linux-kernel - Re: [PATCH v5 0/5] DMA mapping changes for SCSI core

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Date:   Fri, 8 Jul 2022 17:17:06 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        Christoph Hellwig <hch@....de>
CC:     <damien.lemoal@...nsource.wdc.com>, <joro@...tes.org>,
        <will@...nel.org>, <jejb@...ux.ibm.com>,
        <m.szyprowski@...sung.com>, <robin.murphy@....com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <iommu@...ts.linux.dev>, <linux-scsi@...r.kernel.org>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH v5 0/5] DMA mapping changes for SCSI core

On 07/07/2022 21:35, Martin K. Petersen wrote:
> Christoph,
> 
>> Yes, I've mostly been waiting for an ACK from Martin.
> Sorry, I'm on vacation this week. The series looks OK to me although I
> do agree that it would be great if the max was reflected in the queue's
> hard limit and opt in the soft limit.

Ah, I think that I misunderstood Damien's question. I thought he was 
asking why not keep shost max_sectors at dma_max_mapping_size() and then 
init each sdev request queue max hw sectors at dma_opt_mapping_size().

But he seems that you want to know why not have the request queue max 
sectors at dma_opt_mapping_size(). The answer is related to meaning of 
dma_opt_mapping_size(). If we get any mappings which exceed this size 
then it can have a big dma mapping performance hit. So I set max hw 
sectors at this ‘opt’ mapping size to ensure that we get no mappings 
which exceed this size. Indeed, I think max sectors is 128Kb today for 
my host, which would be same as dma_opt_mapping_size() value with an 
IOMMU enabled. And I find that only a small % of request size may exceed 
this 128kb size, but it still has a big performance impact.

> 
> Acked-by: Martin K. Petersen<martin.petersen@...cle.com>

Thanks,
John