mips,s390,sh,sparc: gup: Work around the "COW can break either way" issue

In Linux 4.14 and 4.19 these architectures still have their own
implementations of get_user_pages_fast().  These also need to force
the write flag on when taking the fast path.

Fixes: 407faed9 ("gup: document and work around "COW can break either way" issue")
Fixes: 5e240297

 ("gup: document and work around "COW can break either way" issue")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/mips/mm/gup.c

@@ -272,7 +272,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))
 			goto slow;
-		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+		/*
+		 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+		 * because get_user_pages() may need to cause an early COW in
+		 * order to avoid confusing the normal COW routines. So only
+		 * targets that are already writable are safe to do by just
+		 * looking at the page tables.
+		 */
+		if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
 			goto slow;
 	} while (pgdp++, addr = next, addr != end);
 	local_irq_enable();

arch/s390/mm/gup.c

@@ -285,7 +285,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 
 	might_sleep();
 	start &= PAGE_MASK;
-	nr = __get_user_pages_fast(start, nr_pages, write, pages);
+	/*
+	 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+	 * because get_user_pages() may need to cause an early COW in
+	 * order to avoid confusing the normal COW routines. So only
+	 * targets that are already writable are safe to do by just
+	 * looking at the page tables.
+	 */
+	nr = __get_user_pages_fast(start, nr_pages, 1, pages);
 	if (nr == nr_pages)
 		return nr;
 

arch/sh/mm/gup.c

@@ -240,7 +240,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))
 			goto slow;
-		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+		/*
+		 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+		 * because get_user_pages() may need to cause an early COW in
+		 * order to avoid confusing the normal COW routines. So only
+		 * targets that are already writable are safe to do by just
+		 * looking at the page tables.
+		 */
+		if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
 			goto slow;
 	} while (pgdp++, addr = next, addr != end);
 	local_irq_enable();

arch/sparc/mm/gup.c

@@ -262,7 +262,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))
 			goto slow;
-		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+		/*
+		 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+		 * because get_user_pages() may need to cause an early COW in
+		 * order to avoid confusing the normal COW routines. So only
+		 * targets that are already writable are safe to do by just
+		 * looking at the page tables.
+		 */
+		if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
 			goto slow;
 	} while (pgdp++, addr = next, addr != end);