Re: [PATCHv3 2/2] null_blk: prevent submit and poll queues update for shared tagset

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

On 7/23/25 22:43, Nilay Shroff wrote:
> 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 <nilay@linux.ibm.com>

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>

-- 
Damien Le Moal
Western Digital Research