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

[Anoop C S <anoopcs@cryptolab.net>]

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).

Thanks,
Anoop C S.