[PATCH v2 3/4] mm/gup: introduce pin_user_pages_fast_only()

[John Hubbard <jhubbard@nvidia.com>]

This is the FOLL_PIN equivalent of __get_user_pages_fast(),
except with a more descriptive name, and gup_flags instead of
a boolean "write" in the argument list.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 84b601cab699..98be7289d7e9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1820,6 +1820,8 @@ extern int mprotect_fixup(struct vm_area_struct *vma,
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages);
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages);
 /*
  * per-process(per-mm_struct) statistics.
  */
diff --git a/mm/gup.c b/mm/gup.c
index 4564b0dc7d0b..6fa9b2016a53 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2859,6 +2859,42 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
 }
 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
 
+/*
+ * This is the FOLL_PIN equivalent of __get_user_pages_fast(). Behavior is the
+ * same, except that this one sets FOLL_PIN instead of FOLL_GET.
+ *
+ * The API rules are the same, too: no negative values may be returned.
+ */
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages)
+{
+	int nr_pinned;
+
+	/*
+	 * FOLL_GET and FOLL_PIN are mutually exclusive. Note that the API
+	 * rules require returning 0, rather than -errno:
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return 0;
+	/*
+	 * FOLL_FAST_ONLY is required in order to match the API description of
+	 * this routine: no fall back to regular ("slow") GUP.
+	 */
+	gup_flags |= (FOLL_PIN | FOLL_FAST_ONLY);
+	nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
+						 pages);
+	/*
+	 * This routine is not allowed to return negative values. However,
+	 * internal_get_user_pages_fast() *can* return -errno. Therefore,
+	 * correct for that here:
+	 */
+	if (nr_pinned < 0)
+		nr_pinned = 0;
+
+	return nr_pinned;
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);
+
 /**
  * pin_user_pages_remote() - pin pages of a remote process (task != current)
  *
-- 
2.26.2