linux-kernel - Re: early fixmap causes kmap breakage

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Date:	Mon, 29 Dec 2008 22:22:12 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...e.hu,
	ijc@...lion.org.uk
Subject: Re: early fixmap causes kmap breakage

Nick Piggin <npiggin@...e.de> writes:

> 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.

Where is this?

>> > 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?

Why would we want or need to?

>> 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.

Hmm.

I have taken a quick skim and I am not seeming the part of the code you are
talking about.  Is the problem code in mainline?

I'm guessing it has something to do with reserve_top_address() being called
with a bad value in the normal case.  But I don't see it being called
at all in the normal case.

Eric

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