[RFC PATCH v2 2/8] ovl: implement ->writepages operation

[Chengguang Xu <cgxu519@mykernel.net>]

Implement overlayfs' ->writepages operation so that
we can sync dirty data/metadata to upper filesystem.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
---
 fs/overlayfs/inode.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index b584dca845ba..f27fc5be34df 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -11,6 +11,7 @@
 #include <linux/posix_acl.h>
 #include <linux/ratelimit.h>
 #include <linux/fiemap.h>
+#include <linux/writeback.h>
 #include "overlayfs.h"
 
 
@@ -516,7 +517,32 @@ static const struct inode_operations ovl_special_inode_operations = {
 	.update_time	= ovl_update_time,
 };
 
+static int ovl_writepages(struct address_space *mapping,
+			  struct writeback_control *wbc)
+{
+	struct inode *upper_inode = ovl_inode_upper(mapping->host);
+	struct ovl_fs *ofs =  mapping->host->i_sb->s_fs_info;
+	struct writeback_control tmp_wbc = *wbc;
+
+	if (!ovl_should_sync(ofs))
+		return 0;
+
+	/*
+	 * for sync(2) writeback, it has a separate external IO
+	 * completion path by checking PAGECACHE_TAG_WRITEBACK
+	 * in pagecache, we have to set for_sync to 0 in thie case,
+	 * let writeback waits completion after syncing individual
+	 * dirty inode, because we haven't implemented overlayfs'
+	 * own pagecache yet.
+	 */
+	if (wbc->for_sync && (wbc->sync_mode == WB_SYNC_ALL))
+		tmp_wbc.for_sync = 0;
+
+	return sync_inode(upper_inode, &tmp_wbc);
+}
+
 static const struct address_space_operations ovl_aops = {
+	.writepages		= ovl_writepages,
 	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
 	.direct_IO		= noop_direct_IO,
 };
-- 
2.26.2