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

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Message-Id: <1245092433.6741.201.camel@laptop>
Date:	Mon, 15 Jun 2009 21:00:33 +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 20:52 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Mon, 2009-06-15 at 20:42 +0200, Ingo Molnar wrote:
> > > * 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 ...
> > 
> > How will you manage the max depth?
> 
> 	if (++depth == MAX_DEPTH) {
> 		print_all_entries_and_nasty_warning();
> 		 /* hope we'll live long enough for the syslog to touch disk */
> 		depth = 0;
> 	}

That will only trigger if we hit it, which will be _very_ rare.

> unbalanced kmap is a bad bug - the easier we make it to catch, the 
> better. The system wouldnt survive anyway.

My proposed patch validates strict balance of types. But I can easily
add the above as well.

By removing the types it becomes very difficult to verify the max depth.
I really don't like removing them.



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