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

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Message-ID: <20200621185207.m7535hzpm4ubrk4i@MacBook-Pro.localdomain>
Date:   Sun, 21 Jun 2020 20:52:07 +0200
From:   "javier.gonz@...sung.com" <javier@...igon.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Kanchan Joshi <joshi.k@...sung.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "bcrl@...ck.org" <bcrl@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "selvakuma.s1@...sung.com" <selvakuma.s1@...sung.com>,
        "nj.shetty@...sung.com" <nj.shetty@...sung.com>
Subject: Re: [PATCH 3/3] io_uring: add support for zone-append

On 19.06.2020 09:02, Jens Axboe wrote:
>On 6/19/20 8:59 AM, Pavel Begunkov wrote:
>> On 19/06/2020 17:15, Jens Axboe wrote:
>>> On 6/19/20 3:41 AM, javier.gonz@...sung.com wrote:
>>>> Jens,
>>>>
>>>> Would you have time to answer a question below in this thread?
>>>>
>>>> On 18.06.2020 11:11, javier.gonz@...sung.com wrote:
>>>>> On 18.06.2020 08:47, Damien Le Moal wrote:
>>>>>> On 2020/06/18 17:35, javier.gonz@...sung.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@...sung.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@...sung.com>
>>>>>>>>> Signed-off-by: Kanchan Joshi <joshi.k@...sung.com>
>>>>>>>>> Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
>>>>>>>>> Signed-off-by: Javier Gonzalez <javier.gonz@...sung.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.
>>>
>>> If you need 64-bit of return value, then it's not going to work. Even
>>> with the existing patches, reusing cqe->flags isn't going to fly, as
>>> it would conflict with eg doing zone append writes with automatic
>>> buffer selection.
>>
>> Buffer selection is for reads/recv kind of requests, but appends
>> are writes. In theory they can co-exist using cqe->flags.
>
>Yeah good point, since it's just writes, doesn't matter. But the other
>point still stands, it could potentially conflict with other flags, but
>I guess only to the extent where both flags would need extra storage in
>->flags. So not a huge concern imho.

Very good point Pavel!

If co-existing with the current flags is an option, I'll explore this
for the next version.

Thanks Jens and Pavel for the time and ideas!

Javier