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


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