Hi Ingo,
On Tue, Nov 13, 2012 at 10:54:17AM +0100, Ingo Molnar wrote:
>
> * Mel Gorman <[email protected]> wrote:
>
> > From: Andrea Arcangeli <[email protected]>
> >
> > Implement pte_numa and pmd_numa.
> >
> > <Changlog SNIP>
> > ---
> > arch/x86/include/asm/pgtable.h | 65
> > ++++++++++++++++++++++++++++++++++++++--
> > include/asm-generic/pgtable.h | 12 ++++++++
> > 2 files changed, 75 insertions(+), 2 deletions(-)
> >
> > <Patch SNIP>
>
> Hm, this overcomplicates things quite a bit and adds arch
> specific code, and there's no explanation given for that
> approach that I can see?
>
So there are two possible problems here - the PTE flag naming and how
it's implemented.
On the PTE flag naming front, the changelog explains the disadvantages
to using PROT_NONE and this arrangement allows an architecture to make a
better decision if one is available. The relevant parts of the changelog are
_PAGE_NUMA on x86 shares the same bit number of _PAGE_PROTNONE (but
it could also use a different bitflag, it's up to the architecture
to decide).
and
Sharing the same bitflag with _PAGE_PROTNONE in fact complicates
things: it requires us to ensure the code paths executed by
_PAGE_PROTNONE remains mutually exclusive to the code paths executed
by _PAGE_NUMA at all times, to avoid _PAGE_NUMA and _PAGE_PROTNONE
to step into each other toes.
so I'd like to keep that. Any major objections?
> Basically, what's wrong with the generic approach that numa/core
> has:
>
> __weak bool pte_numa(struct vm_area_struct *vma, pte_t pte)
>
> [see the full function below.]
>
> Then we can reuse existing protection-changing functionality and
> keep it all tidy.
>
I very much like this idea of this approach. Superficially I see nothing
wrong with it. I just didn't think of it when I was trying to resolve
the two trees together.
> an architecture that wants to do something special could
> possibly override it in the future - but we want to keep the
> generic logic in generic code.
>
Sensible and probably less mess in the future.
> __weak bool pte_numa(struct vm_area_struct *vma, pte_t pte)
> {
I'll lift this and see can it be modified to use _PAGE_NUMA instead of
hard-coding for PROT_NONE. Of course if you beat me to it and send a patch,
that'd be cool too :)
Thanks!
--
Mel Gorman
SUSE Labs
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