diff --git a/include/jemalloc/internal/jemalloc_internal.h.in b/include/jemalloc/internal/jemalloc_internal.h.in
index 5b809bfa..8ba4a19a 100644
--- a/include/jemalloc/internal/jemalloc_internal.h.in
+++ b/include/jemalloc/internal/jemalloc_internal.h.in
@@ -813,7 +813,7 @@ sa2u(size_t size, size_t alignment)
 	 * Calculate the multi-page mapping that large_palloc() would need in
 	 * order to guarantee the alignment.
 	 */
-	if (usize + large_pad + PAGE_CEILING(alignment) < usize) {
+	if (usize + large_pad + PAGE_CEILING(alignment) - PAGE < usize) {
 		/* size_t overflow. */
 		return (0);
 	}
diff --git a/src/extent_mmap.c b/src/extent_mmap.c
index 0dd3247e..23dd4f88 100644
--- a/src/extent_mmap.c
+++ b/src/extent_mmap.c
@@ -9,7 +9,7 @@ extent_alloc_mmap_slow(size_t size, size_t alignment, bool *zero, bool *commit)
 	void *ret;
 	size_t alloc_size;
 
-	alloc_size = size + alignment;
+	alloc_size = size + alignment - PAGE;
 	/* Beware size_t wrap-around. */
 	if (alloc_size < size)
 		return (NULL);