Re: [PATCH] io_uring: take page references for NOMMU pbuf_ring mmaps

Jens Axboe
2026-04-21T16:05:58Z

On 4/21/26 10:01 AM, Greg Kroah-Hartman wrote:
> On Tue, Apr 21, 2026 at 03:55:38PM +0200, Greg Kroah-Hartman wrote:
>> On Tue, Apr 21, 2026 at 07:50:32AM -0600, Jens Axboe wrote:
>>> On 4/21/26 7:46 AM, Greg Kroah-Hartman wrote:
>>>> Note, I have no way of testing this, I'm only forwarding this on because
>>>> I got the bug report and was able to generate something that "seems"
>>>
>>> AI bug report I presume? Because I can't imagine anyone ever attempted
>>> to run this.
>>
>> Yes, I got a bunch of "non-mmu" bug reports, which is a bit odd but I
>> guess you can do that with qemu these days? I should dig into that,
>> maybe that way I can test this and get a reproducer for you. If not,
>> let's just bin the thing.
>>
>>>> correct, but it might be a total load of crap here, my knowledge of the
>>>> vm layer is very low so take this for where it is coming from (i.e. a
>>>> non-deterministic pattern matching system.)
>>>>
>>>> I do have another patch that just disables io_uring for !MMU systems, if
>>>> you want that instead? Or is this feature something that !MMU devices
>>>> actually care about?
>>>
>>> I mean, who really cares about !MMU in the first place, we should just
>>> kill that off with a passion.
>>>
>>> Let me take a closer look at this and bounce it past some vm people, my
>>> nommu knowledge is close to zero as it's never been relevant in my
>>> professional life time. Which is saying something...
>>
>> Let me try to get a reproducer going first, let's not waste any more
>> human time on this just yet, sorry for sending this out without that
>> done first...
>
> Ok, attached is a poc.c and a script to run it. If you run this on a
> 7.0 kernel today, it "should" crash. and then if you apply the patch it
> doesn't (or at least that's what happened in my testing.)
>
> Note, I have run this locally, and it seems to work, but be careful, I
> can't guarantee anything, it does seem quite odd in that it "crashes"
> the kernel with a sysrq call to show "proof". Although that is a cool
> trick, I need to remember that...

I'll try and run a nommu qemu and see what pops out on my end. What a
waste of time for a nothing burger ;-)

--
Jens Axboe