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

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

On 22/12/2020 11:04, Dmitry Kadashev wrote:
> On Tue, Dec 22, 2020 at 11:11 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
[...]
>>> 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.
> 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

Then we screw that specific user. Interestingly, if it has CAP_IPC_LOCK
capability we don't even account locked memory.

btw, do you use registered buffers?

> 
> # 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
> 

-- 
Pavel Begunkov