[PATCH] mm: anon is already wrprotected

do_anonymous_page's pte_wrprotect causes some confusion: in such a case,
vm_page_prot must already be forcing COW, so must omit write permission, and
so the pte_wrprotect is redundant.  Replace it by a comment to that effect,
and reword the comment on unuse_pte which also caused confusion.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 7 insertions, 7 deletions

diff --git a/mm/memory.c b/mm/memory.c
index fd5d4c6dc762..13667681cd16 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1768,13 +1768,14 @@ do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                 unsigned long addr)
 {
         pte_t entry;
-        struct page * page = ZERO_PAGE(addr);
 
-        /* Read-only mapping of ZERO_PAGE. */
-        entry = pte_wrprotect(mk_pte(ZERO_PAGE(addr), vma->vm_page_prot));
+        /* Mapping of ZERO_PAGE - vm_page_prot is readonly */
+        entry = mk_pte(ZERO_PAGE(addr), vma->vm_page_prot);
 
         /* ..except if it's a write access */
         if (write_access) {
+                struct page *page;
+
                 /* Allocate our own private page. */
                 pte_unmap(page_table);
                 spin_unlock(&mm->page_table_lock);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1dcaeda039f4..05c851291241 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -398,10 +398,9 @@ void free_swap_and_cache(swp_entry_t entry)
 }
 
 /*
- * Always set the resulting pte to be nowrite (the same as COW pages
- * after one process has exited).  We don't know just how many PTEs will
- * share this swap entry, so be cautious and let do_wp_page work out
- * what to do if a write is requested later.
+ * No need to decide whether this PTE shares the swap entry with others,
+ * just let do_wp_page work it out if a write is requested later - to
+ * force COW, vm_page_prot omits write permission from any private vma.
  *
  * vma->vm_mm->page_table_lock is held.
  */