Re: EIO with io_uring O_DIRECT writes on ext4

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Tue, Jul 23, 2019 at 09:20:05AM -0600, Jens Axboe wrote:
> 
> I actually think it's XFS that's broken here, it's not passing down
> the IOCB_NOWAIT -> IOMAP_NOWAIT -> REQ_NOWAIT. This means we lose that
> important request bit, and we just block instead of triggering the
> not_supported case.
> 
> Outside of that, that case needs similar treatment to what I did for
> the EAGAIN case here:
> 
> http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=893a1c97205a3ece0cbb3f571a3b972080f3b4c7
> 
> It was a big mistake to pass back these values in an async fashion,
> and it also means that some accounting in other drivers are broken
> as we can get completions without the bio actually being submitted.

Hmmm, I had been trying to track down a similar case with virtio-scsi
on top of LVM, using a Google Compute Engine VM.  In that case,
xfstests generic/471 was failing with EIO, when it would pass just
*fine* when I was using KVM with a virtio-scsi device w/o LVM.

So it sounds like that what's going on is if the device driver (or
LVM, or anything else in the storage stack) needs to do a blocking
memory allocation, and NOWAIT is requested, we'll end up returning EIO
because an asynchronous error is getting reported, where as if we
could return it synchronously, the file system could properly return
EOPNOTSUP.  Am I understanding you correctly?

I guess there's a separate question hiding here, which is whether
there's a way to allow dm-linear or dm-crypt to handle NOWAIT requests
without needing to block.

					- Ted