Re: unit tests and get_user_pages_ptes_fast()

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Oct 05, 2010 at 09:36:59AM +0200, Avi Kivity wrote:
>  On 10/05/2010 01:59 AM, Marcelo Tosatti wrote:
> >Yep, the drawback is the unnecessary write fault. What i have here is:
> >
> >--- kvm.orig/virt/kvm/kvm_main.c
> >+++ kvm/virt/kvm/kvm_main.c
> >@@ -827,7 +827,7 @@ unsigned long gfn_to_hva(struct kvm *kvm
> >  }
> >  EXPORT_SYMBOL_GPL(gfn_to_hva);
> >
> >-pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t gfn)
> >+pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t gfn, int *writable)
> >  {
> >         struct page *page[1];
> >         unsigned long addr;
> >@@ -842,8 +842,16 @@ pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t
> >                 return page_to_pfn(bad_page);
> >         }
> >
> >+       *writable = 1;
> >         npages = get_user_pages_fast(addr, 1, 1, page);
> >
> >+       /* attempt to map read-only */
> >+       if (unlikely(npages != 1)) {
> >+               npages = get_user_pages_fast(addr, 1, 0, page);
> >+               if (npages == 1)
> >+                       *writable = 0;
> >+       }
> >+
> >         if (unlikely(npages != 1)) {
> >                 struct vm_area_struct *vma;
> >
> >Can rebase and resend, if you'd like.
> >
> 
> That will work for me but not for ksm.  I guess it's good to get
> things going, so please to post it.

It'll not be so advantageous for ksm because there should be read-faults
very rarely on that case.

Will post.