Re: [PATCH] fix/improve generic page table walker

[Matt Mackall <mpm@selenic.com>]

On Wed, 2009-03-11 at 14:49 +0100, Martin Schwidefsky wrote:
> From: Martin Schwidefsky <schwidefsky@de.ibm.com>
> 
> On s390 the /proc/pid/pagemap interface is currently broken. This is
> caused by the unconditional loop over all pgd/pud entries as specified
> by the address range passed to walk_page_range. The tricky bit here
> is that the pgd++ in the outer loop may only be done if the page table
> really has 4 levels. For the pud++ in the second loop the page table needs
> to have at least 3 levels. With the dynamic page tables on s390 we can have
> page tables with 2, 3 or 4 levels. Which means that the pgd and/or the
> pud pointer can get out-of-bounds causing all kinds of mayhem.

Not sure why this should be a problem without delving into the S390
code. After all, x86 has 2, 3, or 4 levels as well (at compile time) in
a way that's transparent to the walker.

> The proposed solution is to fast-forward over the hole between the start
> address and the first vma and the hole between the last vma and the end
> address. The pgd/pud/pmd/pte loops are used only for the address range
> between the first and last vma. This guarantees that the page table
> pointers stay in range for s390. For the other architectures this is
> a small optimization.

I've gone to lengths to keep VMAs out of the equation, so I can't say
I'm excited about this solution.

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