Re: [PATCH v2] io_uring: fix short read slow path

[Stefan Hajnoczi]

On Fri, Jul 01, 2022 at 07:52:31AM +0900, Dominique Martinet wrote:
> Stefano Garzarella wrote on Thu, Jun 30, 2022 at 05:49:21PM +0200:
> > > so when we ask for more we issue an extra short reads, making sure we go
> > > through the two short reads path.
> > > (Unfortunately I wasn't quite sure what to fiddle with to issue short
> > > reads in the first place, I tried cutting one of the iovs short in
> > > luring_do_submit() but I must not have been doing it properly as I ended
> > > up with 0 return values which are handled by filling in with 0 (reads
> > > after eof) and that didn't work well)
> > 
> > Do you remember the kernel version where you first saw these problems?
> 
> Since you're quoting my paragraph about testing two short reads, I've
> never seen any that I know of; but there's also no reason these couldn't
> happen.
> 
> Single short reads have been happening for me with O_DIRECT (cache=none)
> on btrfs for a while, but unfortunately I cannot remember which was the
> first kernel I've seen this on -- I think rather than a kernel update it
> was due to file manipulations that made the file eligible for short
> reads in the first place (I started running deduplication on the backing
> file)
> 
> The older kernel I have installed right now is 5.16 and that can
> reproduce it --  I'll give my laptop some work over the weekend to test
> still maintained stable branches if that's useful.

Hi Dominique,
Linux 5.16 contains commit 9d93a3f5a0c ("io_uring: punt short reads to
async context"). The comment above QEMU's luring_resubmit_short_read()
claims that short reads are a bug that was fixed by Linux commit
9d93a3f5a0c.

If the comment is inaccurate it needs to be fixed. Maybe short writes
need to be handled too.

I have CCed Jens and the io_uring mailing list to clarify:
1. Are short IORING_OP_READV reads possible on files/block devices?
2. Are short IORING_OP_WRITEV writes possible on files/block devices?

Thanks,
Stefan

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