Re: 3.4-rc7 numa_policy slab poison.

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

Added some more people explicitly to the cc, in case they don't peruse
the mailing lists as carefully as their personal emails.

It certainly looks like some kind of mpol_get/put imbalance.

However, looking at mm/mempolicy.c, I really want to just dig out my
own eyes with a spoon. All the games with MPOL_F_SHARED in particular
look *really* unsafe. In particular, why i it safe to suddenly set
MPOL_F_SHARED in sp_alloc(), when it previously was unshared and might
have random stale refcounts if so?

The locking is also *really* hard to read. It's full of conditional
locks/unlock things, see for example do_mbind(), which really is
inexcusably ugly in just about all respects.

But there's not a lot of recent stuff. The thing that jumps out is Mel
Gorman's recent commit cc9a6c8776615 ("cpuset: mm: reduce large
amounts of memory barrier related damage v3"), which has a whole new
loop with that scary mpol_cond_put() usage. And there's we had
problems with vma merging..

Dave, how recent is this problem? Have you already tried older kernels?

Kosaki, Mel, Christoph, please give Dave's system call fuzzer a test,
maybe you can see what the problem is quickly..

                              Linus

On Mon, May 21, 2012 at 8:47 AM, Dave Jones <davej@redhat.com> wrote:
> On Fri, May 18, 2012 at 02:58:51PM -0400, Dave Jones wrote:
>  > On Thu, May 17, 2012 at 05:31:20PM -0400, Dave Jones wrote:
>  >
>  >  > =============================================================================
>  >  > BUG numa_policy (Not tainted): Poison overwritten
>  >  > -----------------------------------------------------------------------------
>  >  >
>  >  > INFO: 0xffff880146498250-0xffff880146498250. First byte 0x6a instead of 0x6b
>  >  > INFO: Allocated in mpol_new+0xa3/0x140 age=46310 cpu=6 pid=32154
>  >  >   __slab_alloc+0x3d3/0x445
>  >  >   kmem_cache_alloc+0x29d/0x2b0
>  >  >   mpol_new+0xa3/0x140
>  >  >   sys_mbind+0x142/0x620
>  >  >   system_call_fastpath+0x16/0x1b
>  >  > INFO: Freed in __mpol_put+0x27/0x30 age=46268 cpu=6 pid=32154
>  >  >   __slab_free+0x2e/0x1de
>  >  >   kmem_cache_free+0x25a/0x260
>  >  >   __mpol_put+0x27/0x30
>  >  >   remove_vma+0x68/0x90
>  >  >   exit_mmap+0x118/0x140
>  >  >   mmput+0x73/0x110
>  >  >   exit_mm+0x108/0x130
>  >  >   do_exit+0x162/0xb90
>  >  >   do_group_exit+0x4f/0xc0
>  >  >   sys_exit_group+0x17/0x20
>  >  >   system_call_fastpath+0x16/0x1b
>  >  > INFO: Slab 0xffffea0005192600 objects=27 used=27 fp=0x          (null) flags=0x20000000004080
>  >  > INFO: Object 0xffff880146498250 @offset=592 fp=0xffff88014649b9d0
>  >
>  > As I can reproduce this fairly easily, I enabled the dynamic debug prints for mempolicy.c,
>  > and noticed something odd (but different to the above trace..)
>  >
>  > INFO: 0xffff88014649abf0-0xffff88014649abf0. First byte 0x6a instead of 0x6b
>  > INFO: Allocated in mpol_new+0xa3/0x140 age=196087 cpu=7 pid=11496
>  >  __slab_alloc+0x3d3/0x445
>  >  kmem_cache_alloc+0x29d/0x2b0
>  >  mpol_new+0xa3/0x140
>  >  sys_mbind+0x142/0x620
>  >  system_call_fastpath+0x16/0x1b
>  > INFO: Freed in __mpol_put+0x27/0x30 age=40838 cpu=7 pid=20824
>  >  __slab_free+0x2e/0x1de
>  >  kmem_cache_free+0x25a/0x260
>  >  __mpol_put+0x27/0x30
>  >  mpol_set_shared_policy+0xe6/0x280
>  >  shmem_set_policy+0x2a/0x30
>  >  shm_set_policy+0x28/0x30
>  >  sys_mbind+0x4e7/0x620
>  >  system_call_fastpath+0x16/0x1b
>  > INFO: Slab 0xffffea0005192600 objects=27 used=27 fp=0x          (null) flags=0x20000000004080
>  > INFO: Object 0xffff88014649abf0 @offset=11248 fp=0xffff880146498de0
>  >
>  > In this case, it seems the policy was allocated by pid 11496, and freed by a different pid!
>  > How is that possible ?  (Does kind of explain why it looks like a double-free though I guess).
>  >
>  > debug printout for the relevant pids below, in case it yields further clues..
>
> Anyone ?  This can be reproduced very quickly by doing..
>
> $ git clone git://git.codemonkey.org.uk/trinity.git
> $ make
> $ ./trinity -q -c mbind
>
> On my 8-core box, it happens within 30 seconds.
>
> If I run this long enough, the box wedges completely, needing a power cycle to reboot.
>
>        Dave
>

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