Revert "mm: avoid data corruption on CoW fault into PFN-mapped VMA"

This reverts commit e7466cc.

mm/memory.c

@@ -2173,11 +2173,11 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
 	 * On architectures with software "accessed" bits, we would
 	 * take a double page fault, so mark it accessed here.
 	 */
-	if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
+	force_mkyoung = arch_faults_on_old_pte() && !pte_young(vmf->orig_pte);
+	if (force_mkyoung) {
 		pte_t entry;
 
 		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
-		locked = true;
 		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
 			/*
 			 * Other thread has already handled the fault
@@ -2201,31 +2201,12 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
 	 * zeroes.
 	 */
 	if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
-		if (locked)
-			goto warn;
-
-		/* Re-validate under PTL if the page is still mapped */
-		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
-		locked = true;
-		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
-			/* The PTE changed under us. Retry page fault. */
-			ret = false;
-			goto pte_unlock;
-		}
-
 		/*
-		 * The same page can be mapped back since last copy attampt.
-		 * Try to copy again under PTL.
+		 * Give a warn in case there can be some obscure
+		 * use-case
 		 */
-		if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
-			/*
-			 * Give a warn in case there can be some obscure
-			 * use-case
-			 */
-warn:
-			WARN_ON_ONCE(1);
-			clear_page(kaddr);
-		}
+		WARN_ON_ONCE(1);
+		clear_page(kaddr);
 	}
 
 	ret = true;