dm: add cache target

Add a target that allows a fast device such as an SSD to be used as a
cache for a slower device such as a disk.

A plug-in architecture was chosen so that the decisions about which data
to migrate and when are delegated to interchangeable tunable policy
modules.  The first general purpose module we have developed, called
"mq" (multiqueue), follows in the next patch.  Other modules are
under development.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Heinz Mauelshagen <mauelshagen@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

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 */