[PATCH v3 3/3] hugetlbfs: add swap entry check in follow_hugetlb_page()

[Naoya Horiguchi]

With applying the previous patch "hugetlbfs: stop setting VM_DONTDUMP in
initializing vma(VM_HUGETLB)" to reenable hugepage coredump, if a memory
error happens on a hugepage and the affected processes try to access
the error hugepage, we hit VM_BUG_ON(atomic_read(&page->_count) <= 0)
in get_page().

The reason for this bug is that coredump-related code doesn't recognise
"hugepage hwpoison entry" with which a pmd entry is replaced when a memory
error occurs on a hugepage.
In other words, physical address information is stored in different bit layout
between hugepage hwpoison entry and pmd entry, so follow_hugetlb_page()
which is called in get_dump_page() returns a wrong page from a given address.

We need to filter out only hwpoison hugepages to have data on healthy
hugepages in coredump. So this patch makes follow_hugetlb_page() avoid
trying to get page when a pmd is in swap entry like format.

ChangeLog v3:
 - add comment about using is_swap_pte()

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 mm/hugetlb.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git v3.9-rc3.orig/mm/hugetlb.c v3.9-rc3/mm/hugetlb.c
index 0d1705b..3bc20bd 100644
--- v3.9-rc3.orig/mm/hugetlb.c
+++ v3.9-rc3/mm/hugetlb.c
@@ -2966,9 +2966,15 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		 * Some archs (sparc64, sh*) have multiple pte_ts to
 		 * each hugepage.  We have to make sure we get the
 		 * first, for the page indexing below to work.
+		 *
+		 * is_swap_pte test covers both is_hugetlb_entry_hwpoisoned
+		 * and hugepages under migration in which case
+		 * hugetlb_fault waits for the migration and bails out
+		 * properly for HWPosined pages.
 		 */
 		pte = huge_pte_offset(mm, vaddr & huge_page_mask(h));
-		absent = !pte || huge_pte_none(huge_ptep_get(pte));
+		absent = !pte || huge_pte_none(huge_ptep_get(pte)) ||
+			is_swap_pte(huge_ptep_get(pte));
 
 		/*
 		 * When coredumping, it suits get_dump_page if we just return
-- 
1.7.11.7

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