Re: [PATCH 3/3] block: only allocate poll_stats if there's a user of them

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 23, 2021 at 09:18:13AM -0700, Jens Axboe wrote:
> This is essentially never used, yet it's about 1/3rd of the total
> queue size. Allocate it when needed, and don't embed it in the queue.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  block/blk-mq.c         | 20 ++++++++++++++++++--
>  block/blk-stat.c       |  6 ------
>  block/blk-sysfs.c      |  1 +
>  include/linux/blkdev.h |  9 +++++++--
>  4 files changed, 26 insertions(+), 10 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 20a6445f6a01..cb41c441aa8f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -4577,9 +4577,25 @@ EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);
>  /* Enable polling stats and return whether they were already enabled. */
>  static bool blk_poll_stats_enable(struct request_queue *q)
>  {
> -	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags) ||
> -	    blk_queue_flag_test_and_set(QUEUE_FLAG_POLL_STATS, q))
> +	struct blk_rq_stat *poll_stat;
> +
> +	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
>  		return true;

Can't we replace the checks for QUEUE_FLAG_POLL_STATS with checks for
q->poll_stat now?