Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

[Rik van Riel]

On 04/02/2010 02:37 PM, Linus Torvalds wrote:
> On Fri, 2 Apr 2010, Andrew Morton wrote:
> > On Fri, 2 Apr 2010 11:09:14 -0700 (PDT) Linus Torvalds<torvalds@xxxxxxxxxxxxxxxxxxxx>  wrote:
> >
> > > I think this is likely due to the new scalable anon_vma linking by Rik.
> >
> > Similar to https://bugzilla.kernel.org/show_bug.cgi?id=15680
>
> Yup, looks like the same thing, except that bugzilla entry was due to
> swapping rather than hibernation and memory shrinking. But same end
> result, just different reasons for why we were trying to shrink the page
> lists.

Interesting that it is a null pointer dereference, given
that we do not zero out the anon_vma_chain structs before
freeing them.

Page_referenced_anon() takes the anon_vma->lock before
walking the list.  The three places where we modify the
anon_vma_chain->same_anon_vma list, we also hold the
lock.

No doubt something in mm/ is doing something silly, but
I have not found anything yet :(

If I had to guess, I'd say maybe we got one of the
mprotect & vma_adjust cases wrong.  Maybe a page stayed
around in the LRU (and in a process?) after its anon_vma
already got freed?

There has to be a reason why a very heavy AIM7 workload
and some other stress tests did not trigger it, but a few
people are able to trigger it on their systems...
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