linux-kernel - Re: [PATCH v3 3/4] scsi: core: Cap shost max_sectors according to DMA optimum mapping limits

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Date:   Wed, 8 Jun 2022 18:50:29 +0100
From:   John Garry <john.garry@...wei.com>
To:     Bart Van Assche <bvanassche@....org>,
        <damien.lemoal@...nsource.wdc.com>, <joro@...tes.org>,
        <will@...nel.org>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <hch@....de>,
        <m.szyprowski@...sung.com>, <robin.murphy@....com>
CC:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <linux-scsi@...r.kernel.org>, <liyihang6@...ilicon.com>,
        <chenxiang66@...ilicon.com>, <thunder.leizhen@...wei.com>
Subject: Re: [PATCH v3 3/4] scsi: core: Cap shost max_sectors according to DMA
 optimum mapping limits

On 08/06/2022 18:33, Bart Van Assche wrote:
> On 6/6/22 02:30, John Garry wrote:
>> +    if (dma_dev->dma_mask) {
>> +        shost->max_sectors = min_t(unsigned int, shost->max_sectors,
>> +                dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
>> +    }
> 
> Since IOVA_RANGE_CACHE_MAX_SIZE = 6 this limits max_sectors to 2**6 * 
> PAGE_SIZE or 256 KiB if the page size is 4 KiB.

It's actually 128K for 4K page size, as any IOVA size is roundup to 
power-of-2 when testing if we may cache it, which means anything >128K 
would roundup to 256K and cannot be cached.

> I think that's too 
> small. Some (SRP) storage arrays require much larger transfers to 
> achieve optimal performance.

Have you tried this achieve this optimal performance with an IOMMU enabled?

Please note that this limit only applies if we have an IOMMU enabled for 
the scsi host dma device. Otherwise we are limited by dma direct or 
swiotlb max mapping size, as before.

Thanks,
John