Re: [PATCH 5/6] kvm, x86: use ro page and don't copy shared page

Gleb Natapov
2010-07-29T05:56:52Z

On Thu, Jul 29, 2010 at 10:15:22AM +0800, Lai Jiangshan wrote:
> On 07/16/2010 03:19 PM, Gleb Natapov wrote:
>
> >> +/* get a current mapped page fast, and test whether the page is writable. */
> >> +static struct page *get_user_page_and_protection(unsigned long addr,
> >> + int *writable)
> >> +{
> >> + struct page *page[1];
> >> +
> >> + if (__get_user_pages_fast(addr, 1, 1, page) == 1) {
> >> + *writable = 1;
> >> + return page[0];
> >> + }
> >> + if (__get_user_pages_fast(addr, 1, 0, page) == 1) {
> >> + *writable = 0;
> >> + return page[0];
> >> + }
> >> + return NULL;
> >> +}
> >> +
> >> +static pfn_t kvm_get_pfn_for_page_fault(struct kvm *kvm, gfn_t gfn,
> >> + int write_fault, int *host_writable)
> >> +{
> >> + unsigned long addr;
> >> + struct page *page;
> >> +
> >> + if (!write_fault) {
> >> + addr = gfn_to_hva(kvm, gfn);
> >> + if (kvm_is_error_hva(addr)) {
> >> + get_page(bad_page);
> >> + return page_to_pfn(bad_page);
> >> + }
> >> +
> >> + page = get_user_page_and_protection(addr, host_writable);
> >> + if (page)
> >> + return page_to_pfn(page);
> >> + }
> >> +
> >> + *host_writable = 1;
> >> + return kvm_get_pfn_for_gfn(kvm, gfn);
> >> +}
> >> +
> > kvm_get_pfn_for_gfn() returns fault_page if page is mapped RO, so caller
> > of kvm_get_pfn_for_page_fault() and kvm_get_pfn_for_gfn() will get
> > different results when called on the same page. Not good.
> > kvm_get_pfn_for_page_fault() logic should be folded into
> > kvm_get_pfn_for_gfn().
> >
>
>
> The different results are the things we just need.
How so? Users of kvm_get_pfn_for_gfn() will think that page is invalid
and may report misconfiguration to userspace and users of
kvm_get_pfn_for_page_fault() will think that the access to page is OK.
There are no many users of kvm_get_pfn_for_gfn() and may be your patch
replace all of them with kvm_get_pfn_for_page_fault(), but this just
strengthen the point that they should be merged.

> We don't want to copy and write a page which is mapped RO when
> only read fault.
I don't see how returning inconsistent results helps us achieving that.

BTW since kvm_get_pfn_for_gfn() will never map RO page
get_user_page_and_protection() will never find any RO pages. Looks like
kvm_get_pfn_for_page_fault() is equivalent to kvm_get_pfn_for_gfn()
since !write_fault section will at best find mapped RW page.

--
Gleb.