identify multipage ->writepages() calls

NFS needs to be able to distinguish between single-page ->writepage() calls and
 multipage ->writepages() calls.

 For the single-page writepage calls NFS can kick off the I/O within the
 context of ->writepage().

 For multipage ->writepages calls, nfs_writepage() will leave the I/O pending
 and nfs_writepages() will kick off the I/O when it all has been queued up
 within NFS.

 Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
 Signed-off-by: Andrew Morton <akpm@osdl.org>
 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/writeback.h

@@ -53,10 +53,11 @@ struct writeback_control {
 	loff_t start;
 	loff_t end;
 
-	unsigned nonblocking:1;			/* Don't get stuck on request queues */
-	unsigned encountered_congestion:1;	/* An output: a queue is full */
-	unsigned for_kupdate:1;			/* A kupdate writeback */
-	unsigned for_reclaim:1;			/* Invoked from the page allocator */
+	unsigned nonblocking:1;		/* Don't get stuck on request queues */
+	unsigned encountered_congestion:1; /* An output: a queue is full */
+	unsigned for_kupdate:1;		/* A kupdate writeback */
+	unsigned for_reclaim:1;		/* Invoked from the page allocator */
+	unsigned for_writepages:1;	/* This is a writepages() call */
 };
 
 /*

mm/page-writeback.c

@@ -550,11 +550,17 @@ void __init page_writeback_init(void)
 
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
+	int ret;
+
 	if (wbc->nr_to_write <= 0)
 		return 0;
+	wbc->for_writepages = 1;
 	if (mapping->a_ops->writepages)
-		return mapping->a_ops->writepages(mapping, wbc);
-	return generic_writepages(mapping, wbc);
+		ret =  mapping->a_ops->writepages(mapping, wbc);
+	else
+		ret = generic_writepages(mapping, wbc);
+	wbc->for_writepages = 0;
+	return ret;
 }
 
 /**