ZFS kmem_map too small.

[Bakul Shah]

I have an athlon 64 x2 w/ 2GB of memory running in 64 bit mode.
The system dies in a couple of days.  This machine does cvsup
mirroring every few hours + occassional portupgrade + makes.
But I have not been able to panic it deterministically.  Its
loader.conf has

    vfs.zfs.zil_disable=1
    vfs.zfs.prefetch_disable=1

No other changes from defaults for anything relating to vm or
zfs.

The only way to make this more stable was to revert arc.c to
R1.10. There two differences: R1.11 arc_reclaim_needed()
returns 1 when 80% of kmem is used, while R1.10 does so at
50% of kmem.  Second, R1.11 sets arc_c_max to 3/4 of
kmem_size while R1.10 sets it to 1/2 of kmem_size.

> > Tips on how to reproduce it?  Make the kmem_map size small and set the
> > ARC's maximum size to be the same as it ? :)
>
> I'm not really interested in fixing configuration bugs, although we
> should do as much as we can to not allow bugus tunnings.

IMHO bad config can result in suboptimal performance but a
panic seems a bit drastic.  Particularly as things that used
to work a month ago now don't.  It would be nice if
configurations were sanitized during initialization or if
that is not possible take away a tuning knob altogether (or
leave it under debugging).

Still, fiddling with limits to make the panic go away seems
to somehow miss the point as I always worry it will show up
under other conditions.  May be there a way to ensure that
kmem_map is never too small or may be zfs can reserve a few
resources for its own use so that it can get out of a tight
spot?

Thanks!