/*
* Configuration.
*/
int (*emit_config_values)(struct dm_cache_policy *p, char *result,
unsigned maxlen, ssize_t *sz_ptr);
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
#define CACHE_POLICY_VERSION_SIZE 3
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];
unsigned version[CACHE_POLICY_VERSION_SIZE];
/*
* For use by an alias dm_cache_policy_type to point to the
* real dm_cache_policy_type.
*/
struct dm_cache_policy_type *real;
/*
* 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 */
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