[RFC PATCH] futex: replace get_user_pages() with get_user_pages_fast()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hello,

Is there any reason to take mmap_sem explicitly here?
Should we change gup_fast() to allow NULL argument (ie for avoid get_page)?

============================
  down_read(&mm->mmap_sem);
  get_user_pages(current, current->mm)
  up_read(&mm->mmap_sem);

and

  get_user_pages_fast()

make an equivalent result, And latter would be better when mamp_sem
highly contended case, because it can avoid to take mmap_sem if
the target page doesn't need a page fault.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 kernel/futex.c |    8 +++-----
 1 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index fe28dc2..9f7b1ae 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -351,13 +351,11 @@ static inline void put_futex_key(union futex_key *key)
  */
 static int fault_in_user_writeable(u32 __user *uaddr)
 {
-	struct mm_struct *mm = current->mm;
 	int ret;
+	struct page *page;

-	down_read(&mm->mmap_sem);
-	ret = get_user_pages(current, mm, (unsigned long)uaddr,
-			     1, 1, 0, NULL, NULL);
-	up_read(&mm->mmap_sem);
+	ret = get_user_pages_fast((unsigned long)uaddr, 1, 1, &page);
+	put_page(page);

 	return ret < 0 ? ret : 0;
 }
-- 
1.7.3.1