linux-ext4 - RE: Is ext2 freezable?

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Message-ID: <EE124450C0AAF944A40DD71E61F878C99CE2C3@SINEX14MBXC418.southpacific.corp.microsoft.com>
Date:	Thu, 18 Sep 2014 06:46:21 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: RE: Is ext2 freezable?

> -----Original Message-----
> From: Dexuan Cui
> Sent: Thursday, September 18, 2014 13:16 PM
> To: 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":
> ...
> 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've got the answer:
ext2.ko itself does support fsfreeze, but typical linux distros don't supply
ext2.ko at all now -- instead, they usually supply ext3.ko and have ext4 builtin.

So when I mount an ext2 partition, actually the kernel is registering the ext4
driver as an ext2 driver and in this case the ext2's s_op->freeze_fs is NULL --
but, why did ext4 choose this behavior for ext2?

Thanks,
-- Dexuan
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