[PATCH 05/21] mm: anon is already wrprotected

[Hugh Dickins <hugh@veritas.com>]

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>
---

 mm/memory.c   |    7 ++++---
 mm/swapfile.c |    7 +++----
 2 files changed, 7 insertions(+), 7 deletions(-)

--- mm04/mm/memory.c	2005-09-24 19:27:05.000000000 +0100
+++ mm05/mm/memory.c	2005-09-24 19:27:19.000000000 +0100
@@ -1768,13 +1768,14 @@ do_anonymous_page(struct mm_struct *mm, 
 		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);
--- mm04/mm/swapfile.c	2005-09-22 12:32:04.000000000 +0100
+++ mm05/mm/swapfile.c	2005-09-24 19:27:19.000000000 +0100
@@ -396,10 +396,9 @@ void free_swap_and_cache(swp_entry_t ent
 }
 
 /*
- * 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.
  */