[PATCH] blk-mq: create correct map for fallback case

[Daniel Wagner]

The fallback code in blk_mq_map_hw_queues is original from
blk_mq_pci_map_queues and was added to handle the case where
pci_irq_get_affinity will return NULL for !SMP configuration.

blk_mq_map_hw_queues replaces besides blk_mq_pci_map_queues also
blk_mq_virtio_map_queues which used to use blk_mq_map_queues for the
fallback.

It's possible to use blk_mq_map_queues for both cases though.
blk_mq_map_queues creates the same map as blk_mq_clear_mq_map for !SMP
that is CPU 0 will be mapped to hctx 0.

The WARN_ON_ONCE has to be dropped for virtio as the fallback is also
taken for certain configuration on default. Though there is still a
WARN_ON_ONCE check in lib/group_cpus.c:

       WARN_ON(nr_present + nr_others < numgrps);

which will trigger if the caller tries to create more hardware queues
than CPUs. It tests the same as the WARN_ON_ONCE in
blk_mq_pci_map_queues did.

Fixes: a5665c3d150c ("virtio: blk/scsi: replace blk_mq_virtio_map_queues with blk_mq_map_hw_queues")
Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Closes: https://lore.kernel.org/all/20250122093020.6e8a4e5b@xxxxxxxxxxxxxxxxxx/
Signed-off-by: Daniel Wagner <wagi@xxxxxxxxxx>
---
 block/blk-mq-cpumap.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index ad8d6a363f24ae11968b42f7bcfd6a719a0499b7..444798c5374f48088b661b519f2638bda8556cf2 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -87,7 +87,6 @@ void blk_mq_map_hw_queues(struct blk_mq_queue_map *qmap,
 	return;
 
 fallback:
-	WARN_ON_ONCE(qmap->nr_queues > 1);
-	blk_mq_clear_mq_map(qmap);
+	blk_mq_map_queues(qmap);
 }
 EXPORT_SYMBOL_GPL(blk_mq_map_hw_queues);

---
base-commit: c4b9570cfb63501638db720f3bee9f6dfd044b82
change-id: 20250123-fix-blk_mq_map_hw_queues-dc72a6a1c71a

Best regards,
-- 
Daniel Wagner <wagi@xxxxxxxxxx>