[PATCH v2 5/5] null_blk: Support configuring the maximum DMA segment size

[Bart Van Assche <bvanassche@acm.org>]

Add support for configuring the maximum DMA segment size. The maximum DMA
segment size may be set to a value smaller than the virtual memory page
size. No checks are performed on the new max_segment_size parameter
because all positive values are valid. The value 0 is converted into a
valid value by blk_validate_limits().

Since rq_for_each_segment() may yield bvecs larger than the maximum DMA
segment size, add code in the rq_for_each_segment() loop that restricts
the bvec length to the maximum DMA segment size.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/block/null_blk/main.c     | 13 +++++++++++++
 drivers/block/null_blk/null_blk.h |  1 +
 2 files changed, 14 insertions(+)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 677ac829ef80..c43d74cdfd23 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -169,6 +169,10 @@ 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 g_max_segment_size = BLK_MAX_SEGMENT_SIZE;
+module_param_named(max_segment_size, g_max_segment_size, int, 0444);
+MODULE_PARM_DESC(max_segment_size, "Maximum size of a DMA segment in bytes");
+
 static unsigned int nr_devices = 1;
 module_param(nr_devices, uint, 0444);
 MODULE_PARM_DESC(nr_devices, "Number of devices to register");
@@ -450,6 +454,7 @@ 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(max_segment_size, uint, NULL);
 NULLB_DEVICE_ATTR(irqmode, uint, NULL);
 NULLB_DEVICE_ATTR(hw_queue_depth, uint, NULL);
 NULLB_DEVICE_ATTR(index, uint, NULL);
@@ -608,6 +613,7 @@ static struct configfs_attribute *nullb_device_attrs[] = {
 	&nullb_device_attr_index,
 	&nullb_device_attr_irqmode,
 	&nullb_device_attr_max_sectors,
+	&nullb_device_attr_max_segment_size,
 	&nullb_device_attr_mbps,
 	&nullb_device_attr_memory_backed,
 	&nullb_device_attr_no_sched,
@@ -805,6 +811,7 @@ static struct nullb_device *null_alloc_dev(void)
 	dev->queue_mode = g_queue_mode;
 	dev->blocksize = g_bs;
 	dev->max_sectors = g_max_sectors;
+	dev->max_segment_size = g_max_segment_size;
 	dev->irqmode = g_irqmode;
 	dev->hw_queue_depth = g_hw_queue_depth;
 	dev->blocking = g_blocking;
@@ -1251,6 +1258,9 @@ static blk_status_t null_transfer(struct nullb *nullb, struct page *page,
 	unsigned int valid_len = len;
 	void *p;
 
+	WARN_ONCE(len > dev->max_segment_size, "%u > %u\n", len,
+		  dev->max_segment_size);
+
 	p = kmap_local_page(page) + off;
 	if (!is_write) {
 		if (dev->zoned) {
@@ -1298,6 +1308,8 @@ static blk_status_t null_handle_data_transfer(struct nullb_cmd *cmd,
 	spin_lock_irq(&nullb->lock);
 	rq_for_each_segment(bvec, rq, iter) {
 		len = bvec.bv_len;
+		len = min(bvec.bv_len, nullb->dev->max_segment_size);
+		bvec.bv_len = len;
 		if (transferred_bytes + len > max_bytes)
 			len = max_bytes - transferred_bytes;
 		err = null_transfer(nullb, bvec.bv_page, len, bvec.bv_offset,
@@ -1961,6 +1973,7 @@ static int null_add_dev(struct nullb_device *dev)
 		.logical_block_size	= dev->blocksize,
 		.physical_block_size	= dev->blocksize,
 		.max_hw_sectors		= dev->max_sectors,
+		.max_segment_size	= dev->max_segment_size,
 		.dma_alignment		= 1,
 	};
 
diff --git a/drivers/block/null_blk/null_blk.h b/drivers/block/null_blk/null_blk.h
index 6c4c4bbe7dad..43dc47789718 100644
--- a/drivers/block/null_blk/null_blk.h
+++ b/drivers/block/null_blk/null_blk.h
@@ -93,6 +93,7 @@ struct nullb_device {
 	unsigned int queue_mode; /* block interface */
 	unsigned int blocksize; /* block size */
 	unsigned int max_sectors; /* Max sectors per command */
+	unsigned int max_segment_size; /* Max size of a single DMA segment. */
 	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 */