Re: [rfc][patch 1/4] include: add callbacks to toggle reference counting for VM_MIXEDMAP pages

["Jared Hulbert" <jaredeh@gmail.com>]

On Jan 9, 2008 7:14 AM, Carsten Otte <cotte@de.ibm.com> wrote:
> From: Carsten Otte <cotte@de.ibm.com>
>
> include: add callbacks to toggle reference counting for VM_MIXEDMAP pages
>
> This patch introduces two arch callbacks, which may optionally be implemented
> in case the architecutre does define __HAVE_ARCH_PTEP_NOREFCOUNT.
>
> The first callback, pte_set_norefcount(__pte) is called by core-vm to indicate
> that subject page table entry is going to be inserted into a VM_MIXEDMAP vma.
> default implementation:         noop
> s390 implementation:            set sw defined bit in pte
> proposed arm implementation:    noop
>
> The second callback, mixedmap_refcount_pte(__pte) is called by core-vm to
> figure out whether or not subject pte requires reference counting in the
> corresponding struct page entry. A non-zero result indicates reference counting
> is required.
> default implementation:         (1)

I think this should be:

default implementation:   convert pte_t to pfn, use pfn_valid()

Keep in mind the reason we are talking about using anything other than
pfn_valid() in vm_normal_page() is because s390 has a non-standard
pfn_valid() implementation.  It's s390 that's broken, not the rest of
the world.  So lets not break everything else to fix s390:)  Or am I
missing something?

> s390 implementation:            query sw defined bit in pte
> proposed arm implementation:    convert pte_t to pfn, use pfn_valid()

proposed arm implementation: default

> Signed-off-by: Carsten Otte <cotte@de.ibm.com>
> ---
> Index: linux-2.6/include/asm-generic/pgtable.h
> ===================================================================
> --- linux-2.6.orig/include/asm-generic/pgtable.h
> +++ linux-2.6/include/asm-generic/pgtable.h
> @@ -99,6 +99,11 @@ static inline void ptep_set_wrprotect(st
>  }
>  #endif
>
> +#ifndef __HAVE_ARCH_PTEP_NOREFCOUNT
> +#define pte_set_norefcount(__pte)      (__pte)
> +#define mixedmap_refcount_pte(__pte)   (1)

+#define mixedmap_refcount_pte(__pte)   pfn_valid(pte_pfn(__pte))

Should we rename "mixedmap_refcount_pte" to "mixedmap_normal_pte" or
something else more neutral?  To me "mixedmap_refcount_pte" sounds
like it's altering the pte.

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