Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

[Andrea Arcangeli <andrea@qumranet.com>]

On Tue, Jan 29, 2008 at 06:28:05PM -0600, Jack Steiner wrote:
> On Tue, Jan 29, 2008 at 04:20:50PM -0800, Christoph Lameter wrote:
> > On Wed, 30 Jan 2008, Andrea Arcangeli wrote:
> > 
> > > > invalidate_range after populate allows access to memory for which ptes 
> > > > were zapped and the refcount was released.
> > > 
> > > The last refcount is released by the invalidate_range itself.
> > 
> > That is true for your implementation and to address Robin's issues. Jack: 
> > Is that true for the GRU?
> 
> I'm not sure I understand the question. The GRU never (currently) takes
> a reference on a page. It has no mechanism for tracking pages that
> were exported to the external TLBs.

If you don't have a pin, then things like invalidate_range in
remap_file_pages can't be safe as writes through the external TLBs can
keep going on pages in the freelist. For you to be safe w/o a
page-pin, you need to return in the direction of invalidate_page
inside ptep_clear_flush (or anyway before
page_cache_release/__free_page/put_page...). You're generally not safe
with any invalidate_range that may run after the page pointed by the
pte has been freed (or can be freed by the VM anytime because of being
unpinned cache).