Re: panic: Unaligned free (was: kernel panic while copying files)

From: Gary Jennejohn <gljennjohn_at_gmail.com>
Date: Thu, 1 Jul 2021 05:58:00 +0200
On Wed, 30 Jun 2021 10:35:14 -0600
Warner Losh <imp_at_bsdimp.com> wrote:

> On Wed, Jun 30, 2021 at 6:58 AM Gary Jennejohn <gljennjohn_at_gmail.com> wrote:
> 
> > On Wed, 30 Jun 2021 06:02:59 +0100
> > Graham Perrin <grahamperrin_at_gmail.com> wrote:
> >  
> > > On 29/06/2021 10:42, Gary Jennejohn wrote:  
> > > > ___ panic is now the result of an unaligned free.
> > > >
> > > > panic: Unaligned free of 0xfffff800259e2800 from zone
> > > >      0xfffffe00dc9d2000(da_ccb) slab 0xfffff800259e2fd8(3)
> > > >
> > > > I have the crash dump and a debug kernel in case anyone wants more  
> > info.  
> > > Can you post the backtrace etc. here? Thanks
> > >  
> >
> > Sure.  As can be seen from the uma zone being da_ccb, the panic
> > resulted from setting kern.cam.da.enable_uma_ccbs=1.
> >
> > Unread portion of the kernel message buffer:
> > panic: Unaligned free of 0xfffff800259e2800 from zone
> > 0xfffffe00dc9d2000(da_ccb) slab 0xfffff800259e2fd8(3)
> > cpuid = 2
> > time = 1624958650
> > KDB: stack backtrace:
> > db_trace_self_wrapper() at db_trace_self_wrapper+0x2c/frame
> > 0xfffffe00c62687a0
> > kdb_backtrace() at kdb_backtrace+0x46/frame 0xfffffe00c6268850
> > vpanic() at vpanic+0x227/frame 0xfffffe00c62688f0
> > panic() at panic+0x4e/frame 0xfffffe00c6268950
> > uma_dbg_free() at uma_dbg_free+0xfc/frame 0xfffffe00c62689a0
> > item_dtor() at item_dtor+0x7c/frame 0xfffffe00c62689e0
> > uma_zfree_arg() at uma_zfree_arg+0xf0/frame 0xfffffe00c6268a50
> > uma_zfree() at uma_zfree+0x23/frame 0xfffffe00c6268a70
> > xpt_free_ccb() at xpt_free_ccb+0x43/frame 0xfffffe00c6268a90
> > camperiphdone() at camperiphdone+0x211/frame 0xfffffe00c6268ae0
> > xpt_done_process() at xpt_done_process+0x550/frame 0xfffffe00c6268b40
> > xpt_done_td() at xpt_done_td+0x1c0/frame 0xfffffe00c6268b80
> > fork_exit() at fork_exit+0x117/frame 0xfffffe00c6268bf0
> > fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe00c6268bf0
> > --- trap 0, rip = 0, rsp = 0, rbp = 0 ---
> > KDB: enter: panic
> >
> > doadump (textdump=0) at /usr/src/sys/kern/kern_shutdown.c:399
> > 399             dumptid = curthread->td_tid;
> > (kgdb) bt
> > #0  doadump (textdump=0) at /usr/src/sys/kern/kern_shutdown.c:399
> > #1  0xffffffff804d5dd7 in db_dump (dummy=-2138843371, dummy2=false,
> > dummy3=-1,
> >     dummy4=0xfffffe00c6268320 "") at /usr/src/sys/ddb/db_command.c:575
> > #2  0xffffffff804d5bf4 in db_command (
> >     last_cmdp=0xffffffff8114ce80 <db_last_command>, cmd_table=0x0,
> > dopager=1)
> >     at /usr/src/sys/ddb/db_command.c:482
> > #3  0xffffffff804d583c in db_command_loop ()
> >     at /usr/src/sys/ddb/db_command.c:535
> > #4  0xffffffff804da27c in db_trap (type=3, code=0)
> >     at /usr/src/sys/ddb/db_main.c:270
> > #5  0xffffffff8083df9d in kdb_trap (type=3, code=0, tf=0xfffffe00c6268770)
> >     at /usr/src/sys/kern/subr_kdb.c:727
> > #6  0xffffffff80d31494 in trap (frame=0xfffffe00c6268770)
> >     at /usr/src/sys/amd64/amd64/trap.c:604
> > #7  0xffffffff80d32628 in trap_check (frame=0xfffffe00c6268770)
> >     at /usr/src/sys/amd64/amd64/trap.c:664
> > #8  <signal handler called>
> > #9  breakpoint () at /usr/src/sys/amd64/include/cpufunc.h:66
> > #10 0xffffffff8083d3d0 in kdb_enter (why=0xffffffff80e0355b "panic",
> >     msg=0xffffffff80e0355b "panic") at /usr/src/sys/kern/subr_kdb.c:505
> > #11 0xffffffff807d1725 in vpanic (
> >     fmt=0xffffffff80dbca46 "Unaligned free of %p from zone %p(%s) slab
> > %p(%d)", ap=0xfffffe00c6268930) at /usr/src/sys/kern/kern_shutdown.c:906
> > #12 0xffffffff807d120e in panic (
> >     fmt=0xffffffff80dbca46 "Unaligned free of %p from zone %p(%s) slab
> > %p(%d)")
> >     at /usr/src/sys/kern/kern_shutdown.c:843
> > #13 0xffffffff80c16a8c in uma_dbg_free (zone=0xfffffe00dc9d2000,
> >     slab=0xfffff800259e2fd8, item=0xfffff800259e2800)
> >     at /usr/src/sys/vm/uma_core.c:5659
> > #14 0xffffffff80c0c5dc in item_dtor (zone=0xfffffe00dc9d2000,
> >     item=0xfffff800259e2800, size=544, udata=0x0, skip=SKIP_NONE)
> >     at /usr/src/sys/vm/uma_core.c:3418
> > #15 0xffffffff80c0ba60 in uma_zfree_arg (zone=0xfffffe00dc9d2000,
> >     item=0xfffff800259e2800, udata=0x0) at /usr/src/sys/vm/uma_core.c:4374
> > #16 0xffffffff802e45d3 in uma_zfree (zone=0xfffffe00dc9d2000,
> >     item=0xfffff800259e2800) at /usr/src/sys/vm/uma.h:404
> > #17 0xffffffff802dc3c3 in xpt_free_ccb (free_ccb=0xfffff800259e2800)
> >     at /usr/src/sys/cam/cam_xpt.c:4676
> > #18 0xffffffff802dacf1 in camperiphdone (periph=0xfffff80025329b00,
> >     done_ccb=0xfffff80025a24cc0) at /usr/src/sys/cam/cam_periph.c:1427
> > #19 0xffffffff802e4520 in xpt_done_process (ccb_h=0xfffff80025a24cc0)
> >     at /usr/src/sys/cam/cam_xpt.c:5493
> > #20 0xffffffff802e68e0 in xpt_done_td (arg=0xffffffff81143700 <cam_doneqs>)
> >     at /usr/src/sys/cam/cam_xpt.c:5548
> > #21 0xffffffff807673c7 in fork_exit (callout=0xffffffff802e6720
> > <xpt_done_td>,
> >     arg=0xffffffff81143700 <cam_doneqs>, frame=0xfffffe00c6268c00)
> >     at /usr/src/sys/kern/kern_fork.c:1083
> > #22 <signal handler called>
> >
> > [kgdb stuff removed]
> >
> > (kgdb) down
> > #15 0xffffffff80c0ba60 in uma_zfree_arg (zone=0xfffffe00dc9d2000,
> >     item=0xfffff800259e2800, udata=0x0) at /usr/src/sys/vm/uma_core.c:4374
> > 4374                    item_dtor(zone, item, cache_uz_size(cache), udata,
> > SKIP_NONE);
> > (kgdb) down
> > #14 0xffffffff80c0c5dc in item_dtor (zone=0xfffffe00dc9d2000,
> >     item=0xfffff800259e2800, size=544, udata=0x0, skip=SKIP_NONE)
> >     at /usr/src/sys/vm/uma_core.c:3418
> > 3418                            uma_dbg_free(zone, NULL, item);
> > (kgdb) p/x skipdbg
> > $26 = 0x0
> > (kgdb) p/x zone->uz_flags
> > $27 = 0x41000000 (UMA_ZFLAG_TRASH|UMA_ZFLAG_CTORDTOR)
> > (kgdb) down
> > #13 0xffffffff80c16a8c in uma_dbg_free (zone=0xfffffe00dc9d2000,
> >     slab=0xfffff800259e2fd8, item=0xfffff800259e2800)
> >     at /usr/src/sys/vm/uma_core.c:5659
> > 5659                    panic("Unaligned free of %p from zone %p(%s) slab
> > %p(%d)",
> >
> > Note that item is the same as saved_ccb.
> >
> > (kgdb) p/x *zone->uz_keg
> > $28 = {uk_hash = {uh_slab_hash = 0x0, uh_hashsize = 0x0, uh_hashmask =
> > 0x0},
> >   uk_zones = {lh_first = 0xfffffe00dc9d2000}, uk_dr = {
> >     dr_policy = 0xffffffff810010e0, dr_iter = 0x0}, uk_align = 0x7,
> >   uk_reserve = 0x0, uk_size = 0x220, uk_rsize = 0x220,
> >   uk_init = 0xffffffff80c17d50, uk_fini = 0xffffffff80c17d80,
> >   uk_allocf = 0xffffffff80d342f0, uk_freef = 0xffffffff80d346a0,
> >   uk_offset = 0x0, uk_kva = 0x0, uk_pgoff = 0xfd8, uk_ppera = 0x1,
> >   uk_ipers = 0x7, uk_flags = 0x0, uk_name = 0xffffffff80debbac, uk_link = {
> >     le_next = 0xfffff80005968000, le_prev = 0xfffff80005968380},
> >   uk_domain = 0xfffff80005968240}
> > (kgdb) p/x *slab
> > $29 = {us_link = {le_next = 0xdeadc0dedeadc0de, le_prev =
> > 0xdeadc0dedeadc0de},
> >   us_freecount = 0xc0de, us_flags = 0xad, us_domain = 0xde, us_free = {
> >     __bits = 0xfffff800259e2ff0}}
> > (kgdb) p/x *0xfffff800259e2ff0
> > $30 = 0xdeadc0de
> > Don't know whether this matters, but slab seems to be unitialized.
> > (kgdb) p/x freei
> > $31 = 0x3
> >
> > In any case, saved_ccb has an address which lies outside the range
> > covered by slab, i.e. freei is bigger than the number of entries in
> > slab.
> >
> > I suspect that the only way to really figure out what's going on is to
> > run the kernel in kgbd and set lots of breakpoints,
> >  
> 
> What's happening is this, I think.
> 
> (1) We send a request.
> (2) It fails, so we send a start unit
> BUT we do weird things to copy the CCBs around, and the request from (1)
> and (2) have different allocations. Error recovery overwrites the original
> request with a new request after saving it off....
> (3) start unit succeeds, we go to free one of the CCBs and it's marked
> incorrectly, triggering either this panic or the prior one we saw.
> 
> These actions were fine when there was one allocator, but now that there
> are two more care must be taken, and that more care hasn't been taken yet,
> so kern.cam.da.enable_uma_ccbs=1 is unsafe for now and should not be used.
> kern.cam.ada.enable_uma_ccbs=1 is fine as its error recovery doesn't do
> these things.
> 

I noticed that the value of vm.debug.divisor affects what value is
returned in uma_core.c:uma_dbg_kskip(), so I decided to try a few
different values.

The returned value is used to set skipdbg in uma_core.c:item_dtor().

The default is vm.debug.divisor=1.

vm.debug.divisor is only present when INVARIANTS is defined.

kskipdbg eventually affects the value of freei.

With these values:
vm.debug.divisor: 0
kern.cam.da.enable_uma_ccbs: 1
I can turn on the disk and it comes up without a panic!

However, I didn't try to do any large data transfers to the disk.

So, it appears that at least vm.debug.divisor is a big factor in
whether or not a panic happens with INVARIANTS.

-- 
Gary Jennejohn
Received on Thu Jul 01 2021 - 03:58:00 UTC

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