QEMU/KVM Panic on USB Mount

[John Baldwin]

On Friday, July 09, 2010 10:27:46 pm Boris Kochergin wrote:
> Hans Petter Selasky wrote:
> > On Friday 09 July 2010 19:12:59 Sean Bruno wrote:
> >   
> >> In order to come up with the most convoluted problem possible, I present
> >> to you screen shots of a panic from a FreeBSD VM running in QEMU
> >> emulation under RedHat's KVM infrastructure.
> >>
> >> I presented a perfectly functional USB partition from my host machine to
> >> the VM.  Then I attempted to mount the partition from FreeBSD and it
> >> generated the panics located here:
> >>
> >> http://people.freebsd.org/~sbruno/usb_panic1.png
> >> http://people.freebsd.org/~sbruno/usb_panic2.png
> >>
> >> This is 100% reproducible.  But not an emergency or anything.
> >>     
> >
> > It might be that there is a conflict that both the VM and the OS is accessing 
> > / loading drivers on the same USB device. The panic seems not directly related 
> > to USB.
> >
> > --HPS
> >
> >   
> I followed the code for a while and the cause of the panic appears to be 
> that ffs_vgetf() in /usr/src/sys/ufs/ffs/ffs_vfsops.c fails in one of 
> the several ways possible, causing in VFS_ROOT() in 
> /usr/src/sys/kern/vfs_mount.c to return a non-zero value and resulting 
> in the panic. I think the problem calls for someone with serious VFS chops.
> 
> The I/O errors from the USB device are suspicious (in that they may 
> cause the mount code to misbehave if they occur during its course).

Much of the filesystem code doesn't really handle I/O errors very gracefully.
I do think trasz@ did some work a year or so ago to improve this somewhat.

-- 
John Baldwin