If an hctx has no software ctx mapped, blk_mq_map_swqueue() never allocates tags and leaves hctx->tags NULL. The CPU hotplug offline notifier can still run for that hctx, return early since hctx cannot hold any requests. Signed-off-by: Cong Zhang --- This issue was observed during CPU hotplug. If an hctx is not mapped, offlining a CPU can trigger a kernel crash. When a block device does not map all hctx, some hctx instances may remain unused. These unused hctx can still receive CPU offline notifications and enter blk_mq_hctx_notify_offline(). blk_mq_hctx_notify_offline() calls blk_mq_hctx_has_requests() to check whether there are pending requests on the hctx. However, unused hctx do not have tags allocated, which leads to a crash. Since an unused hctx cannot have any requests, fix this by returning early when nr_ctx is zero, skipping blk_mq_hctx_notify_offline(). --- block/blk-mq.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index 1978eef95dca3fb332a73aeff7b9613ee770a8a3..eff4f72ce83be80aac9da86aab35079be7d2b5e4 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -3721,7 +3721,7 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node) struct blk_mq_hw_ctx, cpuhp_online); int ret = 0; - if (blk_mq_hctx_has_online_cpu(hctx, cpu)) + if (!hctx->nr_ctx || blk_mq_hctx_has_online_cpu(hctx, cpu)) return 0; /* --- base-commit: cc3aa43b44bdb43dfbac0fcb51c56594a11338a8 change-id: 20251226-blk_mq_no_ctx_checking-05b68f5b8b9d Best regards, -- Cong Zhang