Re: [PATCH] io_uring: do not charge the SQ/CQ rings to RLIMIT_MEMLOCK

[David Wei <dw@davidwei.uk>]

On 2026-10-06 16:59, Jens Axboe wrote:
> On 10/6/26 6:57 AM, hengyul@cs.unc.edu wrote:
>> From: Hengyu Liang <hengyul@cs.unc.edu>
>>
>> Commit 8078486e1d53 ("io_uring: use region api for SQ") and commit
>> 81a4058e0cd0 ("io_uring: use region api for CQ") made io_uring_setup()
>> allocate the rings with io_create_region().
>>
>> However, io_create_region() charges the memory to RLIMIT_MEMLOCK, and
>> the rings had been exempt from that limit since commit 26bfa89e25f4
>> ("io_uring: place ring SQ/CQ arrays under memcg memory limits"). As of
>> now, a user without CAP_IPC_LOCK gets ENOMEM from io_uring_setup() when
>> their rings exceed the limit, which is 8 MiB by default. PostgreSQL
>> developers have already hit this in their io_uring tests [1].
>>
>> The issue can be reproduced with a simple liburing program, run as an
>> unprivileged user:
>>
>>      #include <liburing.h>
>>      #include <stdio.h>
>>
>>      int main(void)
>>      {
>>              static struct io_uring ring[64];
>>              int i;
>>
>>              for (i = 0; i < 64; i++)
>>                      if (io_uring_queue_init(4096, &ring[i], 0) < 0)
>>                              break;
>>              printf("%d rings\n", i);
>>              return 0;
>>      }
>>
>> Before those commits (v6.13), it prints "64 rings". After those commits
>> (v6.14), it prints "21 rings".
>>
>> This patch makes io_create_region() take the user to charge, and passes
>> no user for the SQ/CQ rings.
> 
> Agree that this is a bug, stricter accounting may break use cases.
> However, I think we can solve this simpler, and actually kill more code.
> How about something like the below instead? Only apply accounting to
> user backed memory, which is how it used to work too. Would be great if
> you could take a look and also run your test case against it.

Tested in a VM and confirmed that with the patch below the reproducer
correctly allocates all 64 w/ a 8 MB RLIMIT_MEMLOCK.

This change is really helpful for me as well, thank you for addressing
this Hengyu.