linux-kernel - Re: [PATCH v3 4/4] io_uring: add support for zone-append

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Message-ID: <20200706141002.GZ25523@casper.infradead.org>
Date:   Mon, 6 Jul 2020 15:10:02 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Kanchan Joshi <joshi.k@...sung.com>, viro@...iv.linux.org.uk,
        bcrl@...ck.org, hch@...radead.org, Damien.LeMoal@....com,
        asml.silence@...il.com, linux-fsdevel@...r.kernel.org,
        mb@...htnvm.io, linux-kernel@...r.kernel.org, linux-aio@...ck.org,
        io-uring@...r.kernel.org, linux-block@...r.kernel.org,
        Selvakumar S <selvakuma.s1@...sung.com>,
        Nitesh Shetty <nj.shetty@...sung.com>,
        Javier Gonzalez <javier.gonz@...sung.com>
Subject: Re: [PATCH v3 4/4] io_uring: add support for zone-append

On Sun, Jul 05, 2020 at 03:12:50PM -0600, Jens Axboe wrote:
> On 7/5/20 3:09 PM, Matthew Wilcox wrote:
> > On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:
> >> On 7/5/20 12:47 PM, Kanchan Joshi wrote:
> >>> From: Selvakumar S <selvakuma.s1@...sung.com>
> >>>
> >>> For zone-append, block-layer will return zone-relative offset via ret2
> >>> of ki_complete interface. Make changes to collect it, and send to
> >>> user-space using cqe->flags.

> > I'm surprised you aren't more upset by the abuse of cqe->flags for the
> > address.
> 
> Yeah, it's not great either, but we have less leeway there in terms of
> how much space is available to pass back extra data.
> 
> > What do you think to my idea of interpreting the user_data as being a
> > pointer to somewhere to store the address?  Obviously other things
> > can be stored after the address in the user_data.
> 
> I don't like that at all, as all other commands just pass user_data
> through. This means the application would have to treat this very
> differently, and potentially not have a way to store any data for
> locating the original command on the user side.

I think you misunderstood me.  You seem to have thought I meant
"use the user_data field to return the address" when I actually meant
"interpret the user_data field as a pointer to where userspace
wants the address stored".