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Subject: Re: [PATCH 3/3] io_uring: add support for zone-append
To:     =?UTF-8?Q?Matias_Bj=c3=b8rling?= <mb@lightnvm.io>,
        "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: <ca513fd1-4ca2-c16e-8b99-73cbd7fe6290@kernel.dk>
Date:   Fri, 19 Jun 2020 08:18:24 -0600
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On 6/19/20 5:15 AM, Matias Bjørling wrote:
> On 19/06/2020 11.41, 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.
> 
> I took a quick look at the code. No expert, but why not use the existing 
> userdata variable? use the lowest bits (40 bits) for the Zone Starting 
> LBA, and use the highest (24 bits) as index into the completion data 
> structure?
> 
> If you want to pass the memory address (same as what fio does) for the 
> data structure used for completion, one may also play some tricks by 
> using a relative memory address to the data structure. For example, the 
> x86_64 architecture uses 48 address bits for its memory addresses. With 
> 24 bit, one can allocate the completion entries in a 32MB memory range, 
> and then use base_address + index to get back to the completion data 
> structure specified in the sqe.

For any current request, sqe->user_data is just provided back as
cqe->user_data. This would make these requests behave differently
from everything else in that sense, which seems very confusing to me
if I was an application writer.

But generally I do agree with you, there are lots of ways to make
< 64-bit work as a tag without losing anything or having to jump
through hoops to do so. The lack of consistency introduced by having
zone append work differently is ugly, though.

-- 
Jens Axboe