Re: [PATCH v2] io_uring/rsrc: fix RLIMIT_MEMLOCK bypass by removing cross-buffer accounting

Jens Axboe
2026-01-23T15:04:51Z

On 1/23/26 7:50 AM, Jens Axboe wrote:
> On 1/23/26 7:26 AM, Pavel Begunkov wrote:
>> On 1/22/26 21:51, Pavel Begunkov wrote:
>> ...
>>>>>> I already briefly touched on that earlier, for sure not going to be of
>>>>>> any practical concern.
>>>>>
>>>>> Modest 16 GB can give 1M entries. Assuming 50ns-100ns per entry for the
>>>>> xarray business, that's 50-100ms. It's all serialised, so multiply by
>>>>> the number of CPUs/threads, e.g. 10-100, that's 0.5-10s. Account sky
>>>>> high spinlock contention, and it jumps again, and there can be more
>>>>> memory / CPUs / numa nodes. Not saying that it's worse than the
>>>>> current O(n^2), I have a test program that borderline hangs the
>>>>> system.
>>>>
>>>> It's definitely not worse than the existing system, which is why I don't
>>>> think it's a big deal. Nobody has ever complained about time to register
>>>> buffers. It's inherently a slow path, and quite slow at that depending
>>>> on the use case. Out of curiosity, I ran some stilly testing on
>>>> registering 16GB of memory, with 1..32 threads. Each will do 16GB, so
>>>> 512GB registered in total for the 32 case. Before is the current kernel,
>>>> after is with per-user xarray accounting:
>>>>
>>>> before
>>>>
>>>> nthreads 1: 646 msec
>>>> nthreads 2: 888 msec
>>>> nthreads 4: 864 msec
>>>> nthreads 8: 1450 msec
>>>> nthreads 16: 2890 msec
>>>> nthreads 32: 4410 msec
>>>>
>>>> after
>>>>
>>>> nthreads 1: 650 msec
>>>> nthreads 2: 888 msec
>>>> nthreads 4: 892 msec
>>>> nthreads 8: 1270 msec
>>>> nthreads 16: 2430 msec
>>>> nthreads 32: 4160 msec
>>>>
>>>> This includes both registering buffers, cloning all of them to another
>>>> ring, and unregistering times, and nowhere is locking scalability an
>>>> issue for the xarray manipulation. The box has 32 nodes and 512 CPUs. So
>>>> no, I strongly believe this isn't an issue.
>>>>
>>>> IOW, accurate accounting is cheaper than the stuff we have now. None of
>>>> them are super cheap. Does it matter? I really don't think so, or people
>>>> would've complained already. The only complaint I got on these kinds of
>>>> things was for cloning, which did get fixed up some releases ago.
>>>
>>> You need compound pages
>>>
>>> always > /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled
>>>
>>> And use update() instead of register() as accounting dedup for
>>> registration is broken-disabled. For the current kernel:
>>>
>>> Single threaded:
>>> 1x1G: 7.5s
>>> 2x1G: 45s
>>> 4x1G: 190s
>>>
>>> 16x should be ~3000s, not going to run it. Uninterruptible and no
>>> cond_resched, so spawn NR_CPUS threads and the system is completely
>>> unresponsive (I guess it depends on the preemption mode).
>> The program is below for reference, but it's trivial. THP setting
>> is done inside for convenience. There are ways to make the runtime
>> even worse, but that should be enough.
>
> Thanks for sending that. Ran it on the same box, on current -git and
> with user_struct xarray accounting. Modified it so that 2nd arg is
> number of threads, for easy running:

Should've tried 32x32 as well, that ends up going deep into "this sucks"
territory:

git

good luck

git + user_struct

axboe@r7625 ~> time ./ppage 32 32
register 32 GB, num threads 32

________________________________________________________
Executed in 16.34 secs fish external
usr time 0.54 secs 497.00 micros 0.54 secs
sys time 451.94 secs 55.00 micros 451.94 secs

--
Jens Axboe