[PATCH] fixup! powerpc/mm: switch device DAX to shared tail vmemmap pages

[Muchun Song]

The PowerPC radix path takes one reference for every PTE that maps the
shared tail page. Unlike the generic path, get_page() lets the counter
continue after it becomes non-positive and eventually cycle to zero.

Use try_get_page() so vmemmap population fails before the shared page
reference count can cycle, matching the generic implementation.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 arch/powerpc/mm/book3s64/radix_pgtable.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 9ca28e4a610a..a3332f32ffb2 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -1042,13 +1042,17 @@ static pte_t * __meminit radix__vmemmap_pte_populate(pmd_t *pmdp, unsigned long
 			/*
 			 * When a PTE/PMD entry is freed from the init_mm
 			 * there's a free_pages() call to this page allocated
-			 * above. Thus this get_page() is paired with the
+			 * above. Thus this try_get_page() is paired with the
 			 * put_page_testzero() on the freeing path.
 			 * This can only called by certain ZONE_DEVICE path,
 			 * and through vmemmap_populate_compound_pages() when
 			 * slab is available.
+			 *
+			 * Use try_get_page() to prevent the shared page refcount
+			 * from overflowing.
 			 */
-			get_page(reuse);
+			if (!try_get_page(reuse))
+				return NULL;
 			p = page_to_virt(reuse);
 			pr_debug("Tail page reuse vmemmap mapping\n");
 		}
-- 
2.54.0