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Date:   Sun, 21 Jun 2020 20:55:05 +0200
From:   "javier.gonz@samsung.com" <javier@javigon.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Matias =?utf-8?B?QmrDuHJsaW5n?= <mb@lightnvm.io>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        Kanchan Joshi <joshi.k@samsung.com>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
        "bcrl@kvack.org" <bcrl@kvack.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "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>
Subject: Re: [PATCH 3/3] io_uring: add support for zone-append
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On 19.06.2020 09:44, Jens Axboe wrote:
>On 6/19/20 9:40 AM, Matias Bjørling wrote:
>> On 19/06/2020 17.20, Jens Axboe wrote:
>>> On 6/19/20 9:14 AM, Matias Bjørling wrote:
>>>> On 19/06/2020 16.18, Jens Axboe wrote:
>>>>> 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.
>>>>>
>>>> Yep, agree, and extending to three cachelines is big no-go. We could add
>>>> a flag that said the kernel has changes the userdata variable. That'll
>>>> make it very explicit.
>>> Don't like that either, as it doesn't really change the fact that you're
>>> now doing something very different with the user_data field, which is
>>> just supposed to be passed in/out directly. Adding a random flag to
>>> signal this behavior isn't very explicit either, imho. It's still some
>>> out-of-band (ish) notification of behavior that is different from any
>>> other command. This is very different from having a flag that says
>>> "there's extra information in this other field", which is much cleaner.
>>>
>> Ok. Then it's pulling in the bits from cqe->res and cqe->flags that you
>> mention in the other mail. Sounds good.
>
>I think that's the best approach, if we need > 32-bits. Maybe we can get
>by just using ->res, if we switch to multiples of 512b instead for the
>result like Pavel suggested. That'd provide enough room in ->res, and
>would be preferable imho. But if we do need > 32-bits, then we can use
>this approach.

Sounds good.

Thanks Matias too for chipping in with more ideas. We have enough for a
v2.

Javier