Re: [origin tree SLAB corruption] BUG kmalloc-64: Poison overwritten, INFO: Allocated in bdi_alloc_work+0x2b/0x100 age=175 cpu=1 pid=3514

[Linus Torvalds <torvalds@linux-foundation.org>]

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.

			Linus