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Date:   Wed, 10 Aug 2022 09:33:23 +0100
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Subject: Re: [ata] 0568e61225: stress-ng.copy-file.ops_per_sec -15.0%
 regression
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        kernel test robot <oliver.sang@intel.com>
CC:     Christoph Hellwig <hch@lst.de>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Linux Memory Management List" <linux-mm@kvack.org>,
        <linux-ide@vger.kernel.org>, <lkp@lists.01.org>, <lkp@intel.com>,
        <ying.huang@intel.com>, <feng.tang@intel.com>,
        <zhengjun.xing@linux.intel.com>, <fengwei.yin@intel.com>
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From:   John Garry <john.garry@huawei.com>
In-Reply-To: <ef84d2dd-84ea-e1c7-05ef-f0a082d4f7d0@opensource.wdc.com>
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On 09/08/2022 15:57, Damien Le Moal wrote:
>>> As far as I can see, this patch should not make a difference unless the
>>> ATA shost driver is setting the max_sectors value unnecessarily low.
>> For __ATA_BASE_SHT, we don't set max_sectors. As such, we default
>> shost->max_sectors = SCSI_DEFAULT_MAX_SECTORS (=1024) in
>> scsi_host_alloc(). I assume no shost dma mapping limit applied.
>>
>> Then - for example - we could select dev->max_sectors =
>> ATA_MAX_SECTORS_LBA48 (=65535) in ata_dev_configure().
>>
>> So with commit 0568e6122574 we would have final max sectors = 1024, as
>> opposed to 65535 previously. I guess that the problem is something like
>> this.
>>
>> If so, it seems that we would need to apply the shost dma mapping limit
>> separately in ata_scsi_dev_config() and not use shost->max_sectors.
> OK. Will have a look at that.
> 

We may need to introduce something like shost->max_hw_sectors, which is 
set according to sht max sectors and dma mapping limits. That could be 
also used in USB scsiglue slave_configure()

Or else set max_sectors value for __ATA_BASE_SHT, but I don't know a 
sane value there considering ATA_MAX_SECTORS_LBA48 gives max_sectors of 
65535.

Damien, please let me know if you need help now. I am just waiting for 
you to test to prove this theory about dev->max_sectors being capped. I 
don't have an AHCI setup readily-available for testing - just SAS cards 
or QEMU.

Thanks,
John