linux-kernel - Re: [git pull] changes for tip, and a nasty x86 page table bug

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Date:	Fri, 20 Feb 2009 13:02:52 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [git pull] changes for tip, and a nasty x86 page table bug

Hi, Linus,

On Fri, 2009-02-20 at 12:17 +0800, Linus Torvalds wrote:
> 
> On Thu, 19 Feb 2009, Steven Rostedt wrote:
> > 
> > Is this something worthy of 29? I could whip up a patch against your 
> > latest tree.
> 
> I think it's a real issue, but I do have to admit that I don't see why it 
> would only trigegr for you. Is it just because the trace stuff ends up 
> setting pages to RW, and you have to have had a lot of read-only stuff to 
> get a whole read-only PMD to begin with?
> 
> So there's two things that make me nervous:
> 
>  - I do think the KERNPG_TABLE thing is the right thing, and I _think_ 
>    that code is just confused, and we should just do KERNPG_TABLE rather 
>    than play with confused bits one by one (PRESENT, RW, NX) to the point 
>    of just making for more confusion.
> 
>    But I'd like some of the people involved with that code confirm that. 
>    Either a "Yeah, we were just confused" or "No, there's this really 
>    subtle thing going on, liek this: ..."
> 
>  - The fact that apparently you're the first one to hit this. I realize 
>    that you do odd things with ftrace. Was it the fact that you made the 
>    "set_memory_ro()" area larger, and then more dynamically mark it back 
>    to read-write that you hit it? Haven't we done things like that before?

In fact, I am the first one to hit a similar bug. I do some odd thing
with EFI to change the page tables to be executable. Unfortunately I
fixed that bug in a confused way.

Yes. I think KERNPG_TABLE fixes all these types of bugs in a more clear
way.

Best Regards,
Huang Ying


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