Re: [PATCH 3/4] KVM: MMU: track dirty page in speculative path properly

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jul 14, 2010 at 09:18:58PM +0800, Xiao Guangrong wrote:
> 
> 
> Marcelo Tosatti wrote:
> > On Wed, Jul 14, 2010 at 09:24:22AM +0800, Xiao Guangrong wrote:
> >>
> >> Marcelo Tosatti wrote:
> >>> On Tue, Jul 13, 2010 at 05:45:27PM +0800, Xiao Guangrong wrote:
> >>>> In speculative path, the page is not real write-access, no need mark it
> >>>> dirty, so clear dirty bit in this path and later examine this bit when
> >>>> we release the page
> >>>>
> >>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> >>>> ---
> >>>>  arch/x86/kvm/mmu.c |   24 +++++++++++-------------
> >>>>  1 files changed, 11 insertions(+), 13 deletions(-)
> >>> Unfortunately all pages that kvm creates translations for are marked
> >>> dirty due to get_user_pages(w=1), except KSM which makes them read-only
> >>> later.
> >> Marcelo, i have looked into get_user_pages() function, but not catch where
> >> to make page dirty, could you point it out for me? :-)
> > 
> > See set_page_dirty call in mm/memory.c::follow_page.
> 
> Yeah, you are right, and i want to use another way to do it since track dirty bit
> is too heavy, also it's dangerous if we miss to set page dirty.
> 
> How about just track access bit for speculative path, we set page both accessed and
> dirty(if it's writable) only if the access bit is set?

A useful thing to do would be to allow read-only mappings, in the fault
path (Lai sent a few patches in that direction sometime ago but there
was no follow up).

So in the case of a read-only fault from the guest, you'd inform
get_user_pages() that read-only access is acceptable (so swapcache pages
can be mapped, or qemu can mprotect(PROT_READ) guest memory).