null_blk: prevent submit and poll queues update for shared tagset

When a user updates the number of submit or poll queues on a null_blk
device, the block layer creates new hardware queues (hctxs). However, if
the device is using a shared tagset, null_blk does not map any software
queues (ctx) to the newly created hctx (via null_map_queues()), resulting
in those hardware queues being left unused for I/O. This behavior is
misleading, as the user may expect the new queues to be functional, even
though they are effectively ignored. To avoid this confusion and potential
misconfiguration:
- Reject runtime updates to submit_queues or poll_queues via sysfs when
  the device uses a shared tagset by returning -EINVAL.
- During configuration validation (prior to powering on the device), reset
  submit_queues and poll_queues to the module parameters (g_submit_queues
  and g_poll_queues) if the shared tagset is enabled.

This ensures consistent behavior and avoids creating unused hardware queues
(hctxs) due to ineffective runtime queue updates.

Signed-off-by: Nilay Shroff <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>

Author: shroffni

drivers/block/null_blk/main.c

@@ -388,6 +388,12 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
 	if (!submit_queues)
 		return -EINVAL;
 
+	/*
+	 * Cannot update queues with shared tagset.
+	 */
+	if (dev->shared_tags)
+		return -EINVAL;
+
 	/*
 	 * Make sure that null_init_hctx() does not access nullb->queues[] past
 	 * the end of that array.
@@ -1884,18 +1890,24 @@ static int null_validate_conf(struct nullb_device *dev)
 		dev->queue_mode = NULL_Q_MQ;
 	}
 
-	if (dev->use_per_node_hctx) {
-		if (dev->submit_queues != nr_online_nodes)
-			dev->submit_queues = nr_online_nodes;
-	} else if (dev->submit_queues > nr_cpu_ids)
-		dev->submit_queues = nr_cpu_ids;
-	else if (dev->submit_queues == 0)
-		dev->submit_queues = 1;
-	dev->prev_submit_queues = dev->submit_queues;
-
-	if (dev->poll_queues > g_poll_queues)
+	if (dev->shared_tags) {
+		dev->submit_queues = g_submit_queues;
 		dev->poll_queues = g_poll_queues;
+	} else {
+		if (dev->use_per_node_hctx) {
+			if (dev->submit_queues != nr_online_nodes)
+				dev->submit_queues = nr_online_nodes;
+		} else if (dev->submit_queues > nr_cpu_ids)
+			dev->submit_queues = nr_cpu_ids;
+		else if (dev->submit_queues == 0)
+			dev->submit_queues = 1;
+
+		if (dev->poll_queues > g_poll_queues)
+			dev->poll_queues = g_poll_queues;
+	}
+	dev->prev_submit_queues = dev->submit_queues;
 	dev->prev_poll_queues = dev->poll_queues;
+
 	dev->irqmode = min_t(unsigned int, dev->irqmode, NULL_IRQ_TIMER);
 
 	/* Do memory allocation, so set blocking */