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Date:      Wed, 04 May 2022 00:44:28 +0000
From:      bugzilla-noreply@freebsd.org
To:        bugs@FreeBSD.org
Subject:   [Bug 263765] panic: ffs_sync: modification on read-only filesystem
Message-ID:  <bug-263765-227@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D263765

            Bug ID: 263765
           Summary: panic: ffs_sync: modification on read-only filesystem
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: markj@FreeBSD.org

I was using a VM to test some code and crashed it many times.  After a whil=
e it
consistently panics during boot, while upgrading the read-only root mount to
read-write:

Trying to mount root from ufs:gpt/rootfs [rw]...=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
WARNING: / was not properly dismounted=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20
WARNING: /: mount pending error: blocks 367136 files 5=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
No suitable dump device was found.=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
Setting hostuuid: 4ff7a735-851c-7030-87f0-cf2c2b9ef44e.=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
Setting hostid: 0x1c981ebb.=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20
Starting file system checks:=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20
WARNING: / was not properly dismounted=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20
panic: /: ffs_sync: modification on read-only filesystem=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
cpuid =3D 1=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
time =3D 1651623448=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
KDB: stack backtrace:=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe007a140=
750=20=20
vpanic() at vpanic+0x17f/frame 0xfffffe007a1407a0=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
panic() at panic+0x43/frame 0xfffffe007a140800=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
ffs_sync() at ffs_sync+0x6e6/frame 0xfffffe007a1408a0=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
vfs_write_suspend() at vfs_write_suspend+0x165/frame 0xfffffe007a1408f0=20=
=20=20=20=20=20=20=20=20
vfs_write_suspend_umnt() at vfs_write_suspend_umnt+0x35/frame
0xfffffe007a140920
ffs_mount() at ffs_mount+0xa2b/frame 0xfffffe007a140a70=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
vfs_domount_update() at vfs_domount_update+0x277/frame 0xfffffe007a140bf0=
=20=20=20=20=20=20=20
vfs_domount() at vfs_domount+0x26b/frame 0xfffffe007a140d20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
vfs_donmount() at vfs_donmount+0x878/frame 0xfffffe007a140dc0=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
sys_nmount() at sys_nmount+0x69/frame 0xfffffe007a140e00=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
amd64_syscall() at amd64_syscall+0x12e/frame 0xfffffe007a140f30=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe007a140f30=
=20=20=20=20=20=20
--- syscall (378, FreeBSD ELF64, sys_nmount), rip =3D 0x28658b33e3da, rsp =
=3D
0x286588ae7e38, rbp =3D 0x286588ae83a0 ---

The problem appears to be that the fmod flag in the superblock got set,
somehow:

magic   19540119 (UFS2)
last mounted time       Mon May  2 21:00:40 2022
last modified time      Mon May  2 21:00:52 2022
superblock location     65536   id      [ 62658b45 0a5e546e ]
ncg     82      size    13107200        blocks  12694551
bsize   32768   shift   15      mask    0xffff8000
fsize   4096    shift   12      mask    0xfffff000
frag    8       shift   3       fsbtodb 3
minfree 8%      optim   time    symlinklen 120
maxbsize 32768  maxbpg  8192    maxcontig 2     contigsumsize 2
nbfree  1377660 ndir    4007    nifree  6527315 nffree  6053
bpg     20042   fpg     160336  ipg     80256   unrefs  0
nindir  4096    inopb   128     maxfilesize     2252349704110079
sbsize  4096    cgsize  32768   csaddr  5056    cssize  4096
sblkno  24      cblkno  32      iblkno  40      dblkno  5056
cgrotor 7       fmod    1       ronly   0       clean   0
metaspace 0     avgfpdir 64     avgfilesize 16384
flags   unclean soft-updates=20

Running fsck from single-user mode fixed the problem, but it reappeared sev=
eral
times.

I'm not really sure how fmod can end up set; it looks like it's cleared any
time FFS updates the superblock.  (But then why is it in the superblock at
all?)

I don't appear to have checksums enabled here, maybe enabling them would he=
lp
narrow down the problem.

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