linux-kernel - Re: [PATCH v20 01/12] block: Introduce queue limits and sysfs for copy-offload support

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Date: Mon, 3 Jun 2024 06:43:56 +0000
From: Nitesh Shetty <nj.shetty@...sung.com>
To: Christoph Hellwig <hch@....de>
Cc: Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>, Alasdair
	Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>, Mikulas Patocka
	<mpatocka@...hat.com>, Keith Busch <kbusch@...nel.org>, Sagi Grimberg
	<sagi@...mberg.me>, Chaitanya Kulkarni <kch@...dia.com>, Alexander Viro
	<viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara
	<jack@...e.cz>, martin.petersen@...cle.com, bvanassche@....org,
	david@...morbit.com, hare@...e.de, damien.lemoal@...nsource.wdc.com,
	anuj20.g@...sung.com, joshi.k@...sung.com, nitheshshetty@...il.com,
	gost.dev@...sung.com, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	dm-devel@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v20 01/12] block: Introduce queue limits and sysfs for
 copy-offload support

On 01/06/24 07:53AM, Christoph Hellwig wrote:
>On Mon, May 20, 2024 at 03:50:14PM +0530, Nitesh Shetty wrote:
>> Add device limits as sysfs entries,
>> 	- copy_max_bytes (RW)
>> 	- copy_max_hw_bytes (RO)
>>
>> Above limits help to split the copy payload in block layer.
>> copy_max_bytes: maximum total length of copy in single payload.
>> copy_max_hw_bytes: Reflects the device supported maximum limit.
>
>That's a bit of a weird way to phrase the commit log as the queue_limits
>are the main thing (and there are three of them as required for the
>scheme to work).  The sysfs attributes really are just an artifact.
>
Acked, we will update commit log.

>> @@ -231,10 +237,11 @@ int blk_set_default_limits(struct queue_limits *lim)
>>  {
>>  	/*
>>  	 * Most defaults are set by capping the bounds in blk_validate_limits,
>> -	 * but max_user_discard_sectors is special and needs an explicit
>> -	 * initialization to the max value here.
>> +	 * but max_user_discard_sectors and max_user_copy_sectors are special
>> +	 * and needs an explicit initialization to the max value here.
>
>s/needs/need/
>
Acked.

>> +/*
>> + * blk_queue_max_copy_hw_sectors - set max sectors for a single copy payload
>> + * @q:	the request queue for the device
>> + * @max_copy_sectors: maximum number of sectors to copy
>> + */
>> +void blk_queue_max_copy_hw_sectors(struct request_queue *q,
>> +				   unsigned int max_copy_sectors)
>> +{
>> +	struct queue_limits *lim = &q->limits;
>> +
>> +	if (max_copy_sectors > (BLK_COPY_MAX_BYTES >> SECTOR_SHIFT))
>> +		max_copy_sectors = BLK_COPY_MAX_BYTES >> SECTOR_SHIFT;
>> +
>> +	lim->max_copy_hw_sectors = max_copy_sectors;
>> +	lim->max_copy_sectors =
>> +		min(max_copy_sectors, lim->max_user_copy_sectors);
>> +}
>> +EXPORT_SYMBOL_GPL(blk_queue_max_copy_hw_sectors);
>
>Please don't add new blk_queue_* helpers, everything should go through
>the atomic queue limits API now.  Also capping the hardware limit
>here looks odd.
>
This was a mistake, we are not using this function. We will remove this
function in next version.

>> +	if (max_copy_bytes & (queue_logical_block_size(q) - 1))
>> +		return -EINVAL;
>
>This should probably go into blk_validate_limits and just round down.
>
Bart also pointed out similar thing. We do same check before issuing
copy offload, so we will remove this check.

>Also most block limits are in kb.  Not that I really know why we are
>doing that, but is there a good reason to deviate from that scheme?
>
We followed discard as a reference, but we can move to kb, if that helps
with overall readability.

Thank you,
Nitesh Shetty