Commit 66aebce747eaf added code to avoid a race condition by
elevating the page refcount in hugetlb_fault() while calling
hugetlb_cow(). However, one code path in hugetlb_cow() includes
an assertion that the page count is 1, whereas it may now also
have the value 2 in this path.
Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
---
We discovered this while testing the original path; one particular
application triggered this due to the specific number of huge pages
it started with.
mm/hugetlb.c | 9 ++++++++-
1 files changed, 8 insertions(+), 1 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index cd65cb1..d5b0254 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2498,7 +2498,14 @@ retry_avoidcopy:
if (outside_reserve) {
BUG_ON(huge_pte_none(pte));
if (unmap_ref_private(mm, vma, old_page, address)) {
- BUG_ON(page_count(old_page) != 1);
+ /*
+ * Page refcount may be 1 in the common case,
+ * but since we may do an extra get_page()
+ * when called from hugetlb_fault(), we allow
+ * a page refcount of 2 as well.
+ */
+ BUG_ON(page_count(old_page) != 1 &&
+ page_count(old_page) != 2);
BUG_ON(huge_pte_none(pte));
spin_lock(&mm->page_table_lock);
ptep = huge_pte_offset(mm, address & huge_page_mask(h));
--
1.6.5.2
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