Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux2.6.34-rc3)

From: Linus Torvalds
Date: Fri Apr 02 2010 - 20:24:26 EST

On Fri, 2 Apr 2010, Rik van Riel wrote:
> 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.

So let's look at the individual anon_vma_chain entries instead.

What is the protection of the 'vma->anon_vma_chain' list? In
anon_vma_prepare(), the code implies that it is the page_table_lock, but
what about anon_vma_clone()? If I'm reading it correctly, it is some odd
mix of "mmap_sem held for writing" or "mmap_sem held for reading _and_
page_table_lock". And then we have the exit case that apparently has no
locking at all, but that should hopefully be single-threaded.

That thing is subtle. A few more comments about the locking would be good,
so that people like me wouldn't have to try to guess the rules from
reading the source.

> 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...

I don't think AIM7 is at all a very interesting workload, and not likely
to stress anything at all. Did your AIM7 test actually cause heavy
swapping? I doubt it.

Page swapout is where a lot of the magic happens, since that happens
without mmap_sem held etc.

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