Re: [PATCH] mm: io_uring: allow oom-killer from io_uring_setup

[Shakeel Butt <shakeelb@google.com>]

On Tue, Jan 25, 2022 at 10:35 AM David Rientjes <rientjes@google.com> wrote:
>
> On Mon, 24 Jan 2022, Shakeel Butt wrote:
>
> > On an overcommitted system which is running multiple workloads of
> > varying priorities, it is preferred to trigger an oom-killer to kill a
> > low priority workload than to let the high priority workload receiving
> > ENOMEMs. On our memory overcommitted systems, we are seeing a lot of
> > ENOMEMs instead of oom-kills because io_uring_setup callchain is using
> > __GFP_NORETRY gfp flag which avoids the oom-killer. Let's remove it and
> > allow the oom-killer to kill a lower priority job.
> >
>
> What is the size of the allocations that io_mem_alloc() is doing?
>
> If get_order(size) > PAGE_ALLOC_COSTLY_ORDER, then this will fail even
> without the __GFP_NORETRY.  To make the guarantee that workloads are not
> receiving ENOMEM, it seems like we'd need to guarantee that allocations
> going through io_mem_alloc() are sufficiently small.
>
> (And if we're really serious about it, then even something like a
> BUILD_BUG_ON().)
>

The test case provided to me for which the user was seeing ENOMEMs was
io_uring_setup() with 64 entries (nothing else).

If I understand rings_size() calculations correctly then the 0 order
allocation was requested in io_mem_alloc().

For order > PAGE_ALLOC_COSTLY_ORDER, maybe we can use
__GFP_RETRY_MAYFAIL. It will at least do more aggressive reclaim
though I think that is a separate discussion. For this issue, we are
seeing ENOMEMs even for order 0 allocations.