[patch 22/28] mm: migrate: fix an incorrect call of prep_transhuge_page()

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From: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
Subject: 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@xxxxxxxx
Fixes: 8135d8926c08 ("mm: memory_hotplug: memory hotremove supports thp migration")
Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
Reported-by: Andrea Reale <ar@xxxxxxxxxxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.14]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 include/linux/migrate.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN include/linux/migrate.h~mm-migrate-fix-an-incorrect-call-of-prep_transhuge_page include/linux/migrate.h
--- a/include/linux/migrate.h~mm-migrate-fix-an-incorrect-call-of-prep_transhuge_page
+++ a/include/linux/migrate.h
@@ -54,7 +54,7 @@ static inline struct page *new_page_node
 	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;
_
--
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