mm/gup: require FOLL_GET for get_user_pages_fast()

Internal to mm/gup.c, require that get_user_pages_fast() and
__get_user_pages_fast() identify themselves, by setting FOLL_GET.  This is
required in order to be able to make decisions based on "FOLL_PIN, or
FOLL_GET, or both or neither are set", in upcoming patches.
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: http://lkml.kernel.org/r/20200211001536.1027652-6-jhubbard@nvidia.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -2390,6 +2390,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	unsigned long len, end;
 	unsigned long flags;
 	int nr = 0;
+	/*
+	 * 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.
+	 */
+	unsigned int gup_flags = FOLL_GET;
+
+	if (write)
+		gup_flags |= FOLL_WRITE;
 
 	start = untagged_addr(start) & PAGE_MASK;
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
@@ -2415,7 +2423,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
 		local_irq_save(flags);
-		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
+		gup_pgd_range(start, end, gup_flags, pages, &nr);
 		local_irq_restore(flags);
 	}
 
@@ -2454,7 +2462,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 	int nr = 0, ret = 0;
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
-				       FOLL_FORCE | FOLL_PIN)))
+				       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
 		return -EINVAL;
 
 	start = untagged_addr(start) & PAGE_MASK;
@@ -2521,6 +2529,13 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
 		return -EINVAL;
 
+	/*
+	 * The caller may or may not have explicitly set FOLL_GET; either way is
+	 * OK. However, 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.
+	 */
+	gup_flags |= FOLL_GET;
 	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(get_user_pages_fast);