[PATCH v6 05/12] mm/gup: require FOLL_GET for get_user_pages_fast()

[John Hubbard <jhubbard@xxxxxxxxxx>]

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.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 mm/gup.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e5f75e886663..c8affbea2019 100644
--- a/mm/gup.c
+++ b/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);
-- 
2.25.0