linux-kernel - Re: [PATCH v8 1/9] block: Introduce queue limits for copy-offload support

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Message-ID: <03c647ff-3c4f-a810-12c4-06a9dc62c90e@opensource.wdc.com>
Date:   Wed, 29 Mar 2023 21:24:11 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Nitesh Shetty <nj.shetty@...sung.com>
Cc:     Anuj Gupta <anuj20.g@...sung.com>, Jens Axboe <axboe@...nel.dk>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
        Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        James Smart <james.smart@...adcom.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, bvanassche@....org,
        hare@...e.de, ming.lei@...hat.com, joshi.k@...sung.com,
        nitheshshetty@...il.com, gost.dev@...sung.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v8 1/9] block: Introduce queue limits for copy-offload
 support

On 3/29/23 19:41, Nitesh Shetty wrote:
>>> +What:		/sys/block/<disk>/queue/copy_max_bytes
>>> +Date:		November 2022
>>> +Contact:	linux-block@...r.kernel.org
>>> +Description:
>>> +		[RW] While 'copy_max_bytes_hw' is the hardware limit for the
>>> +		device, 'copy_max_bytes' setting is the software limit.
>>> +		Setting this value lower will make Linux issue smaller size
>>> +		copies from block layer.
>>
>> 		This is the maximum number of bytes that the block
>>                 layer will allow for a copy request. Must be smaller than
>>                 or equal to the maximum size allowed by the hardware indicated
> 
> Looks good.  Will update in next version. We took reference from discard. 
> 
>> 		by copy_max_bytes_hw. Write 0 to use the default kernel
>> 		settings.
>>
> 
> Nack, writing 0 will not set it to default value. (default value is
> copy_max_bytes = copy_max_bytes_hw)

It is trivial to make it work that way, which would match how max_sectors_kb
works. Write 0 to return copy_max_bytes being equal to the default
copy_max_bytes_hw.

The other possibility that is also interesting is "write 0 to disable copy
offload and use emulation". This one may actually be more useful.

> 
>>> +
>>> +
>>> +What:		/sys/block/<disk>/queue/copy_max_bytes_hw
>>> +Date:		November 2022
>>> +Contact:	linux-block@...r.kernel.org
>>> +Description:
>>> +		[RO] Devices that support offloading copy functionality may have
>>> +		internal limits on the number of bytes that can be offloaded
>>> +		in a single operation. The `copy_max_bytes_hw`
>>> +		parameter is set by the device driver to the maximum number of
>>> +		bytes that can be copied in a single operation. Copy
>>> +		requests issued to the device must not exceed this limit.
>>> +		A value of 0 means that the device does not
>>> +		support copy offload.
>>
>> 		[RO] This is the maximum number of kilobytes supported in a
>>                 single data copy offload operation. A value of 0 means that the
>> 		device does not support copy offload.
>>
> 
> Nack, value is in bytes. Same as discard.

Typo. I meant Bytes. Your text is too long an too convoluted, so unclear.

-- 
Damien Le Moal
Western Digital Research