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Date:      Mon, 22 Jun 2026 19:30:53 -0700
From:      Ryan Libby <rlibby@freebsd.org>
To:        Alan Somers <asomers@freebsd.org>
Cc:        src-committers@freebsd.org, dev-commits-src-all@freebsd.org,  dev-commits-src-main@freebsd.org
Subject:   Re: git: 08180f1b613b - main - witness: actually set read-only tunables in time for witness_startup
Message-ID:  <CAHgpiFxW-MFWnVyug8NzJmWH1nPQsm94bwKwkGBtxmvKBmuPcw@mail.gmail.com>
In-Reply-To: <CAOtMX2jUu73E6_vmE%2BOsBF6AcNKFMTxPChyS92LyVSHCm0q4Vw@mail.gmail.com>
References:  <6a33714c.187fe.20ec4acc@gitrepo.freebsd.org> <CAOtMX2jUu73E6_vmE%2BOsBF6AcNKFMTxPChyS92LyVSHCm0q4Vw@mail.gmail.com>
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On Mon, Jun 22, 2026 at 4:08 PM Alan Somers <asomers@freebsd.org> wrote:
>
> On Wed, Jun 17, 2026 at 10:18 PM Ryan Libby <rlibby@freebsd.org> wrote:
> >
> > The branch main has been updated by rlibby:
> >
> > URL: https://cgit.FreeBSD.org/src/commit/?id=08180f1b613b6fe000135efa28b7c3293cbf1683
> >
> > commit 08180f1b613b6fe000135efa28b7c3293cbf1683
> > Author:     Ryan Libby <rlibby@FreeBSD.org>
> > AuthorDate: 2026-06-18 03:52:53 +0000
> > Commit:     Ryan Libby <rlibby@FreeBSD.org>
> > CommitDate: 2026-06-18 04:05:43 +0000
> >
> >     witness: actually set read-only tunables in time for witness_startup
> >
> >     SYSCTL_XXX with CTLFLAG_RDTUN and without CTLFLAG_NOFETCH should not be
> >     used for values that are needed before SI_SUB_KLD.  Otherwise they are
> >     tuned after they are needed.  Set CTLFLAG_RDTUN | CTLFLAG_NOFETCH for
> >     the debug.witness.witness_count and debug.witness.skipspin sysctls and
> >     add separate tunables for them, which run at SI_SUB_TUNABLES time, i.e.,
> >     in time for witness_startup.
> >
> >     Reviewed by:    kib, markj
> >     Sponsored by:   Dell Inc.
> >     Differential Revision:  https://reviews.freebsd.org/D57613
>
> My dev VM instapanics on boot, and I've bisected it to this change.
> Apparently it doesn't affect everybody, or it would've been reported
> already.  So I'm not asking you to revert it.  But could you please
> take a look at the problem?
>
> Stack trace:
>
> ---<<BOOT>>---
> Copyright (c) 1992-2026 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 16.0-CURRENT #25 n286717-a9e12d3ec8bd: Mon Jun 22 16:50:06 MDT 2026
>     somers@fbsd-head.lauralan.noip.me:/usr/obj/usr/home/somers/src/freebsd.org/src/main/amd64.amd64/sys/GENERIC
> amd64
> FreeBSD clang version 21.1.8 (https://github.com/llvm/llvm-project.git
> llvmorg-21.1.8-0-g2078da43e25a)
> WARNING: WITNESS option enabled, expect reduced performance.
> SRAT: Ignoring memory at addr 0xff000000
> panic: physical address 0x4af918000 not covered by the DMAP
> cpuid = 0
> time = 1
> KDB: stack backtrace:
> db_trace_self_wrapper() at db_trace_self_wrapper+0x36/frame 0xffffffff8293bd60
> vpanic() at vpanic+0x149/frame 0xffffffff8293be90
> panic() at panic+0x43/frame 0xffffffff8293bef0
> pmap_map() at pmap_map+0x2c/frame 0xffffffff8293bf00
> vm_page_startup() at vm_page_startup+0x71c/frame 0xffffffff8293bf90
> vm_mem_init() at vm_mem_init+0x1a/frame 0xffffffff8293bfa0
> mi_startup() at mi_startup+0x1ed/frame 0xffffffff8293bff0
> KDB: enter: panic
> [ thread pid 0 tid 0 ]
> Stopped at      kdb_enter+0x33: movq    $0,0x15ad202(%rip)
>
> /boot/loader.conf contents:
>
> kern.boottrace.enabled=1
> debug.kasan.panic_on_violation=0
> debug.kmsan.panic_on_violation=0
> debug.witness.witness_count=49152

Thanks for the report.

I'm able to repro with
    set debug.witness.witness_count=49152
at the loader prompt.

The WITNESS_COUNT default is 1536.  There is an allocation that is
O(n^2) of witness_count.  The setting of 49152 should end up
attempting to allocate over 2.25 GiB... but actually it will overflow
a 32-bit int to become negative, leading to us trying to pmap_map
memory somewhere after the end of the biggest phys segment.

I think a minimum fix would change the types to prevent overflow.
Actually we also don't check that we aren't just exceeding the size of
the segment, so presumably we could also hit another panic in just
trying to allocate too much.

This was a sort of latent issue: before your setting of witness_count
was just being ignored.

Ryan
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