Re: "Cannot allocate memory" on ring creation (not RLIMIT_MEMLOCK)

[Dmitry Kadashev <dkadashev@gmail.com>]

On Tue, Dec 22, 2020 at 11:11 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 22/12/2020 03:35, Pavel Begunkov wrote:
> > On 21/12/2020 11:00, Dmitry Kadashev wrote:
> > [snip]
> >>> We do not share rings between processes. Our rings are accessible from different
> >>> threads (under locks), but nothing fancy.
> >>>
> >>>> In other words, if you kill all your io_uring applications, does it
> >>>> go back to normal?
> >>>
> >>> I'm pretty sure it does not, the only fix is to reboot the box. But I'll find an
> >>> affected box and double check just in case.
> >
> > I can't spot any misaccounting, but I wonder if it can be that your memory is
> > getting fragmented enough to be unable make an allocation of 16 __contiguous__
> > pages, i.e. sizeof(sqe) * 1024
> >
> > That's how it's allocated internally:
> >
> > static void *io_mem_alloc(size_t size)
> > {
> >       gfp_t gfp_flags = GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN | __GFP_COMP |
> >                               __GFP_NORETRY;
> >
> >       return (void *) __get_free_pages(gfp_flags, get_order(size));
> > }
> >
> > What about smaller rings? Can you check io_uring of what SQ size it can allocate?
> > That can be a different program, e.g. modify a bit liburing/test/nop.
>
> Even better to allocate N smaller rings, where N = 1024 / SQ_size
>
> static int try_size(int sq_size)
> {
>         int ret = 0, i, n = 1024 / sq_size;
>         static struct io_uring rings[128];
>
>         for (i = 0; i < n; ++i) {
>                 if (io_uring_queue_init(sq_size, &rings[i], 0) < 0) {
>                         ret = -1;
>                         break;
>                 }
>         }
>         for (i -= 1; i >= 0; i--)
>                 io_uring_queue_exit(&rings[i]);
>         return ret;
> }
>
> int main()
> {
>         int size;
>
>         for (size = 1024; size >= 2; size /= 2) {
>                 if (!try_size(size)) {
>                         printf("max size %i\n", size);
>                         return 0;
>                 }
>         }
>
>         printf("can't allocate %i\n", size);
>         return 0;
> }

Unfortunately I've rebooted the box I've used for tests yesterday, so I can't
try this there. Also I was not able to come up with an isolated reproducer for
this yet.

The good news is I've found a relatively easy way to provoke this on a test VM
using our software. Our app runs with "admin" user perms (plus some
capabilities), it bumps RLIMIT_MEMLOCK to infinity on start. I've also created
an user called 'ioutest' to run the check for ring sizes using a different user.

I've modified the test program slightly, to show the number of rings
successfully
created on each iteration and the actual error message (to debug a problem I was
having with it, but I've kept this after that). Here is the output:

# sudo -u admin bash -c 'ulimit -a' | grep locked
max locked memory       (kbytes, -l) 1024

# sudo -u ioutest bash -c 'ulimit -a' | grep locked
max locked memory       (kbytes, -l) 1024

# sudo -u admin ./iou-test1
Failed after 0 rings with 1024 size: Cannot allocate memory
Failed after 0 rings with 512 size: Cannot allocate memory
Failed after 0 rings with 256 size: Cannot allocate memory
Failed after 0 rings with 128 size: Cannot allocate memory
Failed after 0 rings with 64 size: Cannot allocate memory
Failed after 0 rings with 32 size: Cannot allocate memory
Failed after 0 rings with 16 size: Cannot allocate memory
Failed after 0 rings with 8 size: Cannot allocate memory
Failed after 0 rings with 4 size: Cannot allocate memory
Failed after 0 rings with 2 size: Cannot allocate memory
can't allocate 1

# sudo -u ioutest ./iou-test1
max size 1024

# ps ax | grep wq
    8 ?        I<     0:00 [mm_percpu_wq]
  121 ?        I<     0:00 [tpm_dev_wq]
  124 ?        I<     0:00 [devfreq_wq]
20593 pts/1    S+     0:00 grep --color=auto wq

-- 
Dmitry Kadashev