linux-kernel - Re: [PATCH v12 5/9] nvme: add copy offload support

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Date:   Thu, 8 Jun 2023 21:24:38 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nitesh Shetty <nj.shetty@...sung.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        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>,
        linux-scsi@...r.kernel.org, willy@...radead.org, hare@...e.de,
        djwong@...nel.org, bvanassche@....org, ming.lei@...hat.com,
        dlemoal@...nel.org, nitheshshetty@...il.com, gost.dev@...sung.com,
        Kanchan Joshi <joshi.k@...sung.com>,
        Javier González <javier.gonz@...sung.com>,
        Anuj Gupta <anuj20.g@...sung.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v12 5/9] nvme: add copy offload support

On Thu, Jun 08, 2023 at 05:38:17PM +0530, Nitesh Shetty wrote:
> Sure, we can do away with subsys and realign more on single namespace copy.
> We are planning to use token to store source info, such as src sector,
> len and namespace. Something like below,
> 
> struct nvme_copy_token {
> 	struct nvme_ns *ns; // to make sure we are copying within same namespace
> /* store source info during *IN operation, will be used by *OUT operation */
> 	sector_t src_sector;
> 	sector_t sectors;
> };
> Do you have any better way to handle this in mind ?

In general every time we tried to come up with a request payload that is
not just data passed to the device it has been a nightmare.

So my gut feeling would be that bi_sector and bi_iter.bi_size are the
ranges, with multiple bios being allowed to form the input data, similar
to how we implement discard merging.

The interesting part is how we'd match up these bios.  One idea would
be that since copy by definition doesn't need integrity data we just
add a copy_id that unions it, and use a simple per-gendisk copy I/D
allocator, but I'm not entirely sure how well that interacts stacking
drivers.