Re: kern/60531: looping "vflushbuf: dirty" message

The following reply was made to PR kern/60531; it has been noted by GNATS.

From: Taylor R Campbell <[email protected]>
To: [email protected], [email protected]
Cc: [email protected], [email protected]
Subject: Re: kern/60531: looping "vflushbuf: dirty" message
Date: Sun, 2 Aug 2026 20:20:48 +0000

 Found another path to reassignbuf called without the vnode lock, this
 time for a VREG vnode instead of VBLK -- this one is definitely
 snapshot-related (presumably the vnode is the snapshot backing store,
 since it's marked VV_SYSTEM), and could cause the vflushbuf warning
 message and/or almost unbounded running time for the vflushbuf loop
 (really, bounded by the file system size; once the whole fs is backed
 up, there won't be any more writes to it):
 
 # mount
 /dev/dk1 on / type ffs (log, local)
 ptyfs on /dev/pts type ptyfs (local)
 tmpfs on /tmp type tmpfs (local)
 tmpfs on /var/run type tmpfs (local)
 tmpfs on /var/shm type tmpfs (local)
 /dev/vio9p0 on /mnt type puffs|9p
 /dev/ld4a on /mnt type ffs (local)
 # dump -X -0ant -f - / | gzip -nc >/mnt/var/tmp/dump.0
   [20:10:11 UTC] DUMP: Found /dev/rdk1 on / in /etc/fstab
   [20:10:11 UTC] DUMP: Date of this level 0 dump: Sun Aug  2 20:10:11 2026
   [20:10:11 UTC] DUMP: Date of last level 0 dump: the epoch
   [20:10:11 UTC] DUMP: Dumping a snapshot of /dev/rdk1 (/) to standard outp=
 ut
   [20:10:11 UTC] DUMP: Label: none
   [20:10:13 UTC] DUMP: mapping (Pass I) [regular files]
   [20:10:19 UTC] DUMP: mapping (Pass II) [directories]
   [20:10:19 UTC] DUMP: estimated 3140383 tape blocks.
   [20:10:19 UTC] DUMP: Volume 1 started at: Sun Aug  2 20:10:19 2026
   [20:10:19 UTC] DUMP: dumping (Pass III) [directories]
 [ 479.4390328] panic: kernel diagnostic assertion "vp->v_type =3D=3D VBLK |=
 | VOP_ISLOCKED(vp)" failed: file "/home/riastradh/netbsd/current/src/sys/ke=
 rn/vfs_subr.c", line 562 vp=3D0xffff00007cf02800 state=3D3
 [ 479.4577654] cpu0: Begin traceback...
 [ 479.4577654] trace fp ffffc000c63c78f0
 [ 479.4790797] fp ffffc000c63c7920 vpanic() at ffffc0000061f864 netbsd:vpan=
 ic+0x1bc
 [ 479.5110599] fp ffffc000c63c7980 kern_assert() at ffffc000008c6958 netbsd=
 :kern_assert+0x58
 [ 479.5425878] fp ffffc000c63c7a10 reassignbuf() at ffffc00000686434 netbsd=
 :reassignbuf+0x2d4
 [ 479.5740262] fp ffffc000c63c7a40 bdwrite() at ffffc000006743e8 netbsd:bdw=
 rite+0x128
 [ 479.5961369] fp ffffc000c63c7a70 ffs_balloc() at ffffc0000053c240 netbsd:=
 ffs_balloc+0x15e0
 [ 479.6380279] fp ffffc000c63c7bd0 wrsnapblk() at ffffc000005401c4 netbsd:w=
 rsnapblk+0x44
 [ 479.6800518] fp ffffc000c63c7c10 ffs_copyonwrite() at ffffc00000540b50 ne=
 tbsd:ffs_copyonwrite+0x1a4
 [ 479.7140520] fp ffffc000c63c7ca0 fscow_run() at ffffc0000069236c netbsd:f=
 scow_run+0xbc
 [ 479.7349917] fp ffffc000c63c7ce0 bread() at ffffc00000676278 netbsd:bread=
 +0x58
 [ 479.7680479] fp ffffc000c63c7d10 ffs_update() at ffffc0000053ed94 netbsd:=
 ffs_update+0x128
 [ 479.8000553] fp ffffc000c63c7d70 ffs_sync() at ffffc00000548ae8 netbsd:ff=
 s_sync+0x1e8
 [ 479.8209946] fp ffffc000c63c7df0 VFS_SYNC() at ffffc000006874e4 netbsd:VF=
 S_SYNC+0x24
 [ 479.8540963] fp ffffc000c63c7e20 sched_sync() at ffffc00000687670 netbsd:=
 sched_sync+0x110
 [ 479.8860508] tf ffffc000c63c7ed0 el0_trap() at ffffc000000c0ff0 netbsd:el=
 1_trap_exit+0x68
 [ 479.8960548] cpu0: End traceback...
 Stopped in pid 0.124 (system) at        netbsd:cpu_Debugger+0xc:        ldp
 x29, x30, [sp],#16
 db{0}> show vnode ffff00007cf02800
 OBJECT 0xffff00007cf02800: locked=3D0, pgops=3D0xffffc00000aebb48, npages=
 =3D0, refs=3D1
 
 vnode 0xffff00007cf02800 flags 0x12<SYSTEM,MPSAFE>
 tag VT_UFS(1) type VREG(1) mount 0xffff00007daf2000 typedata 0x0
 usecount 1 writecount 0 holdcount 410
 size f9c00050 writesize f9c00050 numoutput 0
 data 0xffff00007d0d94c0 lock 0xffff00007cf02a80
 state LOADED key(0xffff00007daf2000 8) 67 1f 00 00 00 00 00 00
 lrulisthd 0xffffc00001265e10
 db{0}> call vprint(0,ffff00007cf02800)
 vnode 0xffff00007cf02800 flags 0x12<SYSTEM,MPSAFE>
 [ 479.8960548]  tag VT_UFS(1) type VREG(1) mount 0xffff00007daf2000 typedat=
 a 0x0
 [ 479.8960548]  usecount 1 writecount 0 holdcount 410
 [ 479.8960548]  size f9c00050 writesize f9c00050 numoutput 0
 [ 479.8960548]  data 0xffff00007d0d94c0 lock 0xffff00007cf02a80
 [ 479.8960548]  state LOADED key(0xffff00007daf2000 8) 67 1f 00 00 00 00 00=
  00
 [ 479.8960548]  lrulisthd 0xffffc00001265e10
 [ 479.8960548]  tag VT_UFS, ino 8039, on dev 168, 1 flags 0x6, nlink 0
 [ 479.8960548]  mode 0100600, owner 0, group 0, size 4190109776
 0