linux-ext4 - Re: [RFC] ext3 freeze feature

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Date:	Fri, 15 Feb 2008 20:51:15 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Christoph Hellwig" <hch@...radead.org>,
	"Andreas Dilger" <adilger@....com>
Cc:	"Theodore Tso" <tytso@....EDU>, <linux-ext4@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] ext3 freeze feature

Hi,

Christoph Hellwig wrote:
> On Fri, Feb 08, 2008 at 08:26:57AM -0500, Andreas Dilger wrote:
>> You may as well make the common ioctl the same as the XFS version,
>> both by number and parameters, so that applications which already
>> understand the XFS ioctl will work on other filesystems.
>
> Yes.  In facy you should be able to lift the implementations of
> XFS_IOC_FREEZE and XFS_IOC_THAW to generic code, there's nothing
> XFS-specific in there.

According to Documentation/ioctl-number.txt,
XFS_IOC_XXXs (_IOWR('X', aa, bb)) are defined for XFS like below.
>From Documentation/ioctl-number.txt:
----------------------------------------------------------------------------
Code    Seq#    Include File            Comments
========================================================
:                 :
'X'     all     linux/xfs_fs.h
----------------------------------------------------------------------------
So XFS_IOC_FREEZE and XFS_IOC_THAW cannot be lifted to generic code simply.
I think we should create new generic numbers for freeze and thaw
like FIBMAP as followings.
linux/fs.h:
#define FIFREEZE _IO(0x00,3)
#define FITHAW   _IO(0x00,4)

And xfs_freeze calls XFS_IOC_FREEZE with a magic number 1, but what is 1?
Instead, I'd like the sec to timeout on freeze API in order to thaw
the filesystem automatically.  It can prevent a filesystem from staying
frozen forever.
(Because a freezer may cause a deadlock by accessing the frozen filesystem.)

Any comments are very welcome.

Cheers, Takashi 

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