Re: [PATCH v2] io_uring: provide fallback request for OOM situations

[Jens Axboe <axboe@kernel.dk>]

On 11/17/19 11:57 PM, Bob Liu wrote:
> On 11/9/19 5:25 AM, Jens Axboe wrote:
>> One thing that really sucks for userspace APIs is if the kernel passes
>> back -ENOMEM/-EAGAIN for resource shortages. The application really has
>> no idea of what to do in those cases. Should it try and reap
>> completions? Probably a good idea. Will it solve the issue? Who knows.
>>
>> This patch adds a simple fallback mechanism if we fail to allocate
>> memory for a request. If we fail allocating memory from the slab for a
>> request, we punt to a pre-allocated request. There's just one of these
>> per io_ring_ctx, but the important part is if we ever return -EBUSY to
>> the application, the applications knows that it can wait for events and
>> make forward progress when events have completed. This is the important
>> part.
>>
> 
> I'm lost how -EBUSY will be returned if allocating from the pre-allocated request.
> Could you please explain a bit more?

The patch actually returns -EAGAIN, not -EBUSY... The last -EBUSY
mention in that commit message should be -EAGAIN.

But the point is that if you get a busy return back, then you know that
things are moving forward as we have a backup request. This is a similar
concept to the mempools we have in the kernel, have any kind of reserve
guarantees forward progress.

-- 
Jens Axboe