Ensure that get_user_pages() evaluates len upon entry into the while loops.

A BUG_ON check is added so that it will catch potential bugs when it is asked

to grab zero pages. follow_hugetlb_page() is modified to adapt the changes

made in get_user_pages().
Signed-off-by: Eugene Teo <eugeneteo@kernel.sg>

---

 include/linux/hugetlb.h |    2 +-

 mm/hugetlb.c            |   10 +++-------

 mm/memory.c             |   15 ++++++---------

 3 files changed, 10 insertions(+), 17 deletions(-)
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h

index 7ca198b..2e8a01d 100644

--- a/include/linux/hugetlb.h

+++ b/include/linux/hugetlb.h

@@ -20,7 +20,7 @@ int hugetlb_sysctl_handler(struct ctl_table *, int, struct file *, void __user *

 int hugetlb_overcommit_handler(struct ctl_table *, int, struct file *, void __user *, size_t *, loff_t *);

 int hugetlb_treat_movable_handler(struct ctl_table *, int, struct file *, void __user *, size_t *, loff_t *);

 int copy_hugetlb_page_range(struct mm_struct *, struct mm_struct *, struct vm_area_struct *);

-int follow_hugetlb_page(struct mm_struct *, struct vm_area_struct *, struct page **, struct vm_area_struct **, unsigned long *, int *, int, int);

+int follow_hugetlb_page(struct mm_struct *, struct vm_area_struct *, struct page **, struct vm_area_struct **, unsigned long *, int, int, int);

 void unmap_hugepage_range(struct vm_area_struct *, unsigned long, unsigned long);

 void __unmap_hugepage_range(struct vm_area_struct *, unsigned long, unsigned long);

 int hugetlb_prefault(struct address_space *, struct vm_area_struct *);

diff --git a/mm/hugetlb.c b/mm/hugetlb.c

index d9a3803..5bb8130 100644

--- a/mm/hugetlb.c

+++ b/mm/hugetlb.c

@@ -960,15 +960,14 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,

 			struct page **pages, struct vm_area_struct **vmas,

-			unsigned long *position, int *length, int i,

+			unsigned long *position, int length, in...
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