Re: [PATCH V6 02/12] block: define 'struct bvec_iter' as packed

From: Hannes Reinecke <hare@suse.de>
To: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org,
	Jeffle Xu <jefflexu@linux.alibaba.com>,
	Mike Snitzer <snitzer@redhat.com>,
	dm-devel@redhat.com, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH V6 02/12] block: define 'struct bvec_iter' as packed
Date: Thu, 22 Apr 2021 15:18:24 +0200	[thread overview]
Message-ID: <2ae12dc1-804f-59f9-6837-3508d6176804@suse.de> (raw)
In-Reply-To: <20210422122038.2192933-3-ming.lei@redhat.com>

On 4/22/21 2:20 PM, Ming Lei wrote:
> 'struct bvec_iter' is embedded into 'struct bio', define it as packed
> so that we can get one extra 4bytes for other uses without expanding
> bio.
> 
> 'struct bvec_iter' is often allocated on stack, so making it packed
> doesn't affect performance. Also I have run io_uring on both
> nvme/null_blk, and not observe performance effect in this way.
> 
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   include/linux/bvec.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index ff832e698efb..a0c4f41dfc83 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -43,7 +43,7 @@ struct bvec_iter {
>   
>   	unsigned int            bi_bvec_done;	/* number of bytes completed in
>   						   current bvec */
> -};
> +} __packed;
>   
>   struct bvec_iter_all {
>   	struct bio_vec	bv;
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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