ptep_establish() is used to establish a new mapping at COW time,
and it always replaces a non-writable page mapping with a totally
new page mapping that is dirty (and likely writable, although ptrace
may cause a non-writable new mapping). Because it was nonwritable,
we don't have to worry about losing concurrent dirty page bit updates.

ptep_update_access_flags() leaves the same page mapping, but updates
the accessed/dirty/writable bits (it only ever sets them, and never
removes any permissions). Often easier, but it may race with a dirty
bit update on another CPU.

Booted on x86 and ppc64.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>