mm: check against orig_pte for finish_fault()

mm: check against orig_pte for finish_fault()

We used to check against none pte in finish_fault(), with the assumption
that the orig_pte is always none pte.

This change prepares us to be able to call do_fault() on !none ptes.  For
example, we should allow that to happen for pte marker so that we can
restore information out of the pte markers.

Let's change the "pte_none" check into detecting changes since we fetched
orig_pte.  One trivial thing to take care of here is, when pmd==NULL for
the pgtable we may not initialize orig_pte at all in handle_pte_fault().

By default orig_pte will be all zeros however the problem is not all
architectures are using all-zeros for a none pte.  pte_clear() will be the
right thing to use here so that we'll always have a valid orig_pte value
for the whole handle_pte_fault() call.

Link: https://lkml.kernel.org/r/20220405014836.14077-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>