Re: Can not set high msize with virtio-9p (Was: Re: virtiofs vs 9p performance)

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Montag, 22. Februar 2021 13:18:14 CET Greg Kurz wrote:
> On Sat, 20 Feb 2021 16:38:35 +0100
> 
> Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > On Freitag, 19. Februar 2021 20:01:12 CET Vivek Goyal wrote:
> > > On Fri, Feb 19, 2021 at 06:33:46PM +0100, Christian Schoenebeck wrote:
> > > > On Freitag, 19. Februar 2021 17:08:48 CET Vivek Goyal wrote:
> > > > > On Fri, Sep 25, 2020 at 10:06:41AM +0200, Christian Schoenebeck 
wrote:
> > > > > > On Freitag, 25. September 2020 00:10:23 CEST Vivek Goyal wrote:
> > > > > > > In my testing, with cache=none, virtiofs performed better than
> > > > > > > 9p in
> > > > > > > all the fio jobs I was running. For the case of cache=auto  for
> > > > > > > virtiofs
> > > > > > > (with xattr enabled), 9p performed better in certain write
> > > > > > > workloads. I
> > > > > > > have identified root cause of that problem and working on
> > > > > > > HANDLE_KILLPRIV_V2 patches to improve WRITE performance of
> > > > > > > virtiofs
> > > > > > > with cache=auto and xattr enabled.
> > > > > > 
> > > > > > Please note, when it comes to performance aspects, you should set
> > > > > > a
> > > > > > reasonable high value for 'msize' on 9p client side:
> > > > > > https://wiki.qemu.org/Documentation/9psetup#msize
> > > > > 
> > > > > Hi Christian,
> > > > > 
> > > > > I am not able to set msize to a higher value. If I try to specify
> > > > > msize
> > > > > 16MB, and then read back msize from /proc/mounts, it sees to cap it
> > > > > at 512000. Is that intended?
> > > > 
> > > > 9p server side in QEMU does not perform any msize capping. The code in
> > > > this
> > > > case is very simple, it's just what you see in function
> > > > v9fs_version():
> > > > 
> > > > https://github.com/qemu/qemu/blob/6de76c5f324904c93e69f9a1e8e4fd0bd6f6
> > > > b57a
> > > > /hw/9pfs/9p.c#L1332>
> > > > 
> > > > > $ mount -t 9p -o
> > > > > trans=virtio,version=9p2000.L,cache=none,msize=16777216
> > > > > hostShared /mnt/virtio-9p
> > > > > 
> > > > > $ cat /proc/mounts | grep 9p
> > > > > hostShared /mnt/virtio-9p 9p
> > > > > rw,sync,dirsync,relatime,access=client,msize=512000,trans=virtio 0 0
> > > > > 
> > > > > I am using 5.11 kernel.
> > > > 
> > > > Must be something on client (guest kernel) side. I don't see this here
> > > > with
> > > > guest kernel 4.9.0 happening with my setup in a quick test:
> > > > 
> > > > $ cat /etc/mtab | grep 9p
> > > > svnRoot / 9p
> > > > rw,dirsync,relatime,trans=virtio,version=9p2000.L,msize=104857600,cach
> > > > e=m
> > > > map 0 0 $
> > > > 
> > > > Looks like the root cause of your issue is this:
> > > > 
> > > > struct p9_client *p9_client_create(const char *dev_name, char
> > > > *options)
> > > > {
> > > > 
> > > > 	...
> > > > 	if (clnt->msize > clnt->trans_mod->maxsize)
> > > > 	
> > > > 		clnt->msize = clnt->trans_mod->maxsize;
> > > > 
> > > > https://github.com/torvalds/linux/blob/f40ddce88593482919761f74910f42f
> > > > 4b84
> > > > c004b/net/9p/client.c#L1045
> > > 
> > > That was introduced by a patch 2011.
> > > 
> > > commit c9ffb05ca5b5098d6ea468c909dd384d90da7d54
> > > Author: Venkateswararao Jujjuri (JV) <jvrao@linux.vnet.ibm.com>
> > > Date:   Wed Jun 29 18:06:33 2011 -0700
> > > 
> > >     net/9p: Fix the msize calculation.
> > >     
> > >     msize represents the maximum PDU size that includes P9_IOHDRSZ.
> > > 
> > > You kernel 4.9 is newer than this. So most likely you have this commit
> > > too. I will spend some time later trying to debug this.
> > > 
> > > Vivek
> 
> Hi Vivek and Christian,
> 
> I reproduce with an up-to-date fedora rawhide guest.
> 
> Capping comes from here:
> 
> net/9p/trans_virtio.c:  .maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3),
> 
> i.e. 4096 * (128 - 3) == 512000
> 
> AFAICT this has been around since 2011, i.e. always for me as a
> maintainer and I admit I had never tried such high msize settings
> before.
> 
> commit b49d8b5d7007a673796f3f99688b46931293873e
> Author: Aneesh Kumar K.V <aneesh.kumar@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.kumar@linux.vnet.ibm.com>
>     Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
> 
> Changelog doesn't help much but it looks like it was a bandaid
> for some more severe issues.

I did not ever have a kernel crash when I boot a Linux guest with a 9pfs root 
fs and 100 MiB msize. Should we ask virtio or 9p Linux client maintainers if 
they can add some info what this is about?

> > 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. :/

Best regards,
Christian Schoenebeck