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

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Date:	Sat, 2 Apr 2011 12:01:24 +0800
From:	Hui Zhu <teawater@...il.com>
To:	Robert Święcki <robert@...ecki.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Sat, Apr 2, 2011 at 00:35, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Apr 1, 2011 at 9:21 AM, Robert Święcki <robert@...ecki.net> wrote:
>>
>> 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.
>
> Ok, if you have CONFIG_DEBUG_BUGVERBOSE then, you do have the bug-table.
>
> Maybe it's just kdb that is broken, and doesn't print it. I wouldn't
> be surprised. It's not the first time I've seen debugging features
> that just make debugging a mess.
>
>> Anything that could help you debugging this? Uploading kernel image
>> (unfortunately I've overwritten this one), dumping more kgdb data?
>
> So in this case kgdb just dropped the most important data on the floor.
>
> But if you have kdb active next time, print out the vma/old contents
> in that function that has the BUG() in it.
>
>> 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?)

Hi Robert,

I am not sure you can success with build trunk with  -O0 -fno-inline.
I suggest you try the patch in
http://code.google.com/p/kgtp/downloads/detail?name=co.patch.
It add a option in "Kernel hacking" called "Compile with almost no
optimization". It will make kernel be built without -O2. It support
x86_32, x86_64 and arm.

PS, maybe you can try kgtp (https://code.google.com/p/kgtp/)  debug your kernel.

Thanks,
Hui

>
> Oh, don't do that. -O0 makes the code totally unreadable (the compiler
> just does _stupid_ things, making the asm code look so horrible that
> you can't match it up against anything sane), and -fno-inline isn't
> worth the pain either.
>
> -Os is much better than those.
>
> But in this case, just getting the filename and line number would have
> made the thing moot anyway - without kdb it _should_ have said
> something clear like
>
>   kernel BUG at %s:%u!
>
> where %s:%u is the filename and line number.
>
>                          Linus
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