Re: [PATCH 2/2] null_blk: fix set->driver_data while setting up tagset

[Damien Le Moal <dlemoal@kernel.org>]

On 2025/07/20 20:35, Nilay Shroff wrote:
> When setting up a null block device, we initialize a tagset that
> includes a driver_data field—typically used by block drivers to
> store a pointer to driver-specific data. In the case of null_blk,
> this should point to the struct nullb instance.
> 
> However, due to recent tagset refactoring in the null_blk driver, we
> missed initializing driver_data when creating a shared tagset. As a
> result, software queues (ctx) fail to map correctly to new hardware
> queues (hctx). For example, increasing the number of submit queues
> triggers an nr_hw_queues update, which invokes null_map_queues() to
> remap queues. Since set->driver_data is unset, null_map_queues()
> fails to map any ctx to the new hctxs, leading to hctx->nr_ctx == 0,
> effectively making the hardware queues unusable for I/O.
> 
> This patch fixes the issue by ensuring that set->driver_data is properly
> initialized to point to the struct nullb during tagset setup.
> 
> Fixes: 72ca28765fc4 ("null_blk: refactor tag_set setup")
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>  drivers/block/null_blk/main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index aa163ae9b2aa..9e1c4ce6fc42 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -1854,13 +1854,14 @@ static int null_init_global_tag_set(void)
>  
>  static int null_setup_tagset(struct nullb *nullb)
>  {
> +	nullb->tag_set->driver_data = nullb;
> +

How can this be correct since the tag_set pointer is initialized below ?

>  	if (nullb->dev->shared_tags) {
>  		nullb->tag_set = &tag_set;

Shouldn't you add:

		nullb->tag_set->driver_data = nullb;

here instead ?

>  		return null_init_global_tag_set();
>  	}
>  
>  	nullb->tag_set = &nullb->__tag_set;
> -	nullb->tag_set->driver_data = nullb;
>  	nullb->tag_set->nr_hw_queues = nullb->dev->submit_queues;
>  	nullb->tag_set->queue_depth = nullb->dev->hw_queue_depth;
>  	nullb->tag_set->numa_node = nullb->dev->home_node;


-- 
Damien Le Moal
Western Digital Research