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

Jens Axboe
2020-08-18T15:23:35Z

On 8/18/20 7:53 AM, Jens Axboe wrote:
> 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!

Ran these through the liburing regression testing as well, and found a
case where 'ret2' isn't initialized. So pushed out new branches. The
correct sha for testing should be:

5.7-stable: a451911d530075352fbc7ef9bb2df68145a747ad
5.8-stable: b101e651782a60eb1e96b64e523e51358b77f94f

--
Jens Axboe