On Wed, Jun 15, 2022 at 10:37:12AM +0800, Ming Lei wrote:
> commit 364b61818f65 ("blk-mq: clearing flush request reference in
> tags->rqs[]") is added to clear the to-be-free flush request from
> tags->rqs[] for avoiding use-after-free on the flush rq.
>
> Yu Kuai reported that blk_mq_clear_flush_rq_mapping() slows down boot time
> by ~8s because running scsi probe which may create and remove lots of
> unpresent LUNs on megaraid-sas which uses BLK_MQ_F_TAG_HCTX_SHARED and
> each request queue has lots of hw queues.
>
> Improve the situation by not running blk_mq_clear_flush_rq_mapping if
> disk isn't added when there can't be any flush request issued.
This looks ok. Another optimization would be to never do this if we
don't have a write cache enabled and thus never ever use the flush_rq.
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
