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

[Dmitry Kadashev <dkadashev@gmail.com>]

On Tue, Dec 22, 2020 at 6:04 PM Dmitry Kadashev <dkadashev@gmail.com> wrote:
>
> 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

This was on kernel 5.6.7, I'm going to try this on 5.10.1 now.

-- 
Dmitry Kadashev