linux-kernel - Re: [origin tree SLAB corruption] BUG kmalloc-64: Poison overwritten, INFO: Allocated in bdi_alloc

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Date:	Tue, 15 Sep 2009 08:57:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Paris <eparis@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	James Morris <jmorris@...ei.org>, Thomas Liu <tliu@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [origin tree SLAB corruption] BUG kmalloc-64: Poison
	overwritten, INFO: Allocated in bdi_alloc_work+0x2b/0x100 age=175
	cpu=1 pid=3514


* Jens Axboe <jens.axboe@...cle.com> wrote:

> On Mon, Sep 14 2009, Paul E. McKenney wrote:
> > On Mon, Sep 14, 2009 at 07:40:27AM -0700, Linus Torvalds wrote:
> > > 
> > > 
> > > On Mon, 14 Sep 2009, Ingo Molnar wrote:
> > > > 
> > > > BUG kmalloc-64: Poison overwritten
> > > > -----------------------------------------------------------------------------
> > > > 
> > > > INFO: 0xf498f6a0-0xf498f6a7. First byte 0x90 instead of 0x6b
> > > > INFO: Allocated in bdi_alloc_work+0x2b/0x100 age=175 cpu=1 pid=3514
> > > > INFO: Freed in bdi_work_free+0x45/0x60 age=9 cpu=1 pid=3509
> > > > INFO: Slab 0xc3257d84 objects=36 used=11 fp=0xf498f690 flags=0x400000c3
> > > > INFO: Object 0xf498f690 @offset=1680 fp=0xf498fe00
> > > > 
> > > > Bytes b4 0xf498f680:  ab 0d 00 00 9c 27 ff ff 5a 5a 5a 5a 5a 5a 5a 5a ?....'??ZZZZZZZZ
> > > >   Object 0xf498f690:  6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
> > > >   Object 0xf498f6a0:  90 f3 98 f4 60 3c 11 c1 6b 6b 6b 6b 6b 6b 6b 6b .?.?`<.?kkkkkkkk
> > > 
> > > That's 8 bytes of 0xf498f398 and 0xc1113c60. Doesn't look like much, but 
> > > they're both valid kernel pointers, and the 0xf498f398 one is actually 
> > > into the same page as the corruption, so it's a pointer to the same slab 
> > > type (or at least same size). Which is a good hint in itself: we're 
> > > looking at a list or something.
> > > 
> > > And it's at offset 16 in the structure. 
> > > 
> > > That's almost certainly a "struct bdi_work", and the use-aftr-free thing 
> > > is the "struct rcu_head rcu_head" part of it. That first thing (pointer to 
> > > the same page) is 'next', and the second thing is a pointer to kernel text 
> > > (and I can pretty much guarantee that 0xc1113c60 is 'bdi_work_free').
> > > 
> > > So this is either a fs/fs-writeback.c bug, or it's a problem with RCU. 
> > > Both of them are new or hugely changed since 2.6.31.
> > 
> > If this run had used CONFIG_TREE_PREEMPT_RCU rather than the 
> > CONFIG_TREE_RCU that it actually had used, I would suggest applying 
> > the patchset I submitted yesterday (Sept 13).
> > 
> > 	http://thread.gmane.org/gmane.linux.kernel/888803
> 
> Ingo, did it? [...]

The config i attached to the bugreport has:

 #
 # RCU Subsystem
 #
 CONFIG_TREE_RCU=y
 # CONFIG_TREE_PREEMPT_RCU is not set
 CONFIG_RCU_TRACE=y
 CONFIG_RCU_FANOUT=64
 CONFIG_RCU_FANOUT_EXACT=y
 CONFIG_TREE_RCU_TRACE=y

So TREE_PREEMPT_RCU & the synchronize_rcu() bug Paul fixed is out.

> [...] I'll dive into this tonight, Linus' analysis and just a general 
> feel does point in the direction of the bdi work.

Hard to tell whether it's BDI, RCU or something else - sadly this is the 
only incident i've managed to log so far. (We'd be all much happier if 
boxes crashed left and right! ;)

-tip's been carrying the RCU changes for a long(er) time which would 
reduce the chance of this being RCU related. [ It's still possible 
though: if it's a bug with a probability of hitting this box on these 
workloads with a chance of 1:20,000 or worse. ]

Plus it triggered shortly after i updated -tip to latest -git which had 
the BDI bits - which would indicate the BDI stuff - or just about 
anything else in -git for that matter - or something older in -tip. 
Every day without having hit this crash once more broadens the range of 
plausible possibilities.

In any case, i'll refrain from trying to fit a line on a single point of 
measurement ;-)

	Ingo
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