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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,
        bcrl@kvack.org, Damien.LeMoal@wdc.com, asml.silence@gmail.com,
        linux-fsdevel@vger.kernel.org, mb@lightnvm.io,
        linux-kernel@vger.kernel.org, linux-aio@kvack.org,
        io-uring@vger.kernel.org, linux-block@vger.kernel.org,
        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: <adc14700-8e95-10b2-d914-afa5029ae80c@kernel.dk>
Date:   Thu, 9 Jul 2020 07:58:04 -0600
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On 7/9/20 4:15 AM, Christoph Hellwig wrote:
> On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 155f3d8..cbde4df 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -402,6 +402,8 @@ struct io_rw {
>>>  	struct kiocb			kiocb;
>>>  	u64				addr;
>>>  	u64				len;
>>> +	/* zone-relative offset for append, in sectors */
>>> +	u32			append_offset;
>>>  };
>>
>> I don't like this very much at all. As it stands, the first cacheline
>> of io_kiocb is set aside for request-private data. io_rw is already
>> exactly 64 bytes, which means that you're now growing io_rw beyond
>> a cacheline and increasing the size of io_kiocb as a whole.
>>
>> Maybe you can reuse io_rw->len for this, as that is only used on the
>> submission side of things.
> 
> 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.

-- 
Jens Axboe