[PATCH] mm: new_vma_page cannot see NULL vma for hugetlb pages

[Michal Hocko]

11c731e81bb0 (mm/mempolicy: fix !vma in new_vma_page()) has removed
BUG_ON(!vma) from new_vma_page which is partially correct because
page_address_in_vma will return EFAULT for non-linear mappings and at
least shared shmem might be mapped this way.

The patch also tried to prevent NULL ptr for hugetlb pages which is not
correct AFAICS because hugetlb pages cannot be mapped as VM_NONLINEAR
and other conditions in page_address_in_vma seem to be legit and catch
real bugs.

This patch restores BUG_ON for PageHuge to catch potential issues when
the to-be-migrated page is not setup properly.

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/mempolicy.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 9e8d2d86978a..f3f51464a23b 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1199,10 +1199,8 @@ static struct page *new_vma_page(struct page *page, unsigned long private, int *
 	}
 
 	if (PageHuge(page)) {
-		if (vma)
-			return alloc_huge_page_noerr(vma, address, 1);
-		else
-			return NULL;
+		BUG_ON(vma)
+		return alloc_huge_page_noerr(vma, address, 1);
 	}
 	/*
 	 * if !vma, alloc_page_vma() will use task or system default policy
-- 
1.8.5.2

-- 
Michal Hocko
SUSE Labs
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