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

[Jens Axboe <axboe@kernel.dk>]

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:

current -git

axboe@r7625 ~> cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled
[always] inherit madvise never
axboe@r7625 ~> for i in 1 2 4 8 16; time ./ppage $i $i; end
register 1 GB, num threads 1

________________________________________________________
Executed in  178.91 millis    fish           external
   usr time    9.82 millis  313.00 micros    9.51 millis
   sys time  161.83 millis  149.00 micros  161.68 millis

register 2 GB, num threads 2

________________________________________________________
Executed in  638.49 millis    fish           external
   usr time    0.03 secs    285.00 micros    0.03 secs
   sys time    1.14 secs    135.00 micros    1.14 secs

register 4 GB, num threads 4

________________________________________________________
Executed in    2.17 secs    fish           external
   usr time    0.05 secs  314.00 micros    0.05 secs
   sys time    6.31 secs  150.00 micros    6.31 secs

register 8 GB, num threads 8

________________________________________________________
Executed in    4.97 secs    fish           external
   usr time    0.12 secs  299.00 micros    0.12 secs
   sys time   28.97 secs  142.00 micros   28.97 secs

register 16 GB, num threads 16

________________________________________________________
Executed in   10.34 secs    fish           external
   usr time    0.20 secs  294.00 micros    0.20 secs
   sys time  126.42 secs  140.00 micros  126.42 secs


-git + user_struct xarray for accounting

axboe@r7625 ~> cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled
[always] inherit madvise never
axboe@r7625 ~> for i in 1 2 4 8 16; time ./ppage $i $i; end
register 1 GB, num threads 1

________________________________________________________
Executed in   54.05 millis    fish           external
   usr time   10.66 millis  327.00 micros   10.34 millis
   sys time   41.60 millis  259.00 micros   41.34 millis

register 2 GB, num threads 2

________________________________________________________
Executed in  105.70 millis    fish           external
   usr time   34.38 millis  206.00 micros   34.17 millis
   sys time   68.55 millis  206.00 micros   68.35 millis

register 4 GB, num threads 4

________________________________________________________
Executed in  214.72 millis    fish           external
   usr time   48.10 millis  193.00 micros   47.91 millis
   sys time  182.25 millis  193.00 micros  182.06 millis

register 8 GB, num threads 8

________________________________________________________
Executed in  441.96 millis    fish           external
   usr time  123.26 millis  195.00 micros  123.07 millis
   sys time  568.20 millis  195.00 micros  568.00 millis

register 16 GB, num threads 16

________________________________________________________
Executed in  917.70 millis    fish           external
   usr time    0.17 secs    202.00 micros    0.17 secs
   sys time    2.48 secs    202.00 micros    2.48 secs


-- 
Jens Axboe