On Tue, Sep 20, 2016 at 07:00:28PM -0700, Yuriy Romanenko wrote:

+int get_user_pages_fast(unsigned long start, int nr_pages, int write,
+ struct page **pages)
+{
+ struct mm_struct *mm = current->mm;
+ int ret;
+ unsigned long page_addr = (start & PAGE_MASK);
+ int nr = 0;
+
+ struct gup_private_data private_data = {
+ .nr = 0,
+ .pages = pages,
+ .write = write
+ };
+
+ struct mm_walk gup_walk = {
+ .pte_entry = gup_pte_entry,
+ .pte_hole = gup_pte_hole_entry,
+ .mm = mm,
+ .private = (void *)&private_data
+ };
+
+ ret = walk_page_range(page_addr,
+ page_addr + nr_pages * PAGE_SIZE,
+ &gup_walk);
+ nr = ret ? ret : nr_pages;

walk_page_range() can't be called without the mmap_sem.

  * Locking:
  *   Callers of walk_page_range() and walk_page_vma() should hold
  *   @walk->mm->mmap_sem, because these function traverse vma list and/or
  *   access to vma's data.
  */
 int walk_page_range(unsigned long start, unsigned long end,
 		    struct mm_walk *walk)
 {
 	int err = 0;
 	unsigned long next;
 	struct vm_area_struct *vma;
 
 	if (start >= end)
 		return -EINVAL;
 
 	if (!walk->mm)
 		return -EINVAL;
 
 	VM_BUG_ON_MM(!rwsem_is_locked(&walk->mm->mmap_sem), walk->mm);