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xen-devel
Re: [Xen-devel] [Linux PATCH] Fix to hugepages to work around new PWT ha
On Thursday, June 24, 2010, Jeremy Fitzhardinge wrote:
> > Um, this is the upper level code. The entire purpose of make_huge_pte is
> > to construct a present huge pte from page and pgprot. The problem is
> > that the original code makes the pte, then sets the present bit via
> > pte_mkhuge(). This means the Xen-specific macro that triggers on
> > present is misled and doesn't do the pfn_to_mfn(). Without this patch
> > hugepages is handing pfns to the hypervisor to map instead of mfns.
> >
> >
>
> In principle, setting present should cause the pte to be converted from
> pfn to mfn, but I don't think that ever happens with normal ptes (since
> non-present ptes contain swap info). But I don't see where a huge pte
> gets present set; pte_mkhuge itself doesn't do anything except set PSE.
Wow. I just dug through the code. The landscape has sure changed since the
last time I followed this path.
It used to be that vma->vm_page_prot only contained the various read/write
flags for that vma. At that time pte_mkhuge() did in fact add _PAGE_PRESENT|
_PAGE_PSE to the pte.
Now it appears that vma->vm_page_prot does include _PAGE_PRESENT in all its
various states. So this part of the patch is in fact unnecessary.
It's what I get for not rechecking my facts to be sure they haven't changed.
Sorry.
Dave McCracken
Oracle Corp.
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