Date: Mon, 29 Jun 2026 10:37:26 -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: <CAHgpiFyG2DuBuUy-Oieu4NyCJyCz=PNJphovKKmo=NvtSw2WwQ@mail.gmail.com> In-Reply-To: <CAHgpiFxW-MFWnVyug8NzJmWH1nPQsm94bwKwkGBtxmvKBmuPcw@mail.gmail.com> References: <6a33714c.187fe.20ec4acc@gitrepo.freebsd.org> <CAOtMX2jUu73E6_vmE%2BOsBF6AcNKFMTxPChyS92LyVSHCm0q4Vw@mail.gmail.com> <CAHgpiFxW-MFWnVyug8NzJmWH1nPQsm94bwKwkGBtxmvKBmuPcw@mail.gmail.com>
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On Mon, Jun 22, 2026 at 7:30 PM Ryan Libby <rlibby@freebsd.org> wrote: > > 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 asomers, I just pushed: 2407abc9fb3b ("witness: harden tunables for large settings") That should avoid the panic with your settings. Thanks to kib's suggestion, if you set boot verbose you should be able to see a log line in /var/log/messages about how much startup memory witness wants. In any case you should also see log lines if it can't be satisfied and is automatically reduced. Your setting of debug.witness.witness_count=49152 should result in a request of over 2.25 GiB of startup allocations because witness uses an O(n^2) adjacency matrix. I'm curious whether you are testing something with that many unique lock names and if so whether witness is solving your use case. I wonder if a different graph representation might be better for that use case. I assume witness is designed with an adjacency matrix for speed. Then again I'm unsure if this has been examined from a performance perspective, and specifically I wonder about the pointer indirection vs just calculating indices. Ryanhome | help
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