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

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Date:   Fri, 31 Jul 2020 13:28:24 +0530
From:   Kanchan Joshi <joshiiitr@...il.com>
To:     Damien Le Moal <Damien.LeMoal@....com>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        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 Fri, Jul 31, 2020 at 12:29 PM Damien Le Moal <Damien.LeMoal@....com> wrote:
>
> 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.

generic_write_check_limits (called by generic_write_checks, used by
most FS) may make it short, and AFAIK it does not depend on
async/sync.
This was one of the reason why we chose to isolate the operation by a
different IOCB flag and not by IOCB_APPEND alone.

For block-device these checks are not done, but there is another place
when it receives writes spanning beyond EOF and iov_iter_truncate()
adjusts it before sending it down.
And we return failure for that case in V4-  "Ref: [PATCH v4 3/6] uio:
return status with iov truncation"


-- 
Joshi