Hi Jens, These patches are what I came up with while analyzing syzbot and blktests complaints related to dynamically changing the number of hardware queues. Please consider these patches for the upstream kernel. Thanks, Bart. Changes compared to v2: - Changed patch 2/8 such that .nr_queues is only set if nr_maps == 1. Changes compared to v1: - Reworked patch 2/8 as requested by Ming. - Added three new null_blk patches. Bart Van Assche (8): blk-mq: Fix a comment in include/linux/blk-mq.h blk-mq: Keep set->nr_hw_queues and set->map[].nr_queues in sync blk-mq: Fix a recently introduced regression in blk_mq_realloc_hw_ctxs() null_blk: Suppress an UBSAN complaint triggered when setting 'memory_backed' null_blk: Fix changing the number of hardware queues null_blk: Fix the null_add_dev() error path null_blk: Handle null_add_dev() failures properly null_blk: Add support for init_hctx() fault injection block/blk-mq.c | 9 ++- drivers/block/null_blk_main.c | 124 +++++++++++++++++++++++----------- include/linux/blk-mq.h | 5 +- 3 files changed, 97 insertions(+), 41 deletions(-)