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@xxxxxxxxxxxxxx>
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
---
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 */
