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

Jens Axboe
2020-08-18T14:54:05Z

On 8/18/20 7:49 AM, Anoop C S wrote:
> On Tue, 2020-08-18 at 07:44 -0700, Jens Axboe wrote:
>> On 8/18/20 12:40 AM, Stefan Metzmacher wrote:
>>> Hi Jens,
>>>
>>>>>>> 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.
>>>>>
>>>>> Here are the two patches against latest 5.7-stable (the rc
>>>>> branch, as
>>>>> we had quite a few queued up after 5.9-rc1). Totally untested,
>>>>> just
>>>>> wanted to see if it was doable.
>>>>>
>>>>> First patch is mostly just applied, with various bits removed
>>>>> that we
>>>>> don't have in 5.7. The second patch just does -EAGAIN punt for
>>>>> the
>>>>> short read case, which will queue the remainder with io-wq for
>>>>> async execution.
>>>>>
>>>>> Obviously needs quite a bit of testing before it can go
>>>>> anywhere else,
>>>>> but wanted to throw this out there in case you were interested
>>>>> in
>>>>> giving it a go...
>>>>
>>>> Actually passes basic testing, and doesn't return short reads. So
>>>> at
>>>> least it's not half bad, and it should be safe for you to test.
>>>>
>>>> I quickly looked at 5.8 as well, and the good news is that the
>>>> same
>>>> patches will apply there without changes.
>>>
>>> Thanks, but I was just curios and I currently don't have the
>>> environment to test, sorry.
>>>
>>> Anoop: you helped a lot reproducing the problem with 5.6, would you
>>> be able to
>>> test the kernel patches against 5.7 or 5.8, while reverting the
>>> samba patches?
>>> See
>>> https://lore.kernel.org/io-uring/e22220a8-669a-d302-f454-03a35c9582b4@kernel.dk/T/#t
>>> for the
>>> whole discussion?
>>
>> I'm actually not too worried about the short reads not working, it'll
>> naturally fall out correctly if the rest of the path is sane. The
>> latter
>> is what I'd be worried about! I ran some synthetic testing and
>> haven't
>> seen any issues so far, so maybe (just maybe) it's actually good.
>>
>> I can setup two branches with the 5.7-stable + patches and 5.8-stable
>> +
>> patches if that helps facilitate testing?
>
> That would be great.
>
> I took those two patches and tried to apply on top of 5.7.y. I had to
> manually resolve very few conflicts. I am not sure whether it is OK or
> not to test such a patched version(because of conflicts).

I pushed out two branches:

5.8-stable: current 5.8-stable rc queue + the three patches for this
5.7-stable: 5.7 ditto

So pick which one you want to use, and then pull it.

git://git.kernel.dk/linux-block 5.8-stable

git://git.kernel.dk/linux-block 5.7-stable

Hope that helps!

--
Jens Axboe