block: Support block devices that preserve the order of write requests

Some storage controllers preserve the request order per hardware queue.
Some but not all device mapper drivers preserve the bio order. Introduce
the feature flag BLK_FEAT_ORDERED_HWQ to allow block drivers and stacked
drivers to indicate that the order of write commands is preserved per
hardware queue and hence that serialization of writes per zone is not
required if all pending writes are submitted to the same hardware queue.

Cc: Damien Le Moal <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Bart Van Assche <[email protected]>

Author: bvanassche

block/blk-settings.c

@@ -761,6 +761,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 		t->features &= ~BLK_FEAT_NOWAIT;
 	if (!(b->features & BLK_FEAT_POLL))
 		t->features &= ~BLK_FEAT_POLL;
+	if (!(b->features & BLK_FEAT_ORDERED_HWQ))
+		t->features &= ~BLK_FEAT_ORDERED_HWQ;
 
 	t->flags |= (b->flags & BLK_FLAG_MISALIGNED);
 

include/linux/blkdev.h

@@ -338,6 +338,12 @@ typedef unsigned int __bitwise blk_features_t;
 /* skip this queue in blk_mq_(un)quiesce_tagset */
 #define BLK_FEAT_SKIP_TAGSET_QUIESCE	((__force blk_features_t)(1u << 13))
 
+/*
+ * The request order is preserved per hardware queue by the block driver and by
+ * the block device.
+ */
+#define BLK_FEAT_ORDERED_HWQ		((__force blk_features_t)(1u << 14))
+
 /* undocumented magic for bcache */
 #define BLK_FEAT_RAID_PARTIAL_STRIPES_EXPENSIVE \
 	((__force blk_features_t)(1u << 15))