mm/gup: introduce pin_user_pages_fast_only()

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: "Joonas Lahtinen" <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://lkml.kernel.org/r/20200519002124.2025955-4-jhubbard@nvidia.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -1827,6 +1827,8 @@ 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.
  */

mm/gup.c

@@ -2917,6 +2917,42 @@ 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)
  *