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

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Message-Id: <1245089943.13761.19334.camel@twins>
Date:	Mon, 15 Jun 2009 20:19:03 +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 20:15 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > +static int kmap_type_to_context(enum km_type type)
> > +{
> > +	switch (type) {
> > +	case KM_BOUNCE_READ:
> > +		return KM_CTX_USER;
> > +	case KM_SKB_SUNRPC_DATA:
> > +		return KM_CTX_USER;
> > +	case KM_SKB_DATA_SOFTIRQ:
> > +		return KM_CTX_SOFTIRQ;
> > +	case KM_USER0:
> > +		return KM_CTX_USER;
> > +	case KM_USER1:
> > +		return KM_CTX_USER;
> > +	case KM_BIO_SRC_IRQ:
> > +		return KM_CTX_IRQ;
> > +	case KM_BIO_DST_IRQ:
> > +		return KM_CTX_IRQ;
> > +	case KM_PTE0:
> > +		return KM_CTX_USER;
> > +	case KM_PTE1:
> > +		return KM_CTX_USER;
> > +	case KM_IRQ0:
> > +		return KM_CTX_IRQ;
> > +	case KM_IRQ1:
> > +		return KM_CTX_IRQ;
> > +	case KM_SOFTIRQ0:
> > +		return KM_CTX_SOFTIRQ;
> > +	case KM_SOFTIRQ1:
> > +		return KM_CTX_SOFTIRQ;
> > +	case KM_NMI:
> > +		return KM_CTX_NMI;
> > +	case KM_NMI_PTE:
> > +		return KM_CTX_NMI;
> > +	}
> > +
> > +	return KM_CTX_MAX;
> 
> why not do a very simple stack of atomic kmaps, like Hugh suggested?
> 
> That would mean a much simpler interface:
> 
> 	kaddr = kmap_atomic(page);
> 
> no index needed. The kunmap pops the entry off the stack:
> 
> 	kunmap_atomic(kaddr);
> 
> This becomes simpler too.
> 
> Now, a stack can be overflown by imbalance - but that's easy to 
> detect and existing entries are easily printed and thus the source 
> of the leak is easily identified.
> 
> In my book this design beats the current enumeration of kmap types 
> indices hands down ... It would likely be much more robust as well, 
> and much more easy to extend.
> 
> Am i missing any subtlety?

The above is mostly debug code used to validate the kmap_atomic
conditions.

KM_CTX_NMI nests in KM_CTX_IRQ nests in KM_CTX_SOFTIRQ nests in
KM_CTX_USER.

And validate that we indeed are in the context specified by the type.
That is, it will warn if we use KM_IRQ1 with KM_CTX_IRQ from user
context.

Some of this was already captured in the old kmap debug code which I
removed.

But yes, I should write that nicer..

/me goes clean up
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