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

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

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;
}


> 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