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

Jens Axboe
2026-04-21T16:21:06Z

On 4/21/26 10:05 AM, Jens Axboe wrote:
> 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 ;-)

What is fix-paddr.py? It's referenced in the build script.

--
Jens Axboe