[PATCH v3 2/8] ovl: setup overlayfs' private bdi

[Josef Bacik <josef@toxicpanda.com>]

From: Chengguang Xu <charliecgxu@tencent.com>

Setup overlayfs' private bdi so that we can collect
overlayfs' own dirty inodes.

Signed-off-by: Chengguang Xu <charliecgxu@tencent.com>
[josef: only for an overlay that tracks dirty inodes, add
 ovl_tracks_dirty()]
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/overlayfs/overlayfs.h | 18 ++++++++++++++++++
 fs/overlayfs/super.c     | 10 ++++++++++
 2 files changed, 28 insertions(+)

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 444e28c09145..0ef81c7a9701 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -7,6 +7,7 @@
 #include <linux/kernel.h>
 #include <linux/uuid.h>
 #include <linux/fs.h>
+#include <linux/backing-dev.h>
 #include <linux/fsverity.h>
 #include <linux/namei.h>
 #include <linux/posix_acl.h>
@@ -683,6 +684,23 @@ static inline bool ovl_should_sync_upper_fs(struct ovl_fs *ofs)
 	       ofs->config.fsync_mode == OVL_FSYNC_STRICT;
 }
 
+/*
+ * Does this overlay track the upper inodes that are modified through it, so
+ * that syncfs() and unmount only have to sync those?
+ *
+ * Only with fsync=relaxed.  In the other modes the overlay never syncs, or it
+ * syncs the entire upper filesystem.  fsync=auto does not count until it has
+ * been resolved.  sync_filesystem() does not write any inodes of a filesystem
+ * that has no writeback, like tmpfs, and nothing ever clears their dirty flags.
+ */
+static inline bool ovl_tracks_dirty(struct ovl_fs *ofs)
+{
+	if (!ovl_upper_mnt(ofs) || ofs->config.fsync_mode != OVL_FSYNC_RELAXED)
+		return false;
+
+	return ovl_upper_mnt(ofs)->mnt_sb->s_bdi != &noop_backing_dev_info;
+}
+
 /*
  * To avoid regressions in existing setups with overlay lower offline changes,
  * we allow lower changes only if none of the new features are used.
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 4500038d6941..efa4d862039c 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1479,6 +1479,16 @@ static int ovl_fill_super_creds(struct fs_context *fc, struct super_block *sb)
 		if (!ofs->workdir)
 			sb->s_flags |= SB_RDONLY;
 
+		/*
+		 * Dirty overlay inodes go on the lists of this bdi.
+		 * ovl_get_workdir() has resolved fsync=auto.
+		 */
+		if (ovl_tracks_dirty(ofs)) {
+			err = super_setup_bdi(sb);
+			if (err)
+				return err;
+		}
+
 		sb->s_stack_depth = upper_sb->s_stack_depth;
 		sb->s_time_gran = upper_sb->s_time_gran;
 	}

-- 
2.55.0