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Message-ID: <6725df4f-4e27-3320-8b7b-22ba15a07866@opensource.wdc.com>
Date:   Mon, 18 Jul 2022 19:47:29 +0900
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Subject: Re: [PATCH v6 4/6] scsi: sd: Allow max_sectors be capped at DMA
 optimal size limit
Content-Language: en-US
To:     John Garry <john.garry@huawei.com>, joro@8bytes.org,
        will@kernel.org, jejb@linux.ibm.com, martin.petersen@oracle.com,
        hch@lst.de, m.szyprowski@samsung.com, robin.murphy@arm.com
Cc:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-ide@vger.kernel.org, iommu@lists.linux.dev,
        linux-scsi@vger.kernel.org, linuxarm@huawei.com
References: <1657797329-98541-1-git-send-email-john.garry@huawei.com>
 <1657797329-98541-5-git-send-email-john.garry@huawei.com>
From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <1657797329-98541-5-git-send-email-john.garry@huawei.com>
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Precedence: bulk

On 7/14/22 20:15, John Garry wrote:
> Streaming DMA mappings may be considerably slower when mappings go through
> an IOMMU and the total mapping length is somewhat long. This is because the
> IOMMU IOVA code allocates and free an IOVA for each mapping, which may
> affect performance.
> 
> New member Scsi_Host.opt_sectors is added, which is the optimal host
> max_sectors, and use this value to cap the request queue max_sectors when
> set.
> 
> It could be considered to have request queues io_opt value initially
> set at Scsi_Host.opt_sectors in __scsi_init_queue(), but that is not
> really the purpose of io_opt.
> 
> Finally, even though Scsi_Host.opt_sectors value should never be greater
> than the request queue max_hw_sectors value, continue to limit to this
> value for safety.
> 
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
>  drivers/scsi/sd.c        | 2 ++
>  include/scsi/scsi_host.h | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index a1a2ac09066f..3eaee1f7aaca 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -3296,6 +3296,8 @@ static int sd_revalidate_disk(struct gendisk *disk)
>  				      (sector_t)BLK_DEF_MAX_SECTORS);
>  	}
>  
> +	rw_max = min_not_zero(rw_max, sdp->host->opt_sectors);
> +

Adding a comment explaining what the cap is would be nice.

>  	/* Do not exceed controller limit */
>  	rw_max = min(rw_max, queue_max_hw_sectors(q));
>  
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 667d889b92b5..d32a84b2bb40 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -607,6 +607,7 @@ struct Scsi_Host {
>  	short unsigned int sg_tablesize;
>  	short unsigned int sg_prot_tablesize;
>  	unsigned int max_sectors;
> +	unsigned int opt_sectors;
>  	unsigned int max_segment_size;
>  	unsigned long dma_boundary;
>  	unsigned long virt_boundary_mask;

Otherwise, looks good.

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>

-- 
Damien Le Moal
Western Digital Research