[PATCH] ovl: skip overlayfs superblocks at global sync

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Stacked filesystems like overlayfs has no own writeback, but they have to
forward syncfs() requests to backend for keeping data integrity.

During global sync() each overlayfs instance calls method ->sync_fs()
for backend although it itself is in global list of superblocks too.
As a result one syscall sync() could write one superblock several times
and send multiple disk barriers.

This patch adds flag SB_I_SKIP_SYNC into sb->sb_iflags to avoid that.

Reported-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 fs/overlayfs/super.c |    5 +++--
 fs/sync.c            |    3 ++-
 include/linux/fs.h   |    2 ++
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 732ad5495c92..59df13d16280 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -261,8 +261,8 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
 		return 0;
 
 	/*
-	 * If this is a sync(2) call or an emergency sync, all the super blocks
-	 * will be iterated, including upper_sb, so no need to do anything.
+	 * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).
+	 * All the super blocks will be iterated, including upper_sb.
 	 *
 	 * If this is a syncfs(2) call, then we do need to call
 	 * sync_filesystem() on upper_sb, but enough if we do it when being
@@ -1818,6 +1818,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	sb->s_xattr = ovl_xattr_handlers;
 	sb->s_fs_info = ofs;
 	sb->s_flags |= SB_POSIXACL;
+	sb->s_iflags |= SB_I_SKIP_SYNC;
 
 	err = -ENOMEM;
 	root_dentry = ovl_get_root(sb, upperpath.dentry, oe);
diff --git a/fs/sync.c b/fs/sync.c
index 4d1ff010bc5a..16c2630ee4bf 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -76,7 +76,8 @@ static void sync_inodes_one_sb(struct super_block *sb, void *arg)
 
 static void sync_fs_one_sb(struct super_block *sb, void *arg)
 {
-	if (!sb_rdonly(sb) && sb->s_op->sync_fs)
+	if (!sb_rdonly(sb) && !(sb->s_iflags & SB_I_SKIP_SYNC) &&
+	    sb->s_op->sync_fs)
 		sb->s_op->sync_fs(sb, *(int *)arg);
 }
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4f6f59b4f22a..f186a966a36c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1409,6 +1409,8 @@ extern int send_sigurg(struct fown_struct *fown);
 #define SB_I_IMA_UNVERIFIABLE_SIGNATURE	0x00000020
 #define SB_I_UNTRUSTED_MOUNTER		0x00000040
 
+#define SB_I_SKIP_SYNC	0x00000100	/* Skip superblock at global sync */
+
 /* Possible states of 'frozen' field */
 enum {
 	SB_UNFROZEN = 0,		/* FS is unfrozen */