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

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Date:	Mon, 15 Jun 2009 20:30:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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.
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