Is ext2 freezable?

Is ext2 freezable?

[Dexuan Cui]

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

RE: Is ext2 freezable?

[Dexuan Cui]

> -----Original Message-----
> From: Dexuan Cui
> Sent: Thursday, September 18, 2014 13:16 PM
> To: linux-ext4@vger.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

Re: Is ext2 freezable?

[Theodore Ts'o]

On Thu, Sep 18, 2014 at 06:46:21AM +0000, Dexuan Cui wrote:
> 
> 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?

It wasn't a deliberate design choice.  It was just that when
no-journal mode was added to ext4, freeeze support was never
implemented, and up until now, no one had asked for it.  We can add it
to ext4; thanks for calling it to our attention.

Cheers,

					- Ted

RE: Is ext2 freezable?

[Dexuan Cui]

> -----Original Message-----
> From: Theodore Ts'o [mailto:tytso@mit.edu]
> Sent: Friday, September 19, 2014 3:18 AM
> To: Dexuan Cui
> Cc: linux-ext4@vger.kernel.org
> Subject: Re: Is ext2 freezable?
>
> On Thu, Sep 18, 2014 at 06:46:21AM +0000, Dexuan Cui wrote:
> >
> > 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?
>
> It wasn't a deliberate design choice.  It was just that when
> no-journal mode was added to ext4, freeeze support was never
> implemented, and up until now, no one had asked for it.  We can add it
> to ext4; thanks for calling it to our attention.
>  - Ted

Hi Ted,
Thanks very much for the clarification!

And thanks a lot for implementing this -- I've seen the patches you sent out
several hours ago.

IMO it's useful to have such compatibility, e.g.,
Hyper-V guest has a fsfreeze-based feature to back up the data; the feature
works fine for ext4 partition, but Ubuntu's installer can create /boot
of ext2 partition by default and hence the feature can't work:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1362574

I believe the bug will go away after Ubuntu integrates your patches.

Thanks!
-- Dexuan

Re: Is ext2 freezable?

[Syed Imtiaz]

Dexuan Cui <decui <at> microsoft.com> writes:
> Hi Ted,
> Thanks very much for the clarification!
> 
> And thanks a lot for implementing this -- I've seen the patches you 
sent out
> several hours ago.
> 
> IMO it's useful to have such compatibility, e.g.,
> Hyper-V guest has a fsfreeze-based feature to back up the data; the 
feature
> works fine for ext4 partition, but Ubuntu's installer can create /boot
> of ext2 partition by default and hence the feature can't work:
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1362574
> 
> I believe the bug will go away after Ubuntu integrates your patches.
> 
> Thanks!


Hi Theodore,
  CAn you please tell me where can i find these patches. I am unable to 
locate it. or the steps the install it ?
Do we have to install it every system that doesnt support fsfreeze in 
the ext2 fs.

Your help will be deeply appreciated

thanks in advance

Re: Is ext2 freezable?

[Eric Sandeen]

On 6/5/15 6:23 AM, Syed Imtiaz wrote:
> Dexuan Cui <decui <at> microsoft.com> writes:
>> Hi Ted,
>> Thanks very much for the clarification!
>>
>> And thanks a lot for implementing this -- I've seen the patches you 
> sent out
>> several hours ago.
>>
>> IMO it's useful to have such compatibility, e.g.,
>> Hyper-V guest has a fsfreeze-based feature to back up the data; the 
> feature
>> works fine for ext4 partition, but Ubuntu's installer can create /boot
>> of ext2 partition by default and hence the feature can't work:
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1362574
>>
>> I believe the bug will go away after Ubuntu integrates your patches.
>>
>> Thanks!
> 
> 
> Hi Theodore,
>   CAn you please tell me where can i find these patches. I am unable to 
> locate it. or the steps the install it ?
> Do we have to install it every system that doesnt support fsfreeze in 
> the ext2 fs.

To be clear, the patches were only for ext2-mounted-with-ext4.ko.
The native ext2 driver freezes w/o any problem.

The upstream commit is here:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=bb04457

and has been fixed upstream since kernel v3.18

-Eric