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>

diff --git a/mm/gup.c b/mm/gup.c
index e54359ef75ac68ed82ac38f0f375390e4d7b79a5..eb7747bc7f7da79db5d4547ec2eeaf4511279844 100644 (file)
--- a/mm/gup.c
+++ b/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);
        }