Re: [dm-devel] [PATCH 3/7] block: copy offload support infrastructure

From: Mikulas Patocka <mpatocka@redhat.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Mike Snitzer <snitzer@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	djwong@kernel.org, linux-nvme@lists.infradead.org,
	dm-devel@redhat.com, hch@lst.de, agk@redhat.com,
	linux-scsi@vger.kernel.org, nitheshshetty@gmail.com,
	willy@infradead.org, nj.shetty@samsung.com, kch@kernel.org,
	SelvaKumar S <selvakuma.s1@samsung.com>,
	selvajove@gmail.com, linux-block@vger.kernel.org,
	javier.gonz@samsung.com, kbusch@kernel.org, axboe@kernel.dk,
	damien.lemoal@wdc.com, joshi.k@samsung.com,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	linux-api@vger.kernel.org, johannes.thumshirn@wdc.com,
	linux-fsdevel@vger.kernel.org, joshiiitr@gmail.com,
	asml.silence@gmail.com
Subject: Re: [dm-devel] [PATCH 3/7] block: copy offload support infrastructure
Date: Tue, 17 Aug 2021 16:41:35 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LRH.2.02.2108171630120.30363@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <ad3561b9-775d-dd4d-0d92-6343440b1f8f@acm.org>

On Tue, 17 Aug 2021, Bart Van Assche wrote:

> On 8/17/21 3:14 AM, SelvaKumar S wrote:
> > Introduce REQ_OP_COPY, a no-merge copy offload operation. Create
> > bio with control information as payload and submit to the device.
> > Larger copy operation may be divided if necessary by looking at device
> > limits. REQ_OP_COPY(19) is a write op and takes zone_write_lock when
> > submitted to zoned device.
> > Native copy offload is not supported for stacked devices.
> 
> Using a single operation for copy-offloading instead of separate operations
> for reading and writing is fundamentally incompatible with the device mapper.
> I think we need a copy-offloading implementation that is compatible with the
> device mapper.

I once wrote a copy offload implementation that is compatible with device 
mapper. The copy operation creates two bios (one for reading and one for 
writing), passes them independently through device mapper and pairs them 
at the physical device driver.

It's here: http://people.redhat.com/~mpatocka/patches/kernel/xcopy/current

I verified that it works with iSCSI. Would you be interested in continuing 
this work?

Mikulas

> Storing the parameters of the copy operation in the bio payload is
> incompatible with the current implementation of bio_split().
> 
> In other words, I think there are fundamental problems with this patch series.
> 
> Bart.
> 

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