mm/gup: Increment the page refcount before the pincount

We should always increase the refcount before doing anything else to
the page so that other page users see the elevated refcount first.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

mm/gup.c

@@ -213,25 +213,23 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 	if (flags & FOLL_GET)
 		return try_get_page(page);
 	else if (flags & FOLL_PIN) {
-		int refs = 1;
-
 		page = compound_head(page);
 
 		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
 			return false;
 
-		if (hpage_pincount_available(page))
-			hpage_pincount_add(page, 1);
-		else
-			refs = GUP_PIN_COUNTING_BIAS;
-
 		/*
 		 * Similar to try_grab_compound_head(): even if using the
 		 * hpage_pincount_add/_sub() routines, be sure to
 		 * *also* increment the normal page refcount field at least
 		 * once, so that the page really is pinned.
 		 */
-		page_ref_add(page, refs);
+		if (hpage_pincount_available(page)) {
+			page_ref_add(page, 1);
+			hpage_pincount_add(page, 1);
+		} else {
+			page_ref_add(page, GUP_PIN_COUNTING_BIAS);
+		}
 
 		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
 	}