block: add notion of a chunk size for request merging

Some drivers have different limits on what size a request should
optimally be, depending on the offset of the request. Similar to
dividing a device into chunks. Add a setting that allows the driver
to inform the block layer of such a chunk size. The block layer will
then prevent merging across the chunks.

This is needed to optimally support NVMe with a non-zero stripe size.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/bio.c

@@ -849,7 +849,8 @@ int bio_add_page(struct bio *bio, struct page *page, unsigned int len,
 		 unsigned int offset)
 {
 	struct request_queue *q = bdev_get_queue(bio->bi_bdev);
-	return __bio_add_page(q, bio, page, len, offset, queue_max_sectors(q));
+
+	return __bio_add_page(q, bio, page, len, offset, blk_max_size_offset(q, bio->bi_iter.bi_sector));
 }
 EXPORT_SYMBOL(bio_add_page);
 

block/blk-settings.c

@@ -113,6 +113,7 @@ void blk_set_default_limits(struct queue_limits *lim)
 	lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK;
 	lim->max_segment_size = BLK_MAX_SEGMENT_SIZE;
 	lim->max_sectors = lim->max_hw_sectors = BLK_SAFE_MAX_SECTORS;
+	lim->chunk_sectors = 0;
 	lim->max_write_same_sectors = 0;
 	lim->max_discard_sectors = 0;
 	lim->discard_granularity = 0;
@@ -276,6 +277,23 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
 }
 EXPORT_SYMBOL(blk_queue_max_hw_sectors);
 
+/**
+ * blk_queue_chunk_sectors - set size of the chunk for this queue
+ * @q:  the request queue for the device
+ * @chunk_sectors:  chunk sectors in the usual 512b unit
+ *
+ * Description:
+ *    If a driver doesn't want IOs to cross a given chunk size, it can set
+ *    this limit and prevent merging across chunks. Note that the chunk size
+ *    must currently be a power-of-2 in sectors.
+ **/
+void blk_queue_chunk_sectors(struct request_queue *q, unsigned int chunk_sectors)
+{
+	BUG_ON(!is_power_of_2(chunk_sectors));
+	q->limits.chunk_sectors = chunk_sectors;
+}
+EXPORT_SYMBOL(blk_queue_chunk_sectors);
+
 /**
  * blk_queue_max_discard_sectors - set max sectors for a single discard
  * @q:  the request queue for the device

include/linux/blkdev.h

@@ -280,6 +280,7 @@ struct queue_limits {
 	unsigned long		seg_boundary_mask;
 
 	unsigned int		max_hw_sectors;
+	unsigned int		chunk_sectors;
 	unsigned int		max_sectors;
 	unsigned int		max_segment_size;
 	unsigned int		physical_block_size;
@@ -910,6 +911,20 @@ static inline unsigned int blk_queue_get_max_sectors(struct request_queue *q,
 	return q->limits.max_sectors;
 }
 
+/*
+ * Return maximum size of a request at given offset. Only valid for
+ * file system requests.
+ */
+static inline unsigned int blk_max_size_offset(struct request_queue *q,
+					       sector_t offset)
+{
+	if (!q->limits.chunk_sectors)
+		return q->limits.max_hw_sectors;
+
+	return q->limits.chunk_sectors -
+			(offset & (q->limits.chunk_sectors - 1));
+}
+
 static inline unsigned int blk_rq_get_max_sectors(struct request *rq)
 {
 	struct request_queue *q = rq->q;
@@ -917,7 +932,11 @@ static inline unsigned int blk_rq_get_max_sectors(struct request *rq)
 	if (unlikely(rq->cmd_type == REQ_TYPE_BLOCK_PC))
 		return q->limits.max_hw_sectors;
 
-	return blk_queue_get_max_sectors(q, rq->cmd_flags);
+	if (!q->limits.chunk_sectors)
+		return blk_queue_get_max_sectors(q, rq->cmd_flags);
+
+	return min(blk_max_size_offset(q, blk_rq_pos(rq)),
+			blk_queue_get_max_sectors(q, rq->cmd_flags));
 }
 
 static inline unsigned int blk_rq_count_bios(struct request *rq)
@@ -983,6 +1002,7 @@ extern void blk_queue_make_request(struct request_queue *, make_request_fn *);
 extern void blk_queue_bounce_limit(struct request_queue *, u64);
 extern void blk_limits_max_hw_sectors(struct queue_limits *, unsigned int);
 extern void blk_queue_max_hw_sectors(struct request_queue *, unsigned int);
+extern void blk_queue_chunk_sectors(struct request_queue *, unsigned int);
 extern void blk_queue_max_segments(struct request_queue *, unsigned short);
 extern void blk_queue_max_segment_size(struct request_queue *, unsigned int);
 extern void blk_queue_max_discard_sectors(struct request_queue *q,