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

From: Nick Piggin
Date: Wed Jun 17 2009 - 03:47:32 EST

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

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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