dm: introduce zone append emulation

For zoned targets that cannot support zone append operations, implement
an emulation using regular write operations. If the original BIO
submitted by the user is a zone append operation, change its clone into
a regular write operation directed at the target zone write pointer
position.

To do so, an array of write pointer offsets (write pointer position
relative to the start of a zone) is added to struct mapped_device. All
operations that modify a sequential zone write pointer (writes, zone
reset, zone finish and zone append) are intersepted in __map_bio() and
processed using the new functions dm_zone_map_bio().

Detection of the target ability to natively support zone append
operations is done from dm_table_set_restrictions() by calling the
function dm_set_zones_restrictions(). A target that does not support
zone append operation, either by explicitly declaring it using the new
struct dm_target field zone_append_not_supported, or because the device
table contains a non-zoned device, has its mapped device marked with the
new flag DMF_ZONE_APPEND_EMULATED. The helper function
dm_emulate_zone_append() is introduced to test a mapped device for this
new flag.

Atomicity of the zones write pointer tracking and updates is done using
a zone write locking mechanism based on a bitmap. This is similar to
the block layer method but based on BIOs rather than struct request.
A zone write lock is taken in dm_zone_map_bio() for any clone BIO with
an operation type that changes the BIO target zone write pointer
position. The zone write lock is released if the clone BIO is failed
before submission or when dm_zone_endio() is called when the clone BIO
completes.

The zone write lock bitmap of the mapped device, together with a bitmap
indicating zone types (conv_zones_bitmap) and the write pointer offset
array (zwp_offset) are allocated and initialized with a full device zone
report in dm_set_zones_restrictions() using the function
dm_revalidate_zones().

For failed operations that may have modified a zone write pointer, the
zone write pointer offset is marked as invalid in dm_zone_endio().
Zones with an invalid write pointer offset are checked and the write
pointer updated using an internal report zone operation when the
faulty zone is accessed again by the user.

All functions added for this emulation have a minimal overhead for
zoned targets natively supporting zone append operations. Regular
device targets are also not affected. The added code also does not
impact builds with CONFIG_BLK_DEV_ZONED disabled by stubbing out all
dm zone related functions.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-core.h

@@ -114,6 +114,11 @@ struct mapped_device {
 	bool init_tio_pdu:1;
 
 	struct srcu_struct io_barrier;
+
+#ifdef CONFIG_BLK_DEV_ZONED
+	unsigned int nr_zones;
+	unsigned int *zwp_offset;
+#endif
 };
 
 /*
@@ -128,6 +133,7 @@ struct mapped_device {
 #define DMF_DEFERRED_REMOVE 6
 #define DMF_SUSPENDED_INTERNALLY 7
 #define DMF_POST_SUSPENDING 8
+#define DMF_EMULATE_ZONE_APPEND 9
 
 void disable_discard(struct mapped_device *md);
 void disable_write_same(struct mapped_device *md);
@@ -143,6 +149,13 @@ static inline struct dm_stats *dm_get_stats(struct mapped_device *md)
 	return &md->stats;
 }
 
+static inline bool dm_emulate_zone_append(struct mapped_device *md)
+{
+	if (blk_queue_is_zoned(md->queue))
+		return test_bit(DMF_EMULATE_ZONE_APPEND, &md->flags);
+	return false;
+}
+
 #define DM_TABLE_MAX_DEPTH 16
 
 struct dm_table {

drivers/md/dm-table.c

@@ -1981,11 +1981,12 @@ static int device_requires_stable_pages(struct dm_target *ti,
 	return blk_queue_stable_writes(q);
 }
 
-void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
-			       struct queue_limits *limits)
+int dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
+			      struct queue_limits *limits)
 {
 	bool wc = false, fua = false;
 	int page_size = PAGE_SIZE;
+	int r;
 
 	/*
 	 * Copy table's limits to the DM device's request_queue
@@ -2064,12 +2065,20 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 	    dm_table_any_dev_attr(t, device_is_not_random, NULL))
 		blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, q);
 
-	/* For a zoned target, setup the zones related queue attributes */
-	if (blk_queue_is_zoned(q))
-		dm_set_zones_restrictions(t, q);
+	/*
+	 * For a zoned target, setup the zones related queue attributes
+	 * and resources necessary for zone append emulation if necessary.
+	 */
+	if (blk_queue_is_zoned(q)) {
+		r = dm_set_zones_restrictions(t, q);
+		if (r)
+			return r;
+	}
 
 	dm_update_keyslot_manager(q, t);
 	blk_queue_update_readahead(q);
+
+	return 0;
 }
 
 unsigned int dm_table_get_num_targets(struct dm_table *t)