linux-kernel - Re: [PATCH v4 6/6] io_uring: add support for zone-append

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Message-ID: <MWHPR04MB37581344328A42EA7F5ED13EE74E0@MWHPR04MB3758.namprd04.prod.outlook.com>
Date:   Fri, 31 Jul 2020 06:59:45 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     "hch@...radead.org" <hch@...radead.org>
CC:     Kanchan Joshi <joshiiitr@...il.com>, Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Kanchan Joshi <joshi.k@...sung.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "bcrl@...ck.org" <bcrl@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        SelvaKumar S <selvakuma.s1@...sung.com>,
        Nitesh Shetty <nj.shetty@...sung.com>,
        Javier Gonzalez <javier.gonz@...sung.com>
Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append

On 2020/07/31 15:45, hch@...radead.org wrote:
> On Fri, Jul 31, 2020 at 06:42:10AM +0000, Damien Le Moal wrote:
>>> - We may not be able to use RWF_APPEND, and need exposing a new
>>> type/flag (RWF_INDIRECT_OFFSET etc.) user-space. Not sure if this
>>> sounds outrageous, but is it OK to have uring-only flag which can be
>>> combined with RWF_APPEND?
>>
>> Why ? Where is the problem ? O_APPEND/RWF_APPEND is currently meaningless for
>> raw block device accesses. We could certainly define a meaning for these in the
>> context of zoned block devices.
> 
> We can't just add a meaning for O_APPEND on block devices now,
> as it was previously silently ignored.  I also really don't think any
> of these semantics even fit the block device to start with.  If you
> want to work on raw zones use zonefs, that's what is exists for.

Which is fine with me. Just trying to say that I think this is exactly the
discussion we need to start with. What interface do we implement...

Allowing zone append only through zonefs as the raw block device equivalent, all
the O_APPEND/RWF_APPEND semantic is defined and the "return written offset"
implementation in VFS would be common for all file systems, including regular
ones. Beside that, there is I think the question of short writes... Not sure if
short writes can currently happen with async RWF_APPEND writes to regular files.
I think not but that may depend on the FS.


-- 
Damien Le Moal
Western Digital Research