Re: [PATCHSET v2 0/2] io_uring: handle short reads internally

Jens Axboe
2020-08-18T03:29:37Z

On 8/17/20 2:25 AM, Stefan Metzmacher wrote:
> Hi Jens,
>
>> Since we've had a few cases of applications not dealing with this
>> appopriately, I believe the safest course of action is to ensure that
>> we don't return short reads when we really don't have to.
>>
>> The first patch is just a prep patch that retains iov_iter state over
>> retries, while the second one actually enables just doing retries if
>> we get a short read back.
>>
>> This passes all my testing, both liburing regression tests but also
>> tests that explicitly trigger internal short reads and hence retry
>> based on current state. No short reads are passed back to the
>> application.
>
> Thanks! I was going to ask about exactly that :-)
>
> It wasn't clear why returning short reads were justified by resulting
> in better performance... As it means the application needs to do
> a lot more work and syscalls.

It mostly boils down to figuring out a good way to do it. With the
task_work based retry, the async buffered reads, we're almost there and
the prep patch adds the last remaining bits to retain the iov_iter state
across issues.

> Will this be backported?

I can, but not really in an efficient manner. It depends on the async
buffered work to make progress, and the task_work handling retry. The
latter means it's 5.7+, while the former is only in 5.9+...

We can make it work for earlier kernels by just using the thread offload
for that, and that may be worth doing. That would enable it in
5.7-stable and 5.8-stable. For that, you just need these two patches.
Patch 1 would work as-is, while patch 2 would need a small bit of
massaging since io_read() doesn't have the retry parts.

I'll give it a whirl just out of curiosity, then we can debate it after
that.

--
Jens Axboe