[PATCH v3 06/12] mm/gup: require FOLL_GET for get_user_pages_fast()

John Hubbard
2020-02-01T03:41:15Z

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>
---
mm/gup.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 83473c2165f4..e899d2e6398c 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