Re: [PATCH] mm: fix possible cause of a page_mapped BUG

["Robert Święcki" <robert@swiecki.net>]

On Sat, Apr 2, 2011 at 3:46 AM, Hugh Dickins <hughd@google.com> wrote:
> On Fri, Apr 1, 2011 at 8:44 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> On Fri, Apr 1, 2011 at 7:34 AM, Robert Święcki <robert@swiecki.net> wrote:
>>>
>>> Hey, I'll apply your patch and check it out. In the meantime I
>>> triggered another Oops (NULL-ptr deref via sys_mprotect).
>>>
>>> The oops is here:
>>>
>>> http://alt.swiecki.net/linux_kernel/sys_mprotect-2.6.38.txt
>>
>> That's not a NULL pointer dereference. That's a BUG_ON().
>>
>> And for some reason you've turned off the BUG_ON() messages, saving
>> some tiny amount of memory.
>>
>> Anyway, it looks like the first BUG_ON() in vma_prio_tree_add(), so it
>> would be this one:
>>
>>        BUG_ON(RADIX_INDEX(vma) != RADIX_INDEX(old));
>>
>> but it is possible that gcc has shuffled things around (so it _might_
>> be the HEAP_INDEX() one). If you had CONFIG_DEBUG_BUGVERBOSE=y, you'd
>> get a filename and line number. One reason I hate -O2 in cases like
>> this is that the basic block movement makes it way harder to actually
>> debug things. I would suggest using -Os too (CONFIG_OPTIMIZE_FOR_SIZE
>> or whatever it's called).
>>
>> Anyway, I do find it worrying. The vma code shouldn't be this fragile.  Hugh?
>>
>> I do wonder what triggers this. Is it a huge-page vma? We seem to be
>> lacking the check to see that mprotect() is on a hugepage boundary -
>> and that seems bogus. Or am I missing some check? The new transparent
>> hugepage support splits the page, but what if it's a _static_ hugepage
>> thing?
>>
>> But why would that affect the radix_index thing? I have no idea. I'd
>> like to blame the anon_vma rewrites last year, but I can't see why
>> that should matter either. Again, hugepages had some special rules, I
>> think (and that would explain why nobody normal sees this).
>>
>> Guys, please give this one a look.
>
> I do intend to look, but I think it makes more sense to wait until
> Robert has reproduced it (or something like it) with my debugging
> patch in.

Hi Hugh,

I did two things, included your patch, and compiled with
CONFIG_CC_OPTIMIZE_FOR_SIZE=y; the kernel didn't BUG() or Oopssed for
~2 days under fuzzing (with getdents and readdir syscalls disabled in
the fuzzer). I don't think -Os has any bigger influence on how mm
internally works therefore I must attribute the change to your patch
(was it patch which fixes something or merely dumps vma structures in
case of any problem?).


==============
BTW, another problem arose in the meantime, not sure if anyhow related
to things we're discussing here, although 'btc 0' in kdb shows that
processor 0 hangs in sys_mlock - I did it in two different moments, to
exclude any coincidences. After those 2 days of fuzzing, 'ps wuax'
stopped working, i.e. it prints some output, then stops, cannot be
killed with -SIGKILL etc. I'll let it run for the time being, I can
dump more data in this PID 17750 if anybody wants:

strace:

# strace -f ps wwuax
....
open("/proc/17750/status", O_RDONLY)    = 6
read(6, "Name:\tiknowthis\nState:\tD (disk s"..., 1023) = 777
close(6)                                = 0
open("/proc/17750/cmdline", O_RDONLY)   = 6
read(6,

Process 17750 also cannot be killed. Attaching more data:

# cat /proc/17750/status
Name:	iknowthis
State:	D (disk sleep)
Tgid:	17750
Pid:	17750
PPid:	1
TracerPid:	0
Uid:	1001	1001	1001	1001
Gid:	1001	1001	1001	1001
FDSize:	64
Groups:	1001
VmPeak:	    7752 kB
VmSize:	    5760 kB
VmLck:	      32 kB
VmHWM:	    4892 kB
VmRSS:	    3068 kB
VmData:	    2472 kB
VmStk:	     408 kB
VmExe:	     160 kB
VmLib:	    2684 kB
VmPTE:	      44 kB
VmSwap:	       0 kB
Threads:	1
SigQ:	218/16382
SigPnd:	0000000000000b00
ShdPnd:	0000400000000503
SigBlk:	0000000000000000
SigIgn:	0000000001001000
SigCgt:	0000000000000000
CapInh:	0000000000000000
CapPrm:	0000000000000000
CapEff:	0000000000000000
CapBnd:	ffffffffffffffff
Cpus_allowed:	01
Cpus_allowed_list:	0
Mems_allowed:	00000000,00000001
Mems_allowed_list:	0
voluntary_ctxt_switches:	43330
nonvoluntary_ctxt_switches:	4436

# cat /proc/17750/wchan
call_rwsem_down_write_failed

# cat /proc/17750/maps (hangs)

(from kdb)

[0]kdb> btp 17750
Stack traceback for pid 17750
0xffff88011e772dc0    17750        1  0    0   D  0xffff88011e773240  iknowthis
<c> ffff88011cbcfb88<c> 0000000000000086<c> ffff88011cbcfb08<c>
ffff88011cbcffd8<c>
<c> 0000000000013f00<c> ffff88011e772dc0<c> ffff88011e773180<c>
ffff88011e773178<c>
<c> ffff88011cbce000<c> ffff88011cbcffd8<c> 0000000000013f00<c>
0000000000013f00<c>
Call Trace:
 [<ffffffff81e286ad>] rwsem_down_failed_common+0xdb/0x10d
 [<ffffffff81e286f2>] rwsem_down_write_failed+0x13/0x15
 [<ffffffff81416953>] call_rwsem_down_write_failed+0x13/0x20
 [<ffffffff81e27da0>] ? down_write+0x25/0x27
 [<ffffffff8115f041>] do_coredump+0x14f/0x9a5
 [<ffffffff8114d4e2>] ? T.1006+0x17/0x32
 [<ffffffff810a45f0>] ? __dequeue_signal+0xfa/0x12f
 [<ffffffff8108a79c>] ? get_parent_ip+0x11/0x42
 [<ffffffff810a6406>] get_signal_to_deliver+0x3be/0x3e6
 [<ffffffff8103e0c1>] do_signal+0x72/0x67d
 [<ffffffff81096807>] ? child_wait_callback+0x0/0x58
 [<ffffffff81e28c28>] ? _raw_spin_unlock_irq+0x36/0x41
 [<ffffffff8108bb06>] ? finish_task_switch+0x4b/0xb9
 [<ffffffff8108c3ed>] ? schedule_tail+0x38/0x68
 [<ffffffff8103eb43>] ? ret_from_fork+0x13/0x80
 [<ffffffff8103e6f8>] do_notify_resume+0x2c/0x6e

[0]kdb>  btc 0
Stack traceback for pid 10350
0xffff88011b6badc0    10350        1  1    0   R  0xffff88011b6bb240 *iknowthis2
<c> ffff8800cfc03db8<c> 0000000000000000<c>
Call Trace:
 <#DB>  <<EOE>>  <IRQ>  [<ffffffff81518b03>] ? __handle_sysrq+0xbf/0x15c
 [<ffffffff81518d7d>] ? handle_sysrq+0x2c/0x2e
 [<ffffffff8152bd90>] ? serial8250_handle_port+0x157/0x2b2
 [<ffffffff810a1be8>] ? run_timer_softirq+0x2b3/0x2c2
 [<ffffffff8152bf4c>] ? serial8250_interrupt+0x61/0x111
 [<ffffffff810e6e52>] ? handle_IRQ_event+0x78/0x150
 [<ffffffff810ea044>] ? move_native_irq+0x19/0x6d
 [<ffffffff810e8d90>] ? handle_edge_irq+0xe3/0x12f
 [<ffffffff8104198f>] ? handle_irq+0x88/0x91
 [<ffffffff81e297a5>] ? do_IRQ+0x4d/0xb3
 [<ffffffff81e29193>] ? ret_from_intr+0x0/0x15
 <EOI>  [<ffffffff811336aa>] ? __mlock_vma_pages_range+0x49/0xad
 [<ffffffff8113370a>] ? __mlock_vma_pages_range+0xa9/0xad
 [<ffffffff811337c0>] ? do_mlock_pages+0xb2/0x118
 [<ffffffff81134002>] ? sys_mlock+0xe8/0xf6
 [<ffffffff8107d7e3>] ? ia32_sysret+0x0/0x5

[0]kdb> btc 1
Stack traceback for pid 9409
0xffff88011c9816e0     9409        1  1    1   R  0xffff88011c981b60  iknowthis2
<c> ffff88011dc21ec8

-- 
Robert Święcki

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