Re: [Linux PATCH] Fix to hugepages to work around new PWT handling

[Dave McCracken <dcm@mccr.org>]

On Wednesday, June 09, 2010, Jeremy Fitzhardinge wrote:
> >>> An additional fix here is to make sure the _PAGE_PRESENT bit is set
> >>> before hugepages does a mk_pte(), since Xen depends on that bit to
> >>> trigger the pfn->mfn translation.
> >>>
> >>>   
> >>>       
> >>>
> >> Why is the kernel creating a non-present mapping?  If it isn't present,
> >> why does it matter whether we do the pfn->mfn conversion?
> >>
> >>     
> >>
> > The hugepage function make_huge_pte() called mk_pte() to turn a page and
> > a  pgprot into a pte before it set PRESENT.  The PRESENT flag was set
> > after the pte was made.  This meant that the Xen version of the macro
> > did not see PRESENT so did not do the pfn_to_mfn().  My patch sets
> > PRESENT first so the right thing will happen.
> >
> >   
> 
> But in general kernel code shouldn't be just nakedly setting present on
> the pte without also remaking the whole thing.  That doesn't happen with
> normal ptes, and it probably shouldn't happen with huge ptes.  Forcing
> present on a pte at this level seems very bogus.   Why not change the
> upper code to set present if that's want it wants?
> 
> I'll skip this chunk for now.

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.

Dave