[PATCH] mm: gup: Re-define follow_page_mask output parameter page_mask usage

[chengang]

From: Chen Gang <cheng...@emindsoft.com.cn>

For a pure output parameter:

 - When callee fails, the caller should not assume the output parameter
   is still valid.

 - And callee should not assume the pure output parameter must be
   provided by caller -- caller has right to pass NULL when caller does
   not care about it.

Signed-off-by: Chen Gang <gang.chen.5...@gmail.com>
---
 include/linux/mm.h | 5 ++---
 mm/gup.c           | 6 +++---
 mm/mlock.c         | 2 +-
 mm/nommu.c         | 1 -
 4 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b21e5f3..5c560fd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2205,10 +2205,9 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
                              unsigned int *page_mask);
 
 static inline struct page *follow_page(struct vm_area_struct *vma,
-               unsigned long address, unsigned int foll_flags)
+                               unsigned long address, unsigned int foll_flags)
 {
-       unsigned int unused_page_mask;
-       return follow_page_mask(vma, address, foll_flags, &unused_page_mask);
+       return follow_page_mask(vma, address, foll_flags, NULL);
 }
 
 #define FOLL_WRITE     0x01    /* check pte is writable */
diff --git a/mm/gup.c b/mm/gup.c
index 96b2b2f..9684b06 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -222,8 +222,6 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
        struct page *page;
        struct mm_struct *mm = vma->vm_mm;
 
-       *page_mask = 0;
-
        page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
        if (!IS_ERR(page)) {
                BUG_ON(flags & FOLL_GET);
@@ -298,7 +296,8 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
 
        page = follow_trans_huge_pmd(vma, address, pmd, flags);
        spin_unlock(ptl);
-       *page_mask = HPAGE_PMD_NR - 1;
+       if (page_mask)
+               *page_mask = HPAGE_PMD_NR - 1;
        return page;
 }
 
@@ -574,6 +573,7 @@ retry:
                if (unlikely(fatal_signal_pending(current)))
                        return i ? i : -ERESTARTSYS;
                cond_resched();
+               page_mask = 0;
                page = follow_page_mask(vma, start, foll_flags, &page_mask);
                if (!page) {
                        int ret;
diff --git a/mm/mlock.c b/mm/mlock.c
index ef8dc9f..626eb58 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -438,7 +438,7 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 
        while (start < end) {
                struct page *page;
-               unsigned int page_mask;
+               unsigned int page_mask = 0;
                unsigned long page_increm;
                struct pagevec pvec;
                struct zone *zone;
diff --git a/mm/nommu.c b/mm/nommu.c
index 95daf81..c1a0a89 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1749,7 +1749,6 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
                              unsigned long address, unsigned int flags,
                              unsigned int *page_mask)
 {
-       *page_mask = 0;
        return NULL;
 }
 
-- 
1.9.3