linux-kernel - Re: early fixmap causes kmap breakage

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Date:	Tue, 30 Dec 2008 07:35:24 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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

On Mon, Dec 29, 2008 at 10:22:12PM -0800, Eric W. Biederman wrote:
> 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?

Where is what?

 
> >> > 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?

So the fixmap allocator allocates contiguous pagetables. Wasn't my
description clear enough?

 
> >> 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.

Yes I think so. one_page_table_init will not allocate a new pte page if
one already exists, so it will not be contiguous with previous/subsequent
pages with the early fixmap pte there. page_table_range_init explains why
that is a problem (this code seems terribly fragile and prone to breakage
but I don't want to expend theeffort on it -- still, I have a patch to
panic the kernel if it breaks its guarantee of contiguous page tables,
which makes breakage easily visible).


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