While working on 64K pages, I found this little buglet in our
update_mmu_cache() implementation.

The code calls __hash_page() passing it an "access" parameter (the type
of access that triggers the hash) containing the bits _PAGE_RW and
_PAGE_USER of the linux PTE.  The latter is useless in this case and the
former is wrong.  In fact, if we have a writeable PTE and we pass
_PAGE_RW to hash_page(), it will set _PAGE_DIRTY (since we track dirty
that way, by hash faulting !dirty) which is not what we want.

In fact, the correct fix is to always pass 0. That means that only
read-only or already dirty read write PTEs will be preloaded. The
(hopefully rare) case of a non dirty read write PTE can't be preloaded
this way, it will have to fault in hash_page on the actual access.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>