dm vdo: add documentation details on zones and locking

Add details describing the vdo zone and thread model to the
documentation comments for major vdo components. Also added
some high-level description of the block map structure.
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-vdo/block-map.h

@@ -19,6 +19,21 @@
 #include "vio.h"
 #include "wait-queue.h"
 
+/*
+ * The block map is responsible for tracking all the logical to physical mappings of a VDO. It
+ * consists of a collection of 60 radix trees gradually allocated as logical addresses are used.
+ * Each tree is assigned to a logical zone such that it is easy to compute which zone must handle
+ * each logical address. Each logical zone also has a dedicated portion of the leaf page cache.
+ *
+ * Each logical zone has a single dedicated queue and thread for performing all updates to the
+ * radix trees assigned to that zone. The concurrency guarantees of this single-threaded model
+ * allow the code to omit more fine-grained locking for the block map structures.
+ *
+ * Load operations must be performed on the admin thread. Normal operations, such as reading and
+ * updating mappings, must be performed on the appropriate logical zone thread. Save operations
+ * must be launched from the same admin thread as the original load operation.
+ */
+
 enum {
 	BLOCK_MAP_VIO_POOL_SIZE = 64,
 };

drivers/md/dm-vdo/dedupe.c

@@ -14,6 +14,11 @@
  * deduplicate against a single block instead of being serialized through a PBN read lock. Only one
  * index query is needed for each hash_lock, instead of one for every data_vio.
  *
+ * Hash_locks are assigned to hash_zones by computing a modulus on the hash itself. Each hash_zone
+ * has a single dedicated queue and thread for performing all operations on the hash_locks assigned
+ * to that zone. The concurrency guarantees of this single-threaded model allow the code to omit
+ * more fine-grained locking for the hash_lock structures.
+ *
  * A hash_lock acts like a state machine perhaps more than as a lock. Other than the starting and
  * ending states INITIALIZING and BYPASSING, every state represents and is held for the duration of
  * an asynchronous operation. All state transitions are performed on the thread of the hash_zone

drivers/md/dm-vdo/recovery-journal.h

@@ -26,6 +26,10 @@
  * write amplification of writes by providing amortization of slab journal and block map page
  * updates.
  *
+ * The recovery journal has a single dedicated queue and thread for performing all journal updates.
+ * The concurrency guarantees of this single-threaded model allow the code to omit more
+ * fine-grained locking for recovery journal structures.
+ *
  * The journal consists of a set of on-disk blocks arranged as a circular log with monotonically
  * increasing sequence numbers. Three sequence numbers serve to define the active extent of the
  * journal. The 'head' is the oldest active block in the journal. The 'tail' is the end of the

drivers/md/dm-vdo/slab-depot.h

@@ -29,11 +29,17 @@
  * a single array of slabs in order to eliminate the need for additional math in order to compute
  * which physical zone a PBN is in. It also has a block_allocator per zone.
  *
- * Load operations are required to be performed on a single thread. Normal operations are assumed
- * to be performed in the appropriate zone. Allocations and reference count updates must be done
- * from the thread of their physical zone. Requests to commit slab journal tail blocks from the
- * recovery journal must be done on the journal zone thread. Save operations are required to be
- * launched from the same thread as the original load operation.
+ * Each physical zone has a single dedicated queue and thread for performing all updates to the
+ * slabs assigned to that zone. The concurrency guarantees of this single-threaded model allow the
+ * code to omit more fine-grained locking for the various slab structures. Each physical zone
+ * maintains a separate copy of the slab summary to remove the need for explicit locking on that
+ * structure as well.
+ *
+ * Load operations must be performed on the admin thread. Normal operations, such as allocations
+ * and reference count updates, must be performed on the appropriate physical zone thread. Requests
+ * from the recovery journal to commit slab journal tail blocks must be scheduled from the recovery
+ * journal thread to run on the appropriate physical zone thread. Save operations must be launched
+ * from the same admin thread as the original load operation.
  */
 
 enum {