mm: migrate: fix an incorrect call of prep_transhuge_page()

In https://lkml.org/lkml/2017/11/20/411, Andrea reported that during
memory hotplug/hot remove prep_transhuge_page() is called incorrectly on
non-THP pages for migration, when THP is on but THP migration is not
enabled.  This leads to a bad state of target pages for migration.

By inspecting the code, if called on a non-THP, prep_transhuge_page()
will

 1) change the value of the mapping of (page + 2), since it is used for
    THP deferred list;

 2) change the lru value of (page + 1), since it is used for THP's dtor.

Both can lead to data corruption of these two pages.

Andrea said:
 "Pragmatically and from the point of view of the memory_hotplug subsys,
  the effect is a kernel crash when pages are being migrated during a
  memory hot remove offline and migration target pages are found in a
  bad state"

This patch fixes it by only calling prep_transhuge_page() when we are
certain that the target page is THP.

Link: http://lkml.kernel.org/r/20171121021855.50525-1-zi.yan@sent.com
Fixes: 8135d8926c ("mm: memory_hotplug: memory hotremove supports thp migration")
Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
Reported-by: Andrea Reale <ar@linux.vnet.ibm.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: <stable@vger.kernel.org>	[4.14]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/migrate.h

@@ -54,7 +54,7 @@ static inline struct page *new_page_nodemask(struct page *page,
 	new_page = __alloc_pages_nodemask(gfp_mask, order,
 				preferred_nid, nodemask);
 
-	if (new_page && PageTransHuge(page))
+	if (new_page && PageTransHuge(new_page))
 		prep_transhuge_page(new_page);
 
 	return new_page;