Re: [patch 2/4] mmu_notifier: Callbacks to invalidate address ranges

[Robin Holt <holt@sgi.com>]

On Fri, Feb 01, 2008 at 04:05:08PM -0800, Christoph Lameter wrote:
> On Fri, 1 Feb 2008, Robin Holt wrote:
> 
> > On Fri, Feb 01, 2008 at 03:19:32PM -0800, Christoph Lameter wrote:
> > > On Fri, 1 Feb 2008, Robin Holt wrote:
> > > 
> > > > We are getting this callout when we transition the pte from a read-only
> > > > to read-write.  Jack and I can not see a reason we would need that
> > > > callout.  It is causing problems for xpmem in that a write fault goes
> > > > to get_user_pages which gets back to do_wp_page that does the callout.
> > > 
> > > Right. You placed it there in the first place. So we can drop the code 
> > > from do_wp_page?
> > 
> > No, we need a callout when we are becoming more restrictive, but not
> > when becoming more permissive.  I would have to guess that is the case
> > for any of these callouts.  It is for both GRU and XPMEM.  I would
> > expect the same is true for KVM, but would like a ruling from Andrea on
> > that.
> 
> do_wp_page is entered when the pte shows that the page is not writeable 
> and it makes the page writable in some situations. Then we do not 
> invalidate the remote reference.
> 
> However, when we do COW then a *new* page is put in place of the existing 
> readonly page. At that point we need to remove the remote pte that is 
> readonly. Then we install a new pte pointing to a *different* page that is 
> writable.
> 
> Are you saying that you get the callback when transitioning from a read 
> only to a read write pte on the *same* page?

I believe that is what we saw.  We have not put in any more debug
information yet.  I will try to squeze it in this weekend.  Otherwise,
I will probably have to wait until early Monday.

Thanks
Robin