On Tue, Feb 23, 2021 at 02:39:48PM +0100, Christian Schoenebeck wrote: > On Montag, 22. Februar 2021 18:11:59 CET Greg Kurz wrote: > > On Mon, 22 Feb 2021 16:08:04 +0100 > > Christian Schoenebeck <qemu_...@crudebyte.com> wrote: > > > > [...] > > > > > I did not ever have a kernel crash when I boot a Linux guest with a 9pfs > > > root fs and 100 MiB msize. > > > > Interesting. > > > > > Should we ask virtio or 9p Linux client maintainers if > > > they can add some info what this is about? > > > > Probably worth to try that first, even if I'm not sure anyone has a > > answer for that since all the people who worked on virtio-9p at > > the time have somehow deserted the project. > > Michael, Dominique, > > we are wondering here about the message size limitation of just 5 kiB in the > 9p Linux client (using virtio transport) which imposes a performance > bottleneck, introduced by this kernel commit: > > commit b49d8b5d7007a673796f3f99688b46931293873e > Author: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com> > Date: Wed Aug 17 16:56:04 2011 +0000 > > net/9p: Fix kernel crash with msize 512K > > With msize equal to 512K (PAGE_SIZE * VIRTQUEUE_NUM), we hit multiple > crashes. This patch fix those. > > Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com> > Signed-off-by: Eric Van Hensbergen <eri...@gmail.com>
Well the change I see is: - .maxsize = PAGE_SIZE*VIRTQUEUE_NUM, + .maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3), so how come you say it changes 512K to 5K? Looks more like 500K to me. > Is this a fundamental maximum message size that cannot be exceeded with > virtio > in general or is there another reason for this limit that still applies? > > Full discussion: > https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg06343.html > > > > > > As the kernel code sais trans_mod->maxsize, maybe its something in > > > > > virtio > > > > > on qemu side that does an automatic step back for some reason. I don't > > > > > see something in the 9pfs virtio transport driver > > > > > (hw/9pfs/virtio-9p-device.c on QEMU side) that would do this, so I > > > > > would > > > > > also need to dig deeper. > > > > > > > > > > Do you have some RAM limitation in your setup somewhere? > > > > > > > > > > For comparison, this is how I started the VM: > > > > > > > > > > ~/git/qemu/build/qemu-system-x86_64 \ > > > > > -machine pc,accel=kvm,usb=off,dump-guest-core=off -m 2048 \ > > > > > -smp 4,sockets=4,cores=1,threads=1 -rtc base=utc \ > > > > > -boot strict=on -kernel > > > > > /home/bee/vm/stretch/boot/vmlinuz-4.9.0-13-amd64 \ > > > > > -initrd /home/bee/vm/stretch/boot/initrd.img-4.9.0-13-amd64 \ > > > > > -append 'root=svnRoot rw rootfstype=9p > > > > > rootflags=trans=virtio,version=9p2000.L,msize=104857600,cache=mmap > > > > > console=ttyS0' \ > > > > > > > > First obvious difference I see between your setup and mine is that > > > > you're mounting the 9pfs as root from the kernel command line. For > > > > some reason, maybe this has an impact on the check in p9_client_create() > > > > ? > > > > > > > > Can you reproduce with a scenario like Vivek's one ? > > > > > > Yep, confirmed. If I boot a guest from an image file first and then try to > > > manually mount a 9pfs share after guest booted, then I get indeed that > > > msize capping of just 512 kiB as well. That's far too small. :/ > > > > Maybe worth digging : > > - why no capping happens in your scenario ? > > Because I was wrong. > > I just figured even in the 9p rootfs scenario it does indeed cap msize to > 5kiB > as well. The output of /etc/mtab on guest side was fooling me. I debugged > this > on 9p server side and the Linux 9p client always connects with a max. msize > of > 5 kiB, no matter what you do. > > > - is capping really needed ? > > > > Cheers, > > That's a good question and probably depends on whether there is a limitation > on virtio side, which I don't have an answer for. Maybe Michael or Dominique > can answer this. > > Best regards, > Christian Schoenebeck >
