Re: [PATCH 1/1] scsi: sd: use max_xfer_blocks for set rw_max if max_xfer_blocks is available

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2021/01/22 16:24, Changheun Lee wrote:
>> On 2021/01/20 15:45, Manjong Lee wrote:
>>> Add recipients for more reviews.
>>
>> Please resend instead of replying to your own patch. The reply quoting corrupts
>> the patch.
>>
>> The patch title is very long.
>>
>>>
>>>> SCSI device has max_xfer_size and opt_xfer_size,
>>>> but current kernel uses only opt_xfer_size.
>>>>
>>>> It causes the limitation on setting IO chunk size,
>>>> although it can support larger one.
>>>>
>>>> So, I propose this patch to use max_xfer_size in case it has valid value.
>>>> It can support to use the larger chunk IO on SCSI device.
>>>>
>>>> For example,
>>>> This patch is effective in case of some SCSI device like UFS
>>>> with opt_xfer_size 512KB, queue depth 32 and max_xfer_size over 512KB.
>>>>
>>>> I expect both the performance improvement
>>>> and the efficiency use of smaller command queue depth.
>>
>> This can be measured, and this commit message should include results to show how
>> effective this change is.
>>
>>>>
>>>> Signed-off-by: Manjong Lee <mj0123.lee@samsung.com>
>>>> ---
>>>> drivers/scsi/sd.c | 56 +++++++++++++++++++++++++++++++++++++++++++----
>>>> 1 file changed, 52 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
>>>> index 679c2c025047..de59f01c1304 100644
>>>> --- a/drivers/scsi/sd.c
>>>> +++ b/drivers/scsi/sd.c
>>>> @@ -3108,6 +3108,53 @@ static void sd_read_security(struct scsi_disk *sdkp, unsigned char *buffer)
>>>> sdkp->security = 1;
>>>> }
>>>>
>>>> +static bool sd_validate_max_xfer_size(struct scsi_disk *sdkp,
>>>> +				      unsigned int dev_max)
>>>> +{
>>>> +	struct scsi_device *sdp = sdkp->device;
>>>> +	unsigned int max_xfer_bytes =
>>>> +		logical_to_bytes(sdp, sdkp->max_xfer_blocks);
>>>> +
>>>> +	if (sdkp->max_xfer_blocks == 0)
>>>> +		return false;
>>>> +
>>>> +	if (sdkp->max_xfer_blocks > SD_MAX_XFER_BLOCKS) {
>>>> +		sd_first_printk(KERN_WARNING, sdkp,
>>>> +				"Maximal transfer size %u logical blocks " \
>>>> +				"> sd driver limit (%u logical blocks)\n",
>>>> +				sdkp->max_xfer_blocks, SD_DEF_XFER_BLOCKS);
>>>> +		return false;
>>>> +	}
>>>> +
>>>> +	if (sdkp->max_xfer_blocks > dev_max) {
>>>> +		sd_first_printk(KERN_WARNING, sdkp,
>>>> +				"Maximal transfer size %u logical blocks "
>>>> +				"> dev_max (%u logical blocks)\n",
>>>> +				sdkp->max_xfer_blocks, dev_max);
>>>> +		return false;
>>>> +	}
>>>> +
>>>> +	if (max_xfer_bytes < PAGE_SIZE) {
>>>> +		sd_first_printk(KERN_WARNING, sdkp,
>>>> +				"Maximal transfer size %u bytes < " \
>>>> +				"PAGE_SIZE (%u bytes)\n",
>>>> +				max_xfer_bytes, (unsigned int)PAGE_SIZE);
>>>> +		return false;
>>>> +	}
>>>> +
>>>> +	if (max_xfer_bytes & (sdkp->physical_block_size - 1)) {
>>>> +		sd_first_printk(KERN_WARNING, sdkp,
>>>> +				"Maximal transfer size %u bytes not a " \
>>>> +				"multiple of physical block size (%u bytes)\n",
>>>> +				max_xfer_bytes, sdkp->physical_block_size);
>>>> +		return false;
>>>> +	}
>>>> +
>>>> +	sd_first_printk(KERN_INFO, sdkp, "Maximal transfer size %u bytes\n",
>>>> +			max_xfer_bytes);
>>>> +	return true;
>>>> +}
>>
>> Except for the order of the comparisons against SD_MAX_XFER_BLOCKS and dev_max,
>> this function looks identical to sd_validate_opt_xfer_size(), modulo the use of
>> max_xfer_blocks instead of opt_xfer_blocks. Can't you turn this into something like:
>>
>> static bool sd_validate_max_xfer_size(struct scsi_disk *sdkp,
>> const char *name,
>> unsigned int xfer_blocks,
>> unsigned int dev_max)
>>
>> To allow checking both opt_xfer_blocks and max_xfer_blocks ?
>>
>>>> +
>>>> /*
>>>> * Determine the device's preferred I/O size for reads and writes
>>>> * unless the reported value is unreasonably small, large, not a
>>>> @@ -3233,12 +3280,13 @@ static int sd_revalidate_disk(struct gendisk *disk)
>>>>
>>>> /* Initial block count limit based on CDB TRANSFER LENGTH field size. */
>>>> dev_max = sdp->use_16_for_rw ? SD_MAX_XFER_BLOCKS : SD_DEF_XFER_BLOCKS;
>>
>> This looks weird: no indentation. Care to resend ?
>>
>>>> -
>>>> -	/* Some devices report a maximum block count for READ/WRITE requests. */
>>>> -	dev_max = min_not_zero(dev_max, sdkp->max_xfer_blocks);
>>>> q->limits.max_dev_sectors = logical_to_sectors(sdp, dev_max);
>>>>
>>>> -	if (sd_validate_opt_xfer_size(sdkp, dev_max)) {
>>>> +	if (sd_validate_max_xfer_size(sdkp, dev_max)) {
>>>> +		q->limits.io_opt = 0;
>>>> +		rw_max = logical_to_sectors(sdp, sdkp->max_xfer_blocks);
>>>> +		q->limits.max_dev_sectors = rw_max;
>>>> +	} else if (sd_validate_opt_xfer_size(sdkp, dev_max)) {
>>
>> This does not look correct to me. This renders the device reported
>> opt_xfer_blocks useless.
>>
>> The unmodified code sets dev_max to the min of SD_MAX_XFER_BLOCKS or
>> SD_DEF_XFER_BLOCKS and of the device reported max_xfer_blocks. The result of
>> this is used as the device max_dev_sectors queue limit, which in turn is used to
>> set the max_hw_sectors queue limit accounting for the adapter limits too.
>>
>> opt_xfer_blocks, if it is valid, will be used to set the io_opt queue limit,
>> which is a hint. This hint is used to optimize the "soft" max_sectors command
>> limit used by the block layer to limit command size if the value of
>> opt_xfer_blocks is smaller than the limit initially set with max_xfer_blocks.
>>
>> So if for your device max_sectors end up being too small, it is likely because
>> the device itself is reporting an opt_xfer_blocks value that is too small for
>> its own good. The max_sectors limit can be manually increased with "echo xxx >
>> /sys/block/sdX/queue/max_sectors_kb". A udev rule can be used to handle this
>> autmatically if needed.
>>
>> But to get a saner default for that device, I do not think that this patch is
>> the right solution. Ideally, the device peculiarity should be handled with a
>> quirk, but that is not used in scsi. So beside the udev rule trick, I am not
>> sure what the right approach is here.
>>
> 
> This approach is for using sdkp->max_xfer_blocks as a rw_max.
> There are no way to use it now when sdkp->opt_xfer_blocks is valid.
> In my case, scsi device reports both of sdkp->max_xfer_blocks, and
> sdkp->opt_xfer_blocks.
> 
> How about set larger valid value between sdkp->max_xfer_blocks,
> and sdkp->opt_xfer_blocks to rw_max?

Again, if your device reports an opt_xfer_blocks value that is too small for its
own good, that is a problem with this device. The solution for that is not to
change something that will affect *all* other storage devices, including those
with a perfectly valid opt_xfer_blocks value.

I think that the solution should be at the LLD level, for that device only. But
I am not sure how to communicate a quirk for opt_xfer_blocks back to the generic
sd driver. You should explore a solution like that. Others may have ideas about
this too. Wait for more comments.

> 
>>>> q->limits.io_opt = logical_to_bytes(sdp, sdkp->opt_xfer_blocks);
>>>> rw_max = logical_to_sectors(sdp, sdkp->opt_xfer_blocks);
>>>> } else {
>>>> -- 
>>>> 2.29.0
>>>>
>>>>
>>>
> 


-- 
Damien Le Moal
Western Digital Research