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

[Pavel Begunkov <asml.silence@gmail.com>]

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.
Also, can you allocate it if you switch a user (preferably to non-root) after it
happens?

> 
> So, I've just tried stopping everything that uses io-uring. No io_wq* processes
> remained:
> 
> $ ps ax | grep wq
>     9 ?        I<     0:00 [mm_percpu_wq]
>   243 ?        I<     0:00 [tpm_dev_wq]
>   246 ?        I<     0:00 [devfreq_wq]
> 27922 pts/4    S+     0:00 grep --colour=auto wq
> $
> 
> But not a single ring (with size 1024) can be created afterwards anyway.
> 
> Apparently the problem netty hit and this one are different?

-- 
Pavel Begunkov