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

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Date:   Fri, 10 Jul 2020 08:09:33 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@...radead.org>,
        Kanchan Joshi <joshiiitr@...il.com>
Cc:     Kanchan Joshi <joshi.k@...sung.com>, viro@...iv.linux.org.uk,
        bcrl@...ck.org, Damien.LeMoal@....com, asml.silence@...il.com,
        linux-fsdevel@...r.kernel.org, Matias Bj??rling <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 7/10/20 7:10 AM, Christoph Hellwig wrote:
> On Fri, Jul 10, 2020 at 12:35:43AM +0530, Kanchan Joshi wrote:
>> Append required special treatment (conversion for sector to bytes) for io_uring.
>> And we were planning a user-space wrapper to abstract that.
>>
>> But good part (as it seems now) was: append result went along with cflags at
>> virtually no additional cost. And uring code changes became super clean/minimal
>> with further revisions.
>> While indirect-offset requires doing allocation/mgmt in application,
>> io-uring submission
>> and in completion path (which seems trickier), and those CQE flags
>> still get written
>> user-space and serve no purpose for append-write.
> 
> I have to say that storing the results in the CQE generally make
> so much more sense.  I wonder if we need a per-fd "large CGE" flag
> that adds two extra u64s to the CQE, and some ops just require this
> version.

I have been pondering the same thing, we could make certain ops consume
two CQEs if it makes sense. It's a bit ugly on the app side with two
different CQEs for a request, though. We can't just treat it as a large
CQE, as they might not be sequential if we happen to wrap. But maybe
it's not too bad.

-- 
Jens Axboe