Re: [PATCH] mm: fix possible cause of a page_mapped BUG

[Linus Torvalds]

On Fri, Apr 1, 2011 at 7:34 AM, Robert ÅwiÄcki <robert@xxxxxxxxxxx> wrote:
>
> Hey, I'll apply your patch and check it out. In the meantime I
> triggered another Oops (NULL-ptr deref via sys_mprotect).
>
> The oops is here:
>
> http://alt.swiecki.net/linux_kernel/sys_mprotect-2.6.38.txt

That's not a NULL pointer dereference. That's a BUG_ON().

And for some reason you've turned off the BUG_ON() messages, saving
some tiny amount of memory.

Anyway, it looks like the first BUG_ON() in vma_prio_tree_add(), so it
would be this one:

        BUG_ON(RADIX_INDEX(vma) != RADIX_INDEX(old));

but it is possible that gcc has shuffled things around (so it _might_
be the HEAP_INDEX() one). If you had CONFIG_DEBUG_BUGVERBOSE=y, you'd
get a filename and line number. One reason I hate -O2 in cases like
this is that the basic block movement makes it way harder to actually
debug things. I would suggest using -Os too (CONFIG_OPTIMIZE_FOR_SIZE
or whatever it's called).

Anyway, I do find it worrying. The vma code shouldn't be this fragile.  Hugh?

I do wonder what triggers this. Is it a huge-page vma? We seem to be
lacking the check to see that mprotect() is on a hugepage boundary -
and that seems bogus. Or am I missing some check? The new transparent
hugepage support splits the page, but what if it's a _static_ hugepage
thing?

But why would that affect the radix_index thing? I have no idea. I'd
like to blame the anon_vma rewrites last year, but I can't see why
that should matter either. Again, hugepages had some special rules, I
think (and that would explain why nobody normal sees this).

Guys, please give this one a look.

                          Linus
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