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Date:   Mon, 11 Jul 2022 15:49:46 +0100
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Subject: Re: [PATCH v5 0/5] DMA mapping changes for SCSI core
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Christoph Hellwig <hch@lst.de>
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        <iommu@lists.linux.dev>, <linux-scsi@vger.kernel.org>,
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From:   John Garry <john.garry@huawei.com>
In-Reply-To: <62b801e8-66b6-0af7-b0c9-195823bf9f62@opensource.wdc.com>
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On 11/07/2022 11:40, Damien Le Moal wrote:
> On 7/11/22 16:36, John Garry wrote:
>> On 11/07/2022 00:08, Damien Le Moal wrote:
>>>> 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().
>>> I was suggesting the reverse:)  Keep the device hard limit
>>> (max_hw_sectors) to the max dma mapping and set the soft limit
>>> (max_sectors) to the optimal dma mapping size.
>>
>> Sure, but as I mentioned below, I only see a small % of requests whose
>> mapping size exceeds max_sectors but that still causes a big performance
>> hit. So that is why I want to set the hard limit as the optimal dma
>> mapping size.
> 
> How can you possibly end-up with requests larger than max_sectors ? BIO
> split is done using this limit, right ? Or is it that request merging is
> allowed up to max_hw_sectors even if the resulting request size exceeds
> max_sectors ?
> 

Ah, I see how I thought that I was seeing requests whose size exceeded 
max_sectors. Somebody must have changed a single disk in my system and 
this odd disk has a higher default max_sectors_kb -- 512 vs 128 for the 
rest.

So ignoring my nonesence that I was seeing oversize requests, as for the 
idea to set default max_sectors at dma_opt_mapping_size(), I see some 
issues:
- for SAS disks I have no common point to impose this limit. Maybe in 
the slave configure callback, but each SAS driver has its own 
implementation generally
- Even if we do config in slave_configure callback the max_sectors value 
is overwritten later in sd_revalidate_disk().

This following change could sort the issue though:

---8<----
diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 895b56c8f25e..bb49bea3d161 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -3214,6 +3214,8 @@ static int sd_revalidate_disk(struct gendisk *disk)
         sector_t old_capacity = sdkp->capacity;
         unsigned char *buffer;
         unsigned int dev_max, rw_max;
+       struct Scsi_Host *host = sdp->host;
+       struct device *dev = host->dma_dev;

         SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp,
                                       "sd_revalidate_disk\n"));
@@ -3296,8 +3298,13 @@ static int sd_revalidate_disk(struct gendisk *disk)
                                       (sector_t)BLK_DEF_MAX_SECTORS);
         }

-       /* Do not exceed controller limit */
-       rw_max = min(rw_max, queue_max_hw_sectors(q));
+       if (dev->dma_mask) {
+               /* Do not exceed dma optimal limit */
+               rw_max = min_t(unsigned int, rw_max,
+                               dma_opt_mapping_size(dev) >> SECTOR_SHIFT);
+       } else {
+               rw_max = min(rw_max, queue_max_hw_sectors(q));
+       }

         /*
          * Only update max_sectors if previously unset or if the 
current value

--->8---

Or I could go with the method in this series, which is not preferred. 
Let me know what you think.

Thanks,
John