linux-kernel - Re: [PATCH v5 02/10] block: Add copy offload support infrastructure

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Message-ID: <Y33UAp6ncSPO84XI@T590>
Date:   Wed, 23 Nov 2022 16:04:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Nitesh Shetty <nj.shetty@...sung.com>
Cc:     axboe@...nel.dk, agk@...hat.com, snitzer@...nel.org,
        dm-devel@...hat.com, kbusch@...nel.org, hch@....de,
        sagi@...mberg.me, james.smart@...adcom.com, kch@...dia.com,
        damien.lemoal@...nsource.wdc.com, naohiro.aota@....com,
        jth@...nel.org, viro@...iv.linux.org.uk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        anuj20.g@...sung.com, joshi.k@...sung.com, p.raghav@...sung.com,
        nitheshshetty@...il.com, gost.dev@...sung.com, ming.lei@...hat.com
Subject: Re: [PATCH v5 02/10] block: Add copy offload support infrastructure

On Wed, Nov 23, 2022 at 11:28:19AM +0530, Nitesh Shetty wrote:
> Introduce blkdev_issue_copy which supports source and destination bdevs,
> and an array of (source, destination and copy length) tuples.
> Introduce REQ_COPY copy offload operation flag. Create a read-write
> bio pair with a token as payload and submitted to the device in order.
> Read request populates token with source specific information which
> is then passed with write request.
> This design is courtesy Mikulas Patocka's token based copy

I thought this patchset is just for enabling copy command which is
supported by hardware. But turns out it isn't, because blk_copy_offload()
still submits read/write bios for doing the copy.

I am just wondering why not let copy_file_range() cover this kind of copy,
and the framework has been there.

When I was researching pipe/splice code for supporting ublk zero copy[1], I
have got idea for async copy_file_range(), such as: io uring based
direct splice, user backed intermediate buffer, still zero copy, if these
ideas are finally implemented, we could get super-fast generic offload copy,
and bdev copy is really covered too.

[1] https://lore.kernel.org/linux-block/20221103085004.1029763-1-ming.lei@redhat.com/

thanks,
Ming