[ofa-general] Re: [PATCH] mmu notifier #v11

[Andrea Arcangeli <andrea@qumranet.com>]

On Fri, Apr 04, 2008 at 03:06:18PM -0700, Christoph Lameter wrote:
> Adds some comments. Still objectionable is the multiple ways of
> invalidating pages in #v11. Callout now has similar locking to emm.

range_begin exists because range_end is called after the page has
already been freed. invalidate_page is called _before_ the page is
freed but _after_ the pte has been zapped.

In short when working with single pages it's a waste to block the
secondary-mmu page fault, because it's zero cost to invalidate_page
before put_page. Not even GRU need to do that.

Instead for the multiple-pte-zapping we have to call range_end _after_
the page is already freed. This is so that there is a single range_end
call for an huge amount of address space. So we need a range_begin for
the subsystems not using page pinning for example. When working with
single pages (try_to_unmap_one, do_wp_page) invalidate_page avoids to
block the secondary mmu page fault, and it's in turn faster.

Besides avoiding need of serializing the secondary mmu page fault,
invalidate_page also reduces the overhead when the mmu notifiers are
disarmed (i.e. kvm not running).