ovl: skip overlayfs superblocks at global sync
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>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 6e3fa96..f57aa34 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
@@ -1870,6 +1870,7 @@ 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 4d1ff01..16c2630 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 4f6f59b..f186a96 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 */
