Messages in this thread | | | Date | Fri, 16 Nov 2012 17:48:53 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH 06/43] mm: numa: Make pte_numa() and pmd_numa() a generic implementation |
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On Fri, Nov 16, 2012 at 06:12:43PM +0100, Ingo Molnar wrote: > > * Mel Gorman <mgorman@suse.de> 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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