[PATCH 6/7] mm/gup: Provide hook to check if __GUP_fast() is allowed for the range

["Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>]

This is preparation patch for transition of x86 to generic GUP_fast()
implementation.

On x86, get_user_pages_fast() does few sanity checks to see if we can
call __get_user_pages_fast() for the range.

Wrapping protection should be useful for generic code too.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/gup.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e04f76594eb9..a6e21380c8b0 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1600,6 +1600,21 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	return nr;
 }
 
+#ifndef gup_fast_permitted
+/*
+ * Check if it's allowed to use __get_user_pages_fast() for the range or
+ * need to fallback to non-fast version.
+ */
+bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
+{
+	unsigned long len, end;
+
+	len = (unsigned long) nr_pages << PAGE_SHIFT;
+	end = start + len;
+	return end >= start;
+}
+#endif
+
 /**
  * get_user_pages_fast() - pin user pages in memory
  * @start:	starting user address
@@ -1619,11 +1634,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			struct page **pages)
 {
-	int nr, ret;
+	int nr = 0, ret = 0;
 
 	start &= PAGE_MASK;
-	nr = __get_user_pages_fast(start, nr_pages, write, pages);
-	ret = nr;
+
+	if (gup_fast_permitted(start, nr_pages, write)) {
+		nr = __get_user_pages_fast(start, nr_pages, write, pages);
+		ret = nr;
+	}
 
 	if (nr < nr_pages) {
 		/* Try to get the remaining pages with get_user_pages */
-- 
2.11.0