null_blk: Allow controlling max_hw_sectors limit

Add the module option and configfs attribute max_sectors to allow
configuring the maximum size of a command issued to a null_blk device.
This allows exercising the block layer BIO splitting with different
limits than the default BLK_SAFE_MAX_SECTORS. This is also useful for
testing the zone append write path of file systems as the max_hw_sectors
limit value is also used for the max_zone_append_sectors limit.

Signed-off-by: Damien Le Moal <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/null_blk.h

···
 	unsigned int home_node; /* home node for the device */
 	unsigned int queue_mode; /* block interface */
 	unsigned int blocksize; /* block size */
+	unsigned int max_sectors; /* Max sectors per command */
 	unsigned int irqmode; /* IRQ completion handler */
 	unsigned int hw_queue_depth; /* queue depth */
 	unsigned int index; /* index of the disk, only valid with a disk */

drivers/block/null_blk_main.c

···
 module_param_named(bs, g_bs, int, 0444);
 MODULE_PARM_DESC(bs, "Block size (in bytes)");
 
+static int g_max_sectors;
+module_param_named(max_sectors, g_max_sectors, int, 0444);
+MODULE_PARM_DESC(max_sectors, "Maximum size of a command (in 512B sectors)");
+
 static unsigned int nr_devices = 1;
 module_param(nr_devices, uint, 0444);
 MODULE_PARM_DESC(nr_devices, "Number of devices to register");
···
 NULLB_DEVICE_ATTR(home_node, uint, NULL);
 NULLB_DEVICE_ATTR(queue_mode, uint, NULL);
 NULLB_DEVICE_ATTR(blocksize, uint, NULL);
+NULLB_DEVICE_ATTR(max_sectors, uint, NULL);
 NULLB_DEVICE_ATTR(irqmode, uint, NULL);
 NULLB_DEVICE_ATTR(hw_queue_depth, uint, NULL);
 NULLB_DEVICE_ATTR(index, uint, NULL);
···
 	&nullb_device_attr_home_node,
 	&nullb_device_attr_queue_mode,
 	&nullb_device_attr_blocksize,
+	&nullb_device_attr_max_sectors,
 	&nullb_device_attr_irqmode,
 	&nullb_device_attr_hw_queue_depth,
 	&nullb_device_attr_index,
···
 static ssize_t memb_group_features_show(struct config_item *item, char *page)
 {
 	return snprintf(page, PAGE_SIZE,
-			"memory_backed,discard,bandwidth,cache,badblocks,zoned,zone_size,zone_capacity,zone_nr_conv,zone_max_open,zone_max_active\n");
+			"memory_backed,discard,bandwidth,cache,badblocks,zoned,zone_size,zone_capacity,zone_nr_conv,zone_max_open,zone_max_active,blocksize,max_sectors\n");
 }
 
 CONFIGFS_ATTR_RO(memb_group_, features);
···
 	dev->home_node = g_home_node;
 	dev->queue_mode = g_queue_mode;
 	dev->blocksize = g_bs;
+	dev->max_sectors = g_max_sectors;
 	dev->irqmode = g_irqmode;
 	dev->hw_queue_depth = g_hw_queue_depth;
 	dev->blocking = g_blocking;
···
 
 	blk_queue_logical_block_size(nullb->q, dev->blocksize);
 	blk_queue_physical_block_size(nullb->q, dev->blocksize);
+	if (!dev->max_sectors)
+		dev->max_sectors = queue_max_hw_sectors(nullb->q);
+	dev->max_sectors = min_t(unsigned int, dev->max_sectors,
+				 BLK_DEF_MAX_SECTORS);
+	blk_queue_max_hw_sectors(nullb->q, dev->max_sectors);
 
 	null_config_discard(nullb);
 
···
 		pr_warn("invalid block size\n");
 		pr_warn("defaults block size to %lu\n", PAGE_SIZE);
 		g_bs = PAGE_SIZE;
+	}
+
+	if (g_max_sectors > BLK_DEF_MAX_SECTORS) {
+		pr_warn("invalid max sectors\n");
+		pr_warn("defaults max sectors to %u\n", BLK_DEF_MAX_SECTORS);
+		g_max_sectors = BLK_DEF_MAX_SECTORS;
 	}
 
 	if (g_home_node != NUMA_NO_NODE && g_home_node >= nr_online_nodes) {