diff --git a/mm/memory.c b/mm/memory.c
index 4bb3acfc3dd9c885956ec02bac7545b5f5e9947f..ead7b4eb2e40460a94a5f33b4fa03cf44e511747 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4922,6 +4922,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
 	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
 		return ret;
 
+	/* We're about to trigger CoW, so never map it through a PMD. */
+	if (is_cow_mapping(vma->vm_flags) &&
+	    (vmf->flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)))
+		return ret;
+
 	if (folio_order(folio) != HPAGE_PMD_ORDER)
 		return ret;
 	page = &folio->page;