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 6e3fa96613f50d606231e52b1957f90b85e3e40c..f57aa348dcd63c56e074e4d83d5c454e8c530b08 100644 (file)
--- 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 4d1ff010bc5afcd6acee1e4dbd736e8bfbd9081a..16c2630ee4bf1d8eeda15bb49e1c1b309c0587a7 100644 (file)
--- 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 4f6f59b4f22a807e55e479468df1f1cb7068cfdc..f186a966a36cbec428b260ef655573d015bda41b 100644 (file)
--- 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 */