From: Zi Yan zi.yan@cs.rutgers.edu 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@sent.com Fixes: 8135d8926c08 ("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 ---

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; _