[PATCH] add shmem_vm_writeback()

Give tmpfs its own shmem_vm_writeback (and empty shmem_writepages):
going through the default mpage_writepages is very wrong for tmpfs,
since that may write nearby pages while still mapped into mms, but
"writing" converts pages from tmpfs file identity to swap backing
identity: doing so while mapped breaks assumptions throughout e.g.  the
shared file is liable to disintegrate into private instances.

mm/shmem.c

@@ -515,9 +515,8 @@ static int shmem_writepage(struct page * page)
 
 	if (!PageLocked(page))
 		BUG();
-
-	if (!(current->flags & PF_MEMALLOC))
-		return fail_writepage(page);
+	if (page_mapped(page))
+		BUG();
 
 	mapping = page->mapping;
 	index = page->index;
@@ -551,6 +550,19 @@ static int shmem_writepage(struct page * page)
 	return fail_writepage(page);
 }
 
+static int shmem_writepages(struct address_space *mapping, struct writeback_control *wbc)
+{
+	return 0;
+}
+
+static int shmem_vm_writeback(struct page *page, struct writeback_control *wbc)
+{
+	clear_page_dirty(page);
+	if (shmem_writepage(page) < 0)
+		set_page_dirty(page);
+	return 0;
+}
+
 /*
  * shmem_getpage_locked - either get the page from swap or allocate a new one
  *
@@ -1524,6 +1536,8 @@ static void destroy_inodecache(void)
 
 static struct address_space_operations shmem_aops = {
 	.writepage	= shmem_writepage,
+	.writepages	= shmem_writepages,
+	.vm_writeback	= shmem_vm_writeback,
 	.set_page_dirty	= __set_page_dirty_nobuffers,
 };