Re: mmap on 9p broken in 6.4

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Wednesday, July 12, 2023 4:14:02 PM CEST Robert Schwebel wrote:
> Christian,
> 
> On Wed, Jul 12, 2023 at 11:01:53AM +0200, Christian Schoenebeck wrote:
> > > TL;DR: mmap() seems to be broken on 9pfs on Linux 6.4. setting
> > > "rootflags=ignoreqv" fixes it as well, but it feels like a regression.
> > 
> > What's the precise set of 9p mount options on guest, which 9p server is used
> > and what are 9p server's options respectively?
> 
> The server is qemu, started like this:
> 
> qemu-system-arm -machine vexpress-a9 -cpu cortex-a9 -smp 4 -m 1024M -nographic -no-reboot -net nic,netdev=net1 -netdev user,id=net1,hostfwd=tcp:127.0.0.1:27737-:22,hostfwd=tcp:127.0.0.1:12345-:12345 -rtc base=2000-01-01 -object rng-random,filename=/dev/random,id=rng0 -device virtio-rng-device,rng=rng0 -device virtio-scsi-device,id=scsi -fsdev local,id=rootfs,path=./platform-v7a/root,security_model=mapped-file -device virtio-9p-device,fsdev=rootfs,mount_tag=/dev/root -kernel ./platform-v7a/images/linuximage -dtb ./platform-v7a/images/vexpress-v2p-ca9.dtb -append 'root=/dev/root rootfstype=9p rootflags=trans=virtio console=ttyAMA0,115200 loglevel=5 systemd.show_status=auto'
> 
> This is qemu 8.0.2.

Does adding QEMU option "multidevs=remap" [1] resolve it?

[1] https://wiki.qemu.org/Documentation/9psetup#Starting_the_Guest_directly

> > > I could track down the breakage to
> > > 
> > >   1543b4c5071c54d76aad7a7a26a6e43082269b0c
> > 
> > This patch is sensitive about 9p server transmitting a meaningful value for
> > file's QID version field. That's why more info about deployed 9p server would
> > help.
> 
> rsc
>