Re: [PATCH 3/3] io_uring: add support for zone-append

["Matias Bjørling" <mb@lightnvm.io>]

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.