linux-kernel - Re: [PATCH] i386: fix assumed to be contiguous leaf page tables for kmap

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Date:	Fri, 16 Jan 2009 13:47:34 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <jbeulich@...ell.com>
Cc:	tglx@...utronix.de, hpa@...or.com, Nick Piggin <npiggin@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386: fix assumed to be contiguous leaf page tables
	for kmap_atomic region (take 2)


* Jan Beulich <jbeulich@...ell.com> wrote:

> Debugging and original patch from Nick Piggin <npiggin@...e.de>
> 
> The early fixmap pmd entry inserted at the very top of the KVA is causing the
> subsequent fixmap mapping code to not provide physically linear pte pages over
> the kmap atomic portion of the fixmap (which relies on said property to
> calculate pte addresses).
> 
> This has caused weird boot failures in kmap_atomic much later in the boot
> process (initial userspace faults) on a 32-bit PAE system with a larger number
> of CPUs (smaller CPU counts tend not to run over into the next page so don't
> show up the problem).
> 
> Solve this by attempting to clear out the page table, and copy any of its
> entries to the new one. Also, add a bug if a nonlinear condition is encountered
> and can't be resolved, which might save some hours of debugging if this fragile
> scheme ever breaks again...
> 
> Signed-off-by: Nick Piggin <npiggin@...e.de>
> 
> Once we have such logic, we can also use it to eliminate the early ioremap
> trickery around the page table setup for the fixmap area. This also fixes
> potential issues with FIX_* entries sharing the leaf page table with the early
> ioremap ones getting discarded by early_ioremap_clear() and not restored by
> early_ioremap_reset(). It at once eliminates the temporary (and configuration,
> namely NR_CPUS, dependent) unavailability of early fixed mappings during the
> time the fixmap area page tables get constructed.
> 
> Finally, also replace the hard coded calculation of the initial table space
> needed for the fixmap area with a proper one, allowing kernels configured for
> large CPU counts to actually boot.
> 
> Signed-off-by: Jan Beulich <jbeulich@...ell.com>
> 
> ---
>  arch/x86/include/asm/io.h |    1 
>  arch/x86/mm/init_32.c     |   48 +++++++++++++++++++++++++++++++++++++++++++---
>  arch/x86/mm/ioremap.c     |   25 -----------------------
>  3 files changed, 45 insertions(+), 29 deletions(-)

Much cleaner - applied to tip/x86/urgent, thanks Jan!

	Ingo
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