Re: fall back from direct to buffered I/O when stable writes are required

hch
2025-11-06T12:49:05Z

On Thu, Nov 06, 2025 at 09:50:10AM +0000, Johannes Thumshirn wrote:
> On 11/5/25 10:44 PM, Darrick J. Wong wrote:
> > Just out of curiosity -- is qemu itself mutating the buffers that it is
> > passing down to the lower levels via dio? Or is it a program in the
> > guest that's mutating buffers that are submitted for dio, which then get
> > zerocopied all the way down to the hypervisor?
>
> If my memory serves me right it is the guest (or at least can be). I
> remember a bug report on btrfs where a Windows guest had messed up
> checksums because of modifying inflight I/O.

qemu passes I/O through, so yes it is guest controller. Windows is most
famous, but the Linux swap code can trigger it easily too.