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Subject: Re: [PATCH 3/3] io_uring: add support for zone-append
To:     Pavel Begunkov <asml.silence@gmail.com>,
        "javier.gonz@samsung.com" <javier@javigon.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>
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        "linux-aio@kvack.org" <linux-aio@kvack.org>,
        "io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "selvakuma.s1@samsung.com" <selvakuma.s1@samsung.com>,
        "nj.shetty@samsung.com" <nj.shetty@samsung.com>
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <a68cb8f6-e17c-9ee3-b732-4be689ffebc3@kernel.dk>
Date:   Fri, 19 Jun 2020 09:02:58 -0600
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On 6/19/20 8:59 AM, Pavel Begunkov wrote:
> On 19/06/2020 17:15, Jens Axboe wrote:
>> On 6/19/20 3:41 AM, javier.gonz@samsung.com wrote:
>>> Jens,
>>>
>>> Would you have time to answer a question below in this thread?
>>>
>>> On 18.06.2020 11:11, javier.gonz@samsung.com wrote:
>>>> On 18.06.2020 08:47, Damien Le Moal wrote:
>>>>> On 2020/06/18 17:35, javier.gonz@samsung.com wrote:
>>>>>> On 18.06.2020 07:39, Damien Le Moal wrote:
>>>>>>> On 2020/06/18 2:27, Kanchan Joshi wrote:
>>>>>>>> From: Selvakumar S <selvakuma.s1@samsung.com>
>>>>>>>>
>>>>>>>> Introduce three new opcodes for zone-append -
>>>>>>>>
>>>>>>>>   IORING_OP_ZONE_APPEND     : non-vectord, similiar to IORING_OP_WRITE
>>>>>>>>   IORING_OP_ZONE_APPENDV    : vectored, similar to IORING_OP_WRITEV
>>>>>>>>   IORING_OP_ZONE_APPEND_FIXED : append using fixed-buffers
>>>>>>>>
>>>>>>>> Repurpose cqe->flags to return zone-relative offset.
>>>>>>>>
>>>>>>>> Signed-off-by: SelvaKumar S <selvakuma.s1@samsung.com>
>>>>>>>> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
>>>>>>>> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>>>>>>>> Signed-off-by: Javier Gonzalez <javier.gonz@samsung.com>
>>>>>>>> ---
>>>>>>>> fs/io_uring.c                 | 72 +++++++++++++++++++++++++++++++++++++++++--
>>>>>>>> include/uapi/linux/io_uring.h |  8 ++++-
>>>>>>>> 2 files changed, 77 insertions(+), 3 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>>>>> index 155f3d8..c14c873 100644
>>>>>>>> --- a/fs/io_uring.c
>>>>>>>> +++ b/fs/io_uring.c
>>>>>>>> @@ -649,6 +649,10 @@ struct io_kiocb {
>>>>>>>> 	unsigned long		fsize;
>>>>>>>> 	u64			user_data;
>>>>>>>> 	u32			result;
>>>>>>>> +#ifdef CONFIG_BLK_DEV_ZONED
>>>>>>>> +	/* zone-relative offset for append, in bytes */
>>>>>>>> +	u32			append_offset;
>>>>>>>
>>>>>>> this can overflow. u64 is needed.
>>>>>>
>>>>>> We chose to do it this way to start with because struct io_uring_cqe
>>>>>> only has space for u32 when we reuse the flags.
>>>>>>
>>>>>> We can of course create a new cqe structure, but that will come with
>>>>>> larger changes to io_uring for supporting append.
>>>>>>
>>>>>> Do you believe this is a better approach?
>>>>>
>>>>> The problem is that zone size are 32 bits in the kernel, as a number
>>>>> of sectors.  So any device that has a zone size smaller or equal to
>>>>> 2^31 512B sectors can be accepted. Using a zone relative offset in
>>>>> bytes for returning zone append result is OK-ish, but to match the
>>>>> kernel supported range of possible zone size, you need 31+9 bits...
>>>>> 32 does not cut it.
>>>>
>>>> Agree. Our initial assumption was that u32 would cover current zone size
>>>> requirements, but if this is a no-go, we will take the longer path.
>>>
>>> Converting to u64 will require a new version of io_uring_cqe, where we
>>> extend at least 32 bits. I believe this will need a whole new allocation
>>> and probably ioctl().
>>>
>>> Is this an acceptable change for you? We will of course add support for
>>> liburing when we agree on the right way to do this.
>>
>> If you need 64-bit of return value, then it's not going to work. Even
>> with the existing patches, reusing cqe->flags isn't going to fly, as
>> it would conflict with eg doing zone append writes with automatic
>> buffer selection.
> 
> Buffer selection is for reads/recv kind of requests, but appends
> are writes. In theory they can co-exist using cqe->flags.

Yeah good point, since it's just writes, doesn't matter. But the other
point still stands, it could potentially conflict with other flags, but
I guess only to the extent where both flags would need extra storage in
->flags. So not a huge concern imho.


-- 
Jens Axboe