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Subject: Re: [PATCH v3 4/4] io_uring: add support for zone-append
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Kanchan Joshi <joshi.k@samsung.com>, viro@zeniv.linux.org.uk,
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        Selvakumar S <selvakuma.s1@samsung.com>,
        Nitesh Shetty <nj.shetty@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <2270907f-670c-5182-f4ec-9756dc645376@kernel.dk>
Date:   Thu, 9 Jul 2020 08:05:46 -0600
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On 7/9/20 8:00 AM, Christoph Hellwig wrote:
> On Thu, Jul 09, 2020 at 07:58:04AM -0600, Jens Axboe wrote:
>>> We don't actually need any new field at all.  By the time the write
>>> returned ki_pos contains the offset after the write, and the res
>>> argument to ->ki_complete contains the amount of bytes written, which
>>> allow us to trivially derive the starting position.
>>
>> Then let's just do that instead of jumping through hoops either
>> justifying growing io_rw/io_kiocb or turning kiocb into a global
>> completion thing.
> 
> Unfortunately that is a totally separate issue - the in-kernel offset
> can be trivially calculated.  But we still need to figure out a way to
> pass it on to userspace.  The current patchset does that by abusing
> the flags, which doesn't really work as the flags are way too small.
> So we somewhere need to have an address to do the put_user to.

Right, we're just trading the 'append_offset' for a 'copy_offset_here'
pointer, which are stored in the same spot...

-- 
Jens Axboe