[RFC PATCH v3 09/10] ovl: introduce helper of syncfs writeback inode waiting

[Chengguang Xu <cgxu519@mykernel.net>]

Introduce a helper ovl_wait_wb_inodes() to wait until all
target upper inodes finish writeback.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
---
 fs/overlayfs/super.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index e5607a908d82..9a535fc11221 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -255,6 +255,36 @@ static void ovl_put_super(struct super_block *sb)
 	ovl_free_fs(ofs);
 }
 
+void ovl_wait_wb_inodes(struct ovl_fs *ofs)
+{
+	LIST_HEAD(tmp_list);
+	struct ovl_inode *oi;
+	struct inode *upper;
+
+	spin_lock(&ofs->syncfs_wait_list_lock);
+	list_splice_init(&ofs->syncfs_wait_list, &tmp_list);
+
+	while (!list_empty(&tmp_list)) {
+		oi = list_first_entry(&tmp_list, struct ovl_inode, wait_list);
+		list_del_init(&oi->wait_list);
+		ihold(&oi->vfs_inode);
+		spin_unlock(&ofs->syncfs_wait_list_lock);
+
+		upper = ovl_inode_upper(&oi->vfs_inode);
+		if (!mapping_tagged(upper->i_mapping, PAGECACHE_TAG_WRITEBACK)) {
+			iput(&oi->vfs_inode);
+			goto wait_next;
+		}
+
+		filemap_fdatawait_keep_errors(upper->i_mapping);
+		iput(&oi->vfs_inode);
+		cond_resched();
+wait_next:
+		spin_lock(&ofs->syncfs_wait_list_lock);
+	}
+	spin_unlock(&ofs->syncfs_wait_list_lock);
+}
+
 /* Sync real dirty inodes in upper filesystem (if it exists) */
 static int ovl_sync_fs(struct super_block *sb, int wait)
 {
-- 
2.26.2