[GFS2] Add writepages for "data=writeback" mounts

It occurred to me that although a gfs2 specific writepages for ordered
writes and journaled data would be tricky, by hooking writepages only
for "data=writeback" mounts we could take advantage of not needing
buffer heads (we don't use them on the read side, nor have we for some
time) and create much larger I/Os for the block layer.

Using blktrace both before and after, its possible to see that for large
I/Os, most of the requests generated through writepages are now 1024
sectors after this patch is applied as opposed to 8 sectors before.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/ops_address.c

@@ -16,6 +16,7 @@
 #include <linux/pagevec.h>
 #include <linux/mpage.h>
 #include <linux/fs.h>
+#include <linux/writeback.h>
 #include <linux/gfs2_ondisk.h>
 #include <linux/lm_interface.h>
 
@@ -156,6 +157,31 @@ static int gfs2_writepage(struct page *page, struct writeback_control *wbc)
 	return 0;
 }
 
+/**
+ * gfs2_writepages - Write a bunch of dirty pages back to disk
+ * @mapping: The mapping to write
+ * @wbc: Write-back control
+ *
+ * For journaled files and/or ordered writes this just falls back to the
+ * kernel's default writepages path for now. We will probably want to change
+ * that eventually (i.e. when we look at allocate on flush).
+ *
+ * For the data=writeback case though we can already ignore buffer heads
+ * and write whole extents at once. This is a big reduction in the
+ * number of I/O requests we send and the bmap calls we make in this case.
+ */
+int gfs2_writepages(struct address_space *mapping, struct writeback_control *wbc)
+{
+	struct inode *inode = mapping->host;
+	struct gfs2_inode *ip = GFS2_I(inode);
+	struct gfs2_sbd *sdp = GFS2_SB(inode);
+
+	if (sdp->sd_args.ar_data == GFS2_DATA_WRITEBACK && !gfs2_is_jdata(ip))
+		return mpage_writepages(mapping, wbc, gfs2_get_block_noalloc);
+
+	return generic_writepages(mapping, wbc);
+}
+
 /**
  * stuffed_readpage - Fill in a Linux page with stuffed file data
  * @ip: the inode
@@ -757,6 +783,7 @@ int gfs2_releasepage(struct page *page, gfp_t gfp_mask)
 
 const struct address_space_operations gfs2_file_aops = {
 	.writepage = gfs2_writepage,
+	.writepages = gfs2_writepages,
 	.readpage = gfs2_readpage,
 	.readpages = gfs2_readpages,
 	.sync_page = block_sync_page,