linux-kernel - Re: [GIT PULL v2] Early SLAB fixes for 2.6.31

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Date:	Wed, 17 Jun 2009 09:47:19 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	cl@...ux-foundation.org, kamezawa.hiroyu@...fujitsu.com,
	lizf@...fujitsu.com, mingo@...e.hu, yinghai@...nel.org
Subject: Re: [GIT PULL v2] Early SLAB fixes for 2.6.31

On Tue, Jun 16, 2009 at 11:45:24AM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 16 Jun 2009, Nick Piggin wrote:
> 
> > On Tue, Jun 16, 2009 at 07:31:38AM +1000, Benjamin Herrenschmidt wrote:
> > > On Mon, 2009-06-15 at 13:23 +0200, Nick Piggin wrote:
> > > > > I think the main problem isn't necessarily init code per se, but the
> > > > > pile of -common- code that can be called both at init time and
> > > > later.
> > > > 
> > > > Just seems bogus argument. Everwhere else that does this (ie.
> > > > allocations that are called from multiple allocation contexts)
> > > > passes correct gfp flags down.
> > > 
> > > So you say we should create new variants of all these APIs that take gfp
> > > flags as arguments just because they might be called early during boot :
> > 
> > No, just create the ones that actually are called in early boot.
> 
> No.
> 
> Nick, I don't think you've follow the problems.
> 
> The thing is, we do end up wanting to do a lot of allocations, and it's 
> not even very "early" - we've already initialized all the allocators. It's 
> just that WE HAVE NOT ENABLED INTERRUPTS YET!

Right, I know.

 
> So the "hack" is to let everybody act as if everything is normal. Which it 
> pretty much is. Just use kmalloc/kfree etc, and use _all_ the regular 
> functions. Setting up the core layers so that we _can_ enable interrupts 
> involves quite a lot of random crud, they should be able to use regular 
> code.

I just don't quite see why the problem got bigger though. Doing
earlier slab allocations than we had previously is going to be
replacing even more specialised code using bootmem right? I know
there are a few hacks for this, but I don't see anywhere getting
*worse* and I don't see anywhere that is all that bad today.


> And the hack is there because we really are in a magic stage. The memory 
> management works, but it just can't do certain things yet. It's not the 
> callers that need to be changed, because the callers are usually regular 
> routines that work perfectly normally long after boot, and having to add a 
> magic "I'm now doign this during early boot" argument to the whole stack 
> is just _stupid_, when the stack itself doesn't actually care - only the 
> allocators do.

In some cases perhaps it is difficult. In others it should be
pretty natural. Lots of memory allocating paths pass gfp
a long way down the stack.

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