[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>
diff --git a/mm/memory.c b/mm/memory.c
index fd5d4c6..1366768 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1768,13 +1768,14 @@
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 1dcaeda..05c8512 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -398,10 +398,9 @@
}
/*
- * 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.
*/