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

["Matias Bjørling" <mb@lightnvm.io>]

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.

Best, Matias