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

["Matias Bjørling" <mb@lightnvm.io>]

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.