Early drop to debugger with DEBUG_MEMGUARD

I first noticed this on my laptop on 08 Aug, after having built & booted

FreeBSD 10.0-CURRENT #975  r253985M/253985:1000041: Tue Aug  6 05:28:39 PDT 2013     root_at_localhost:/common/S4/obj/usr/src/sys/CANARY  i386

OK.  I'm away from home, and Internet access is a bit flaky, so
initially, I suspected that something may have gone wrong with my
source update; I later determined that disabling "options DEBUG_MEMGUARD"
would avoid the panic.

That said, I had been running a kernel with DEBUG_MEMGUARD for quite
a while without issues; I suspect that this drop to debugger either
reflects a real problem that disabling DEBUG_MEMGUARD merely hides
or htat the assert in src/sys/kern/subr_vmem.c:1050 isn't actually
correct in all cases.

So I finally(!) had a chance to try to reproduce the error on a
machine with a serial console; here's a cut/paste from that:

...
 |  7. Boot [V]erbose: NO                  |    `:`                  `:`
 |                                         |      .--             `--.
 |                                         |         .---.....----.
 +-----------------------------------------+
                                          

Booting...
GDB: no debug ports present
KDB: debugger backends: ddb
KDB: current backend: ddb
Copyright (c) 1992-2013 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
        The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 10.0-CURRENT #0  r254245M/254246:1000042: Mon Aug 12 07:20:47 PDT 2013
    root_at_freebeast.catwhisker.org:/common/S3/obj/usr/src/sys/MEMGUARD i386
FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610
WARNING: WITNESS option enabled, expect reduced performance.
panic: Assertion strat == M_BESTFIT || strat == M_FIRSTFIT failed at /usr/src/sys/kern/subr_vmem.c:1050
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper(c116fcdc,73752f20,72732f72,79732f63,656b2f73,...) at db_trace_self_wrapper+0x2d/frame 0xc1820ba0
kdb_backtrace(c11c4b23,0,c0f8a835,c1820c74,c0f8a835,...) at kdb_backtrace+0x30/frame 0xc1820c08
vpanic(c12eea08,100,c0f8a835,c1820c74,c1820c74,...) at vpanic+0x11f/frame 0xc1820c44
kassert_panic(c0f8a835,c1172e98,c1172e39,41a,8,...) at kassert_panic+0xea/frame 0xc1820c68
vmem_alloc(c130d680,6681000,2,c1820cc0,3b5,...) at vmem_alloc+0x53/frame 0xc1820ca0
memguard_init(c130d680,c0a9fa50,c6800000,20281000,1000,10000,0) at memguard_init+0x29/frame 0xc1820cc4
kmeminit(c14b9fd4,c10efc89,0,0,c1820d30,...) at kmeminit+0x171/frame 0xc1820cf0
mallocinit(0,0,2,0,c11d3728,...) at mallocinit+0x32/frame 0xc1820d30
mi_startup() at mi_startup+0xf7/frame 0xc1820d58
begin() at begin+0x2c
KDB: enter: panic
[ thread pid 0 tid 0 ]
Stopped at      kdb_enter+0x3d: movl    $0,kdb_why
db> 

As you can see, this is well before any device probes or much of
anything else.  Thus, I suspect that it's fairly possible that the
assertion may well be OK after a certain point in the boot sequence,
but decidedly *not* OK in this specific instance.  Or perhaps the
assertion just doesn't play well with DEBUG_MEMGUARD.

I'm not about to pretend that I have anywhere near enough familiarity
with what's going on to even suggest a fix, but it seems to me that
Something Is Wrong Here.

The kernel config (in this case) is:

include GENERIC

ident		MEMGUARD

options 	DEBUG_MEMGUARD


The system was running a copy of:

FreeBSD 10.0-CURRENT #1243  r254245M/254246:1000042: Mon Aug 12 05:39:42 PDT 2013     root_at_freebeast.catwhisker.org:/common/S4/obj/usr/src/sys/GENERIC  i386

but with a newly-built MEMGUARD kernel (as above), built from the same
sources.

I have some time to poke at it for the next few hours; subject to
my Internet access & available time, I'm happy to do that, try
patches, or whatever, but I could use a bit of guidance.

Since it's been completely reproducible for me, I suspect that
anyone with sufficiently recenty sources running head can reproduce
it merely by enabling "options DEBUG_MEMGUARD", rebuilding the
kernel, and booting it.

Peace,
david
-- 
David H. Wolfskill				david_at_catwhisker.org
Taliban: Evil men with guns afraid of truth from a 14-year old girl.

See http://www.catwhisker.org/~david/publickey.gpg for my public key.