On Montag, 22. Februar 2021 13:18:14 CET Greg Kurz wrote: > On Sat, 20 Feb 2021 16:38:35 +0100 > > Christian Schoenebeck <qemu_...@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) <jv...@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.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> > > 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
