Re: mmap on 9p broken in 6.4

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Thursday, July 13, 2023 11:51:24 AM CEST Eric Van Hensbergen wrote:
> I’m traveling so haven’t been able to dig in, but something funny going
> on.  I would have expected ignoreqv to not have any impact in default cache
> mode (nocache) - so there’s a bug in there somewhere.  in this case its
> read mmap thats failing and that should work without cache=mmap (which is
> only required for writable mmaps) - I did do bunch of reorganization in
> this code path, I’ll try to eyeball the bug when I get back from travel but
> sounds like I also need to
> integrate a test path with a server that doesn't populate qid.vers so this
> case is covered by our regressions.

Or a runtime warning in general if server always just sends qid.version zero.

About QEMU: unless the (actually unrelated) option `multidevs=remap` option is
used, Rreaddir would return all directory entries with qid.version set to
zero. So that's a bug on QEMU side:

https://github.com/qemu/qemu/blob/71d72ececa086114df80fe4cc04d701b59002eb2/hw/9pfs/9p.c#L2534

I try to send a patch for this next week, such that qid.version is always
filled, no matter what the QEMU command line options are. In the meantime
please use `multidevs=remap` as workaround for testing.

- Christian

> On Thu, Jul 13, 2023 at 8:50 AM Christian Schoenebeck <
> linux_oss@crudebyte.com> wrote:
> 
> > On Thursday, July 13, 2023 12:15:29 AM CEST Robert Schwebel wrote:
> > > On Wed, Jul 12, 2023 at 05:57:40PM +0200, Christian Schoenebeck wrote:
> > > > > 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
> > >
> > > Unfortunately not:
> > >
> > > + exec ./platform-v7a/sysroot-host/bin/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 vi
> > > rtio-rng-device,rng=rng0 -device virtio-scsi-device,id=scsi -fsdev
> > local,id=rootfs,path=./platform-v7a/root,security_model=mapped-file,multidevs=remap
> > -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'
> >
> > I don't see any cache option here. I would expect at least cache=mmap here:
> >
> > https://wiki.qemu.org/Documentation/9p_root_fs#Boot_the_9p_Root_FS_System
> >
> > By default it's cache=none, which means no mmap() would be allowed either.
> >
> > > [    0.000000] L2C: platform modifies aux control register: 0x02020000
> > -> 0x02420000
> > > [    0.000000] L2C: DT/platform modifies aux control register:
> > 0x02020000 -> 0x02420000
> > > [    0.004806] smp_twd: clock not found -2
> > > [    1.069767] simple-pm-bus bus@40000000:motherboard-bus@40000000
> > :iofpga@7,00000000: Failed to create device link (0x180) with dcc:tcrefclk
> > > [    1.085798] simple-pm-bus bus@40000000:motherboard-bus@40000000
> > :iofpga@7,00000000: Failed to create device link (0x180) with dcc:tcrefclk
> > > [    1.153420] physmap-flash 48000000.psram: map_probe failed
> > > [    1.535013] 9pnet_virtio: no channels available for device root
> > >
> > >  ____                        _                   _
> > > |  _ \ ___ _ __   __ _ _   _| |_ _ __ ___  _ __ (_)_  __
> > > | |_) / _ \ '_ \ / _` | | | | __| '__/ _ \| '_ \| \ \/ /
> > > |  __/  __/ | | | (_| | |_| | |_| | | (_) | | | | |>  <
> > > |_|   \___|_| |_|\__, |\__,_|\__|_|  \___/|_| |_|_/_/\_\
> > >                  |___/
> > >
> > >  ____  _     _             _  ___ _
> > > |  _ \(_)___| |_ _ __ ___ | |/ (_) |_
> > > | | | | / __| __| '__/ _ \| ' /| | __|
> > > | |_| | \__ \ |_| | | (_) | . \| | |_
> > > |____/|_|___/\__|_|  \___/|_|\_\_|\__|
> > >
> > >
> > > OSELAS(R)-DistroKit-2019.12.0-00429-g57ffae760eb9 /
> > v7a-2019.12.0-00429-g57ffae760eb9
> > > ptxdist-2023.07.1/2023-07-11T19:56:50+0200
> > >
> > > DistroKit login: root
> > > root@DistroKit:~ mount / -o remount,rw
> > > root@DistroKit:~ ldconfig
> > > ldconfig: Cannot mmap file /lib/libgcc_s.so.1.
> > >
> > > ldconfig: Cannot mmap file /lib/libstdc++.so.
> >
> > Again, effective 9p mount options are always useful, e.g. `mount | grep 9p`
> >
> > >
> > > [...]
> > >
> > > rsc
> > >
> >
> >
> >
>