mm/gup: make get_user_pages_fast_only() return the common return value

There are only two callers, both can handle the common return code:

- get_user_page_fast_only() checks == 1

- gfn_to_page_many_atomic() already returns -1, and the only caller
  checks for negative return values

Remove the restriction against returning negative values.

Link: https://lkml.kernel.org/r/11-v2-987e91b59705+36b-gup_tidy_jgg@nvidia.comSigned-off-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/gup.c

@@ -3005,8 +3005,6 @@ static int internal_get_user_pages_fast(unsigned long start,
  *
  * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
  * the regular GUP.
- * Note a difference with get_user_pages_fast: this always returns the
- * number of pages pinned, 0 if no pages were pinned.
  *
  * If the architecture does not support this function, simply return with no
  * pages pinned.
@@ -3018,7 +3016,6 @@ static int internal_get_user_pages_fast(unsigned long start,
 int get_user_pages_fast_only(unsigned long start, int nr_pages,
 			     unsigned int gup_flags, struct page **pages)
 {
-	int nr_pinned;
 	/*
 	 * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
 	 * because gup fast is always a "pin with a +1 page refcount" request.
@@ -3030,19 +3027,7 @@ int get_user_pages_fast_only(unsigned long start, int nr_pages,
 			       FOLL_GET | FOLL_FAST_ONLY))
 		return -EINVAL;
 
-	nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
-						 pages);
-
-	/*
-	 * As specified in the API description above, this routine is not
-	 * allowed to return negative values. However, the common core
-	 * routine internal_get_user_pages_fast() *can* return -errno.
-	 * Therefore, correct for that here:
-	 */
-	if (nr_pinned < 0)
-		nr_pinned = 0;
-
-	return nr_pinned;
+	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(get_user_pages_fast_only);