linux-kernel - Re: [PATCH v13 2/9] block: Add copy offload support infrastructure

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Message-ID: <CAOSviJ3oDSHk2HXyRZa=A43vCxh-n2YkyuW-qXNq-q=i6bNacQ@mail.gmail.com>
Date:   Thu, 27 Jul 2023 15:59:49 +0530
From:   Nitesh Shetty <nitheshshetty@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Nitesh Shetty <nj.shetty@...sung.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>,
        Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        willy@...radead.org, hare@...e.de, djwong@...nel.org,
        bvanassche@....org, ming.lei@...hat.com, dlemoal@...nel.org,
        gost.dev@...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 v13 2/9] block: Add copy offload support infrastructure

On Thu, Jul 20, 2023 at 1:12 PM Christoph Hellwig <hch@....de> wrote:
> > Suggested-by: Christoph Hellwig <hch@....de>
>
> Hmm, I'm not sure I suggested adding copy offload..
>
We meant for request based design, we will remove it.

> >  static inline unsigned int blk_rq_get_max_segments(struct request *rq)
> >  {
> >       if (req_op(rq) == REQ_OP_DISCARD)
> > @@ -303,6 +310,8 @@ static inline bool bio_may_exceed_limits(struct bio *bio,
> >               break;
> >       }
> >
> > +     if (unlikely(op_is_copy(bio->bi_opf)))
> > +             return false;
>
> This looks wrong to me.  I think the copy ops need to be added to the
> switch statement above as they have non-trivial splitting decisions.
> Or at least should have those as we're missing the code to split
> copy commands right now.
>

Agreed, copy will have non-trivial splitting decisions. But, I
couldn't think of scenarios where this could happen, as we check for
queue limits before issuing a copy. Do you see scenarios where split
could happen for copy here.

Acked for all other review comments.

Thank you,
Nitesh Shetty