linux-kernel - Re: [tip:perfcounters/core] x86: Add NMI types for kmap

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Date:	Mon, 15 Jun 2009 20:42:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	paulus@...ba.org, acme@...hat.com, efault@....de, npiggin@...e.de,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, 2009-06-15 at 20:25 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > but ... look at the APIs i propose above. We dont need _any_ 
> > 'types'.
> > 
> > That type enumeration is basically an open-coded allocator. If we do 
> > a _real_ allocator (a balanced stack of atomic kmaps) we dont need 
> > any of those indices, and all the potential for mismatch goes away 
> > as well - a stack nests trivially with IRQ and NMI and arbitrary 
> > other contexts.
> 
> You want types because:
>  - they encode the intent, and can be verified
>  - they help keep track of the max nesting depth
> 
> In the proposed implementation all type code basically falls away 
> no ! CONFIG_DEBUG_VM, but is kept around for robustness.

But much of the fragility of the types (and their clumsiness - for 
example in highpte ops we have to know at which level of the 
pagetables we are, and use the right kind of index) is _precisely_ 
because we have the types ...

Unbalanced unmaps will be detected under CONFIG_DEBUG_VM: kunmap 
uses 'page' as a parameter which is checked against the pte entry - 
they must match.

I.e. it becomes a symmetric and expressive API:

	kaddr = kmap_atomic(page);
	...
	kunmap_atomic(page);

Hm?

	Ingo
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