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

[Stefan Hajnoczi]

On Thu, Jun 30, 2022 at 10:01:37AM +0900, Dominique Martinet wrote:
> sqeq.off here is the offset to read within the disk image, so obviously
> not 'nread' (the amount we just read), but as the author meant to write
> its current value incremented by the amount we just read.
> 
> Normally recent versions of linux will not issue short reads,
> but it can happen so we should fix this.
> 
> This lead to weird image corruptions when short read happened
> 
> Fixes: 6663a0a33764 ("block/io_uring: implements interfaces for io_uring")
> Link: https://lkml.kernel.org/r/YrrFGO4A1jS0GI0G@atmark-techno.com
> Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
> ---
> v1 -> v2: also updated total_read to use += as suggested by Kevin,
> thank you!
> 
> I've tested this quickly by making short reads "recursives", e.g. added
> the following to luring_resubmit_short_read() after setting 'remaining':
>     if (remaining > 4096) remaining -= 4096;
> 
> 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)
> 
> Anyway, this looks OK to me now.
> 
> Thanks,
> Dominique
> 
>  block/io_uring.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Thanks, applied to my block tree:
https://gitlab.com/stefanha/qemu/commits/block

Stefan

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