Re: [PATCH 1/3] io_uring: don't use req->work.creds for inline requests

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi Pavel,

> On 26/05/2020 17:59, Xiaoguang Wang wrote:
>> hi,
>>
>>> On 26/05/2020 09:43, Xiaoguang Wang wrote:
>>>> In io_init_req(), if uers requires a new credentials, currently we'll
>>>> save it in req->work.creds, but indeed io_wq_work is designed to describe
>>>> needed running environment for requests that will go to io-wq, if one
>>>> request is going to be submitted inline, we'd better not touch io_wq_work.
>>>> Here add a new 'const struct cred *creds' in io_kiocb, if uers requires a
>>>> new credentials, inline requests can use it.
>>>>
>>>> This patch is also a preparation for later patch.
>>>
>>> What's the difference from keeping only one creds field in io_kiocb (i.e.
>>> req->work.creds), but handling it specially (i.e. always initialising)? It will
>>> be a lot easier than tossing it around.
>>>
>>> Also, the patch doubles {get,put}_creds() for sqe->personality case, and that's
>>> extra atomics without a good reason.
>> You're right, thanks.
>> The original motivation for this patch is that it's just a preparation later patch
>> "io_uring: avoid whole io_wq_work copy for inline requests", I can use
>> io_wq_work.func
>> to determine whether to drop io_wq_work in io_req_work_drop_env(), so if
>> io_wq_work.func
>> is NULL, I don't want io_wq_work has a valid creds.
>> I'll look into whether we can just assign req->creds's pointer to
>> io_wq_work.creds to
>> reduce the atomic operations.
> 
> See a comment for the [2/3], can spark some ideas.
> 
> It's a bit messy and makes it more difficult to keep in mind -- all that extra
> state (i.e. initialised or not) + caring whether func was already set. IMHO, the
> nop-test do not really justifies extra complexity, unless the whole stuff is
> pretty and clear. Can you benchmark something more realistic? at least
> reads/writes to null_blk (completion_nsec=0).
Indeed for this patch set, I also don't expect any obvious performance improvement,
just think current codes are not good, so try to improve it.
I will send a v2 version later, in which I'll use null_blk to evaluate performance,
please have a check.

Regards,
Xiaoguang Wang


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