On 2/28/22 05:27, David Hildenbrand wrote:
...

quoted

diff --git a/mm/gup.c b/mm/gup.c
index 5c3f6ede17eb..44446241c3a9 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -3034,6 +3034,40 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
  }
  EXPORT_SYMBOL(pin_user_pages);
  
+/**
+ * pin_user_page() - apply a FOLL_PIN reference to a page ()
+ *
+ * @page: the page to be pinned.
+ *
+ * Similar to get_user_pages(), in that the page's refcount is elevated using
+ * FOLL_PIN rules.
+ *
+ * IMPORTANT: That means that the caller must release the page via
+ * unpin_user_page().
+ *
+ */
+void pin_user_page(struct page *page)
+{
+	struct folio *folio = page_folio(page);
+
+	WARN_ON_ONCE(folio_ref_count(folio) <= 0);
+
+	/*
+	 * Similar to try_grab_page(): be sure to *also*
+	 * increment the normal page refcount field at least once,
+	 * so that the page really is pinned.
+	 */
+	if (folio_test_large(folio)) {
+		folio_ref_add(folio, 1);
+		atomic_add(1, folio_pincount_ptr(folio));
+	} else {
+		folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
+	}
+
+	node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
+}
+EXPORT_SYMBOL(pin_user_page);
+
  /*
   * pin_user_pages_unlocked() is the FOLL_PIN variant of
   * get_user_pages_unlocked(). Behavior is the same, except that this one sets

I assume that function will only get called on a page that has been
obtained by a previous pin_user_pages_fast(), correct?

Well, no. This is meant to be used in place of get_page(), for code that
knows that the pages will be released via unpin_user_page(). So there is
no special prerequisite there.


thanks,
-- 
John Hubbard
NVIDIA