powerpc: get_hugepte() don't put_page() the wrong page

powerpc: get_hugepte() don't put_page() the wrong page

"page" may have changed to point to the next hugepage after the loop
completed, The references have been taken on the head page, so the
put_page must happen there too.

This is a longstanding issue pre-thp inclusion.

It's totally unclear how these page_cache_add_speculative and pte_val(pte)
!= pte_val(*ptep) checks are necessary across all the powerpc gup_fast
code, when x86 doesn't need any of that: there's no way the page can be
freed with irq disabled so we're guaranteed the atomic_inc will happen on
a page with page_count > 0 (so not needing the speculative check).  The
pte check is also meaningless on x86: no need to rollback on x86 if the
pte changed, because the pte can still change a CPU tick after the check
succeeded and it won't be rolled back in that case.  The important thing
is we got a reference on a valid page that was mapped there a CPU tick
ago.  So not knowing the soft tlb refill code of ppc64 in great detail I'm
not removing the "speculative" page_count increase and the pte checks
across all the code, but unless there's a strong reason for it they should
be later cleaned up too.

If a pte can change from huge to non-huge (like it could happen with THP)
passing a pte_t *ptep to gup_hugepte() would also require to repeat the
is_hugepd in gup_hugepte(), but that shouldn't happen with hugetlbfs only
so I'm not altering that.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>