linux-kernel - Re: [PATCH] mm: fix possible cause of a page

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Date:	Fri, 1 Apr 2011 18:21:23 +0200
From:	Robert Święcki <robert@...ecki.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Michel Lespinasse <walken@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] mm: fix possible cause of a page_mapped BUG

On Fri, Apr 1, 2011 at 5:44 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Apr 1, 2011 at 7:34 AM, Robert Święcki <robert@...ecki.net> 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.

Is it possible to turn it off via config flags? Looking into
arch/x86/include/asm/bug.h it seems it's unconditional (as in "it
always manifests itself somehow") and I have
CONFIG_DEBUG_BUGVERBOSE=y.

This BUG/Oopps was triggered before I applied Hugh's patch on a vanilla kernel.

Anything that could help you debugging this? Uploading kernel image
(unfortunately I've overwritten this one), dumping more kgdb data? I
must admit I'm not up-to-date with current linux kernel debugging
techniques. The kernel config is here:
http://alt.swiecki.net/linux_kernel/ise-test-2.6.38-kernel-config.txt

For now I'll compile with -O0 -fno-inline (are you sure you'd like -Os?)

> 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
>



-- 
Robert Święcki
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