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Date:   Sat, 22 Aug 2020 20:14:19 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Bart Van Assche <bvanassche@....org>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        paolo.valente@...aro.org
Cc:     linux-block@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: IOPRIO_CLASS_RT without CAP_SYS_ADMIN?

On 8/22/20 7:58 PM, Bart Van Assche wrote:
> On 2020-08-20 17:35, Khazhismel Kumykov wrote:
>> It'd be nice to allow a process to send RT requests without granting
>> it the wide capabilities of CAP_SYS_ADMIN, and we already have a
>> capability which seems to almost fit this priority idea -
>> CAP_SYS_NICE? Would this fit there?
>>
>> Being capable of setting IO priorities on per request or per thread
>> basis (be it async submission or w/ thread ioprio_set) is useful
>> especially when the userspace has its own prioritization/scheduling
>> before hitting the kernel, allowing us to signal to the kernel how to
>> order certain IOs, and it'd be nice to separate this from ADMIN for
>> non-root processes, in a way that's less error prone than e.g. having
>> a trusted launcher ionice the process and then drop priorities for
>> everything but prio requests.
> 
> Hi Khazhy,
> 
> In include/uapi/linux/capability.h I found the following:
> 
> /* Allow raising priority and setting priority on other (different
>    UID) processes */
> /* Allow use of FIFO and round-robin (realtime) scheduling on own
>    processes and setting the scheduling algorithm used by another
>    process. */
> /* Allow setting cpu affinity on other processes */
> #define CAP_SYS_NICE         23
> 
> If it is acceptable that every process that has permission to submit
> IOPRIO_CLASS_RT I/O also has permission to modify the priority of
> other processes then extending CAP_SYS_NICE is an option. Another
> possibility is to extend the block cgroup controller such that the
> capability to submit IOPRIO_CLASS_RT I/O can be enabled through the
> cgroup interface. There may be other approaches. I'm not sure what
> the best approach is.

I think CAP_SYS_NICE fits pretty nicely, and I was actually planning on
using that for the io_uring SQPOLL side as well. So there is/will be
some precedent for tying it into IO related things, too. For this use
case, I think it's perfect.

-- 
Jens Axboe

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