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

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Message-ID: <20090616052952.GA2924@wotan.suse.de>
Date:	Tue, 16 Jun 2009 07:29:52 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	torvalds@...ux-foundation.org, 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 03:18:06PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2009-06-16 at 06:46 +0200, 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.
> 
> About all of the ones below and more :-) It all depends what you define
> by "early" though :-)
> 
> > >  - All the vmalloc interfaces (__get_vm_area() and it's 5 or 6 variants)
> > >  - Allocation of PCI host bridges data structures in the powerpc code
> > >  - Allocation of interrupt controller domains in the powerpc code
> > >  - Page table allocations (oops ... can't change that arch specific,
> > > would have to be a generic change)
> > 
> > No it would not. If an arch (eg s390) does this in early boot, then
> > it can create its own allocation function which takes a gfp mask, and
> > define the generic one to just pass it a GFP_KERNEL. generic code does
> > not call this in early boot of course.
> 
> Right, which means that ioremap needs a special code path early boot
> since it uses this etc...

You could do it in parts. Having vmalloc layer test for early boot
would capture a lot of these.

 
> We are basically adding special-cases in a whole lot of places, which
> could -ALL- be removed just by having the allocator do the "right
> thing" :-)

I don't know that we're adding special cases. Simply setting up
slab earlier does not require anything more from anyone. In
fact it allows less special case code.

I'm just not convinced that teaching the slab allocator about
early boot context is the right thing to do, so I ask that we
not throw away this context information we already have, if
possible. 

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