linux-kernel - Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL pointer dereference

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Message-Id: <1216927309.15519.253.camel@calx>
Date:	Thu, 24 Jul 2008 14:21:49 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Patrick McHardy <kaber@...sh.net>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>, w@....eu,
	davidn@...idnewall.com, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, stefanr@...6.in-berlin.de,
	rjw@...k.pl, ilpo.jarvinen@...sinki.fi,
	Dave Jones <davej@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL
	pointer dereference


On Thu, 2008-07-24 at 16:32 +0300, Pekka Enberg wrote:
> On Thu, Jul 24, 2008 at 10:56:08PM +1000, Nick Piggin wrote:
> > > > OTOH, skb allocation uses kmalloc don't they? So you could still use
> > > > SLOB ksize for that I guess.
> 
> On Thursday 24 July 2008 23:04, Herbert Xu wrote:
> > > Yes I was referring to the data portion which is kmalloc'ed.
> > > That is also why I'm interested in ksize because a priori we
> > > don't know exactly how big it's going to be.  However, we do
> > > know that statistically 1500 will dominate.
> > >
> > > I'm not interested in ksize for kmem_cache at all.  So in fact
> > > we could have something simpler that's based on kmalloc's rounding
> > > algorithm instead.
> 
> On Thu, 2008-07-24 at 23:13 +1000, Nick Piggin wrote:
> > Yes you could definitely have a function that returns allocated
> > bytes for a given kmalloc size. Should be about as fast or faster
> > than extracting the size from the kaddr...
> 
> Yup, makes sense.

On the other hand, I can imagine useful allocator changes where this
would not be a constant of requested size. For instance, imagine we had
a classless bucket allocator, but with a heuristic to try a larger
bucket when it wasn't cheap/possible to allocate a right-sized object
(because of memory pressure, etc.) and larger ones were available.
This sort of thing is a pretty small change for SLAB/SLUB.

-- 
Mathematics is the supreme nostalgia of our time.

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