linux-kernel - Re: [PATCH] [1/5] CPA: Split static_protections into required_static_prot and advised_static

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Date:	Sat, 9 Feb 2008 16:50:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Andi Kleen <ak@...e.de>, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [1/5] CPA: Split static_protections into required_static_prot
 and advised_static_prot

On Sat, 9 Feb 2008, Andi Kleen wrote:
> On Sat, Feb 09, 2008 at 03:56:02PM +0100, Thomas Gleixner wrote:
> > On Fri, 8 Feb 2008, Andi Kleen wrote:
> > > There is a big difference between NX and RO. NX absolutely has to be cleared
> > > or the kernel will fail while RO just can be set, but does not need to.
> > > And for a large page area not setting NX if there is a area below 
> > > it that needs it is essential, while making it ro is optional again.
> 
> Optional as in it doesn't need to be forced.
> 
> > 
> > No, it's not optional. Making the PMD RO will write protect all 4k
> > PTEs below independent of their setting. So there is the same
> > restriction as we have with NX.
> 
> If there is a boundary between a RO area
> and a RW area and you want to map it with 2MB pages then NX is required,
> but RO is optional on the page and if you prefer TLB use minimalization
> over debugging it is optional. Is it clear now? 

No, it is not clear at all. Why is there a requirement for NX, when I
have an overlapping area of RO and RW in a large page mapping? If I
want to preserve the large page mapping in such a case, then its
required to make the mapping RW and omit the protection of the RO
area, nothing else.

> Note the behaviour for pageattr and thus DEBUG_RODATA / debugging
> sitations where you don't care about your TLB this
> does not change, this makes only a difference for the initial init_32
> direct mapping setup.

Your patches do change the behaviour. The range checking breaks the
enforcement of some restrictions for the sake of keeping the large
page intact.

Thanks,
	tglx
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