linux-kernel - Re: early fixmap causes kmap breakage

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Date:	Tue, 30 Dec 2008 05:01:18 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...e.hu, ijc@...lion.org.uk
Subject: Re: early fixmap causes kmap breakage

On Mon, Dec 29, 2008 at 03:17:31PM -0800, Andrew Morton wrote:
> On Thu, 18 Dec 2008 22:15:43 +0100
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > Hi,
> > 
> > I've debugged a problem where i386+pae systems with more than a few CPUs
> > blow up at boot in the kmap_atomic code.
> 
> ping?

No further progress here, I'm waiting on input for how to fix this
"nicely". Meantime, clearing the early fixmap pte I guess works, but
you lose a page... is it possible to put it into .initdata or is
there some issue with that? (I guess on a PAE kernel, 4K isn't a
big deal).

 
> > The problem is that the kmap_atomic pte pages all need to be contiguous
> > memory because the pte is calculated via the first kmap pte page + an
> > offset (so as not to have to walk the page tables every time).
> > 
> > The fixmap setup code crudely allocates contiguous pte pages, which is fine,
> > but if it finds an already populated pmd entry, then it will not switch it
> > to a new, contiguous pte page. So the early fixmap introduces a discontig
> > page table right in the middle of the kmap atomic fixmaps.
> > 
> > Commenting out the eaarly fixmap setup in head_32.S gets everything working
> > properly. What would be the best way to fix this? Could we put the early
> > fixmap page table in initdata, and then have the fixmap setup proper first
> > clear its corresponding pmd entry?
> 
> How come users/testers aren't reporting this?

Because apparently nobody tests 32-bit PAE systems with more than a couple
of CPUs anymore. This bug comes from HW vendor doing testing of SLES11.


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