Re: panic: Unaligned free (was: kernel panic while copying files)

On 0705T1833, Gary Jennejohn wrote:
> On Mon, 5 Jul 2021 15:04:48 +0100
> Edward Tomasz Napiera__a <trasz@freebsd.org> wrote:
> 
> > On 0701T1330, Gary Jennejohn wrote:
> > > Gary Jennejohn <gljennjohn@gmail.com> wrote:  
> > > > I noticed that the value of vm.debug.divisor affects what value is
> > > > returned in uma_core.c:uma_dbg_kskip(), so I decided to try a few
> > > > different values.
> > > > 
> > > > The returned value is used to set skipdbg in uma_core.c:item_dtor().
> > > > 
> > > > The default is vm.debug.divisor=1.
> > > > 
> > > > vm.debug.divisor is only present when INVARIANTS is defined.
> > > > 
> > > > kskipdbg eventually affects the value of freei.
> > > > 
> > > > With these values:
> > > > vm.debug.divisor: 0
> > > > kern.cam.da.enable_uma_ccbs: 1
> > > > I can turn on the disk and it comes up without a panic!
> > > > 
> > > > However, I didn't try to do any large data transfers to the disk.
> > > > 
> > > > So, it appears that at least vm.debug.divisor is a big factor in
> > > > whether or not a panic happens with INVARIANTS.
> > > >   
> > > 
> > > I decided to do a real test.  So I built a kernel w/o INVARIANTS and
> > > installed it to /boot/test.
> > > 
> > > Then I stuck a 160GB disk I had around into an external USB3 enclosure
> > > and put a filesystem on it.
> > > 
> > > The I booted the new kernel from /boot/test and set the sysctls so:
> > > kern.cam.da.enable_uma_ccbs: 1
> > > kern.cam.ada.enable_uma_ccbs: 1
> > > 
> > > After that I plugged in the external USB3 enclosure and copied about
> > > 114GiB of data from an internal SSD to it - without a kernel panic:
> > > Filesystem    Size    Used   Avail Capacity  Mounted on
> > > /dev/da0p1    144G    114G     18G    86%    /mnt
> > > 
> > > I'm pretty sure that's more than I could copy without a kernel panic
> > > prior to the recent changes made in cam and umass.
> > > 
> > > My test may not be real proof that all bugs have been squashed, but it
> > > certainly seems to be a better situation than we had before.  
> > 
> > I think the vm.debug.divisor simply masks the problem; the underlying
> > bug is still there.
> > 
> > Could you go back to the setup which panics, and then test the patch
> > at https://reviews.freebsd.org/D31054?  It fixes the scenario described
> > by Warner.
> > 
> 
> It looks like this patch fixes things.
> 
> I used the default value vm.debug.divisor=1 and both enable_uma_ccbs=1
> (which are now the default values on my system).
> 
> I used the 8TiB disk, which spins up very slowly and usually resulted very
> quickly in a panic - no panic with the patch.
> 
> Then using dd to /dev/null (bs=1m) I transferred:
> 
> 308755+0 records in
> 308755+0 records out
> 323753082880 bytes transferred in 1366.162410 secs (236979938 bytes/sec)
> 
> from the disk, so about 324GiB without a panic.

Perfect, I've committed the fix.  Thank you!