On Fri, Nov 16, 2012 at 06:12:43PM +0100, Ingo Molnar wrote:
>
> * Mel Gorman <mgorman@xxxxxxx> wrote:
>
> > > Why not use something what we have in numa/core already:
> > >
> > > f05ea0948708 mm/mpol: Create special PROT_NONE infrastructure
> > >
> >
> > Because it's hard-coded to PROT_NONE underneath which I've
> > complained about before. [...]
>
> To which I replied that this is the current generic
> implementation, the moment some different architecture comes
> around we can accomodate it - on a strictly as-needed basis.
>
To which I responded that a new architecutre would have to retrofit and
then change callers like change_prot_none() which is more churn than should
be necessary to add architecture support.
> It is *better* and cleaner to not expose random arch hooks but
> let the core kernel modification be documented in the very patch
> that the architecture support patch makes use of it.
>
And yours requires that arches define pmd_pgprot so there are additional
hooks anyway.
That said, your approach just ends up being heavier. Take this simple
case for what we need for pte_numa.
+static inline pgprot_t vma_prot_none(struct vm_area_struct *vma)
+{
+ /*
+ * obtain PROT_NONE by removing READ|WRITE|EXEC privs
+ */
+ vm_flags_t vmflags = vma->vm_flags & ~(VM_READ|VM_WRITE|VM_EXEC);
+ return pgprot_modify(vma->vm_page_prot, vm_get_page_prot(vmflags));
+}
...
+static bool pte_numa(struct vm_area_struct *vma, pte_t pte)
+{
+ /*
+ * For NUMA page faults, we use PROT_NONE ptes in VMAs with
+ * "normal" vma->vm_page_prot protections. Genuine PROT_NONE
+ * VMAs should never get here, because the fault handling code
+ * will notice that the VMA has no read or write permissions.
+ *
+ * This means we cannot get 'special' PROT_NONE faults from genuine
+ * PROT_NONE maps, nor from PROT_WRITE file maps that do dirty
+ * tracking.
+ *
+ * Neither case is really interesting for our current use though so we
+ * don't care.
+ */
+ if (pte_same(pte, pte_modify(pte, vma->vm_page_prot)))
+ return false;
+
+ return pte_same(pte, pte_modify(pte, vma_prot_none(vma)));
+}
pte_numa requires a call to vma_prot_none which requires a function call
to vm_get_page_prot.
This is the _PAGE_NUMA equivalent.
+__weak int pte_numa(pte_t pte)
+{
+ return (pte_flags(pte) &
+ (_PAGE_NUMA|_PAGE_PRESENT)) == _PAGE_NUMA;
+}
If that was moved to inline as Linus suggests, it becomes one, maybe two
instructions.
--
Mel Gorman
SUSE Labs
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