linux-ext4 - Is ext2 freezable?

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Message-ID: <EE124450C0AAF944A40DD71E61F878C99CE06F@SINEX14MBXC418.southpacific.corp.microsoft.com>
Date:	Thu, 18 Sep 2014 05:15:43 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Is ext2 freezable?

Hi all,
I'm running "fsfreeze  --freeze /mnt" (/mnt is mounted with an ext2 partition)
and getting "fsfreeze: /mnt: freeze failed: Operation not supported":

the strace log shows:

open("/mnt", O_RDONLY|O_LARGEFILE) = 3
fstat64(3, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
 ioctl(3, FIFREEZE, 0)             = -1 EOPNOTSUPP (Operation not supported)

My kernel is 3.16.2.
I know ext3/4 support fs freeze, but I'm not sure about ext2.

Though the above experiment shows it's not supported,  but I do see

struct super_operations ext2_sops defines an ext2_freeze() and the
code of ioctl_fsfreeze() is:

static int ioctl_fsfreeze(struct file *filp)
{
        struct super_block *sb = file_inode(filp)->i_sb;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        /* If filesystem doesn't support freeze feature, return. */
        if (sb->s_op->freeze_fs == NULL)
                return -EOPNOTSUPP;

        /* Freeze */
        return freeze_super(sb);
}

It seems here sb->s_op->freeze_fs is NULL??? why?

I think I must miss something. Please point it out.

Thanks!

-- Dexuan

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