BUG: NULL pointer at IP: blk_mq_map_swqueue+0xbc/0x200 on 4.15.0-rc2

[Ming Lei]

On Tue, Dec 12, 2017 at 12:35:02AM -0800, Christoph Hellwig wrote:
> On Mon, Dec 11, 2017 at 11:58:45AM +0800, Ming Lei wrote:
> > As we talked offline, this IP points to cpumask_set_cpu(), seems this
> > case may happen when one CPU isn't mapped to any hw queue, could you test
> > the following patch to see if it helps your issue?
> 
> This looks odd as mq_map should be pre-zeroed.  If it isn't we have
> a few more problems..

This happens in case of updating nr_hw_queue, for example, when the
number becomes 5 from 12, then the table has to be zeroed for avoiding
obsolete mapping if one CPU isn't mapped to any hw queue.

Thanks,
Ming