[PATCH RFC v3] nbd: skip queue limits update for size-only reconfigure

Commit 242a49e5c878 ("nbd: freeze the queue for queue limits updates")
correctly added queue freezing for live updates of real queue limits.
However, nbd_set_size() is also used for capacity-only generic netlink
updates, so the freeze is unnecessarily broad for the reported request.

When an NBD server becomes unresponsive, it leaves I/O in flight. An
unnecessary queue freeze during a size-only reconfigure will wait forever
for these in-flight I/Os to complete while the global genl_mutex is held,
leading to a system-wide deadlock.

A size-only reconfigure can update config->bytesize and disk capacity
directly because the effective block size and all other queue limits stay
unchanged.

This patch introduces nbd_size_set() to handle size-only updates directly
without freezing the queue or committing queue limits. Capacity-only
generic netlink reconfiguration no longer freezes the queue, while startup
and real block-size changes continue to use the existing frozen limits
update via nbd_set_size().

INFO: task nbd-client:6285 blocked for more than 143 seconds.
Call Trace:
 <TASK>
 blk_mq_freeze_queue_wait+0x101/0x180 block/blk-mq.c:191
 queue_limits_commit_update_frozen+0x55/0xd0 block/blk-settings.c:590
 nbd_set_size+0x454/0x680 drivers/block/nbd.c:374
 nbd_genl_reconfigure+0x7e9/0x1e80 drivers/block/nbd.c:2373
 genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
 </TASK>

INFO: task generic-netlink:6030 blocked for more than 143 seconds.
Call Trace:
 <TASK>
 __mutex_lock+0x7c5/0x1590 kernel/locking/mutex.c:820
 genl_rcv_msg+0x10b/0x7a0 net/netlink/genetlink.c:1208
 </TASK>

Fixes: 2dc691cc4ac2 ("nbd: refactor size updates")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot <[email protected]>
Closes: https://syzkaller.appspot.com/bug?extid=f272bbfbf8498ddadea5
Link: https://syzkaller.appspot.com/ai_job?id=c8611da4-ddd9-47e3-bbe2-8e178beb3f96
To: "Jens Axboe" <[email protected]>
To: "Josef Bacik" <[email protected]>
To: <[email protected]>
To: <[email protected]>
To: "Christoph Hellwig" <[email protected]>
Cc: <[email protected]>

---
v3:
- Removed the unrelated decrement of nbd_total_devices in nbd_dev_remove().
- Corrected the referenced commit title of 242a49e5c878 in the description.
- Clarified that capacity-only generic netlink reconfiguration no longer freezes the queue, while startup and real block-size changes continue to do so.

v2:
- Replaced the approach of using parallel netlink ops and custom timeouts with skipping queue limits updates for size-only reconfigurations.
- Introduced nbd_size_set() and nbd_size_update() to update device capacity directly without freezing the queue when the block size does not change.
- Retained queue freezing only for actual block-size changes.
- Added decrement of nbd_total_devices in nbd_dev_remove().
https://lore.kernel.org/all/[email protected]/T/

v1:
https://lore.kernel.org/all/[email protected]/T/
---
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index fe63f3c55..567b031ec 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -331,6 +331,27 @@ static void nbd_mark_nsock_dead(struct nbd_device *nbd, struct nbd_sock *nsock,
 	nsock->sent = 0;
 }
 
+static void nbd_size_update(struct nbd_device *nbd)
+{
+	struct nbd_config *config = nbd->config;
+
+	if (max_part)
+		set_bit(GD_NEED_PART_SCAN, &nbd->disk->state);
+	if (!set_capacity_and_notify(nbd->disk, config->bytesize >> 9))
+		kobject_uevent(&nbd_to_dev(nbd)->kobj, KOBJ_CHANGE);
+}
+
+static int nbd_size_set(struct nbd_device *nbd, loff_t bytesize)
+{
+	if (bytesize < 0)
+		return -EINVAL;
+
+	nbd->config->bytesize = bytesize;
+	if (nbd->pid)
+		nbd_size_update(nbd);
+	return 0;
+}
+
 static int nbd_set_size(struct nbd_device *nbd, loff_t bytesize, loff_t blksize)
 {
 	struct queue_limits lim;
@@ -375,10 +396,7 @@ static int nbd_set_size(struct nbd_device *nbd, loff_t bytesize, loff_t blksize)
 	if (error)
 		return error;
 
-	if (max_part)
-		set_bit(GD_NEED_PART_SCAN, &nbd->disk->state);
-	if (!set_capacity_and_notify(nbd->disk, bytesize >> 9))
-		kobject_uevent(&nbd_to_dev(nbd)->kobj, KOBJ_CHANGE);
+	nbd_size_update(nbd);
 	return 0;
 }
 
@@ -2062,11 +2080,19 @@ static int nbd_genl_size_set(struct genl_info *info, struct nbd_device *nbd)
 	if (info->attrs[NBD_ATTR_SIZE_BYTES])
 		bytes = nla_get_u64(info->attrs[NBD_ATTR_SIZE_BYTES]);
 
-	if (info->attrs[NBD_ATTR_BLOCK_SIZE_BYTES])
+	if (info->attrs[NBD_ATTR_BLOCK_SIZE_BYTES]) {
 		bsize = nla_get_u64(info->attrs[NBD_ATTR_BLOCK_SIZE_BYTES]);
+		if (!bsize)
+			bsize = 1u << NBD_DEF_BLKSIZE_BITS;
+		if (blk_validate_block_size(bsize))
+			return -EINVAL;
+	}
 
-	if (bytes != config->bytesize || bsize != nbd_blksize(config))
-		return nbd_set_size(nbd, bytes, bsize);
+	if (bytes != config->bytesize || bsize != nbd_blksize(config)) {
+		if (bsize != nbd_blksize(config))
+			return nbd_set_size(nbd, bytes, bsize);
+		return nbd_size_set(nbd, bytes);
+	}
 	return 0;
 }
 


base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
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