linux-ext4 - Re: [RFC PATCH] freeze feature ver 1.0

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Message-Id: <20080328180145t-sato@mail.jp.nec.com>
Date:	Fri, 28 Mar 2008 18:01:45 +0900
From:	Takashi Sato <t-sato@...jp.nec.com>
To:	David Chinner <dgc@....com>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] freeze feature ver 1.0

Hi,

David Chinner wrote:
> Can you please split this into two patches - one which introduces the
> generic functionality *without* the timeout stuff, and a second patch
> that introduces the timeouts.

OK.
I will send the split patches in subsequent mails. 
 
> I think this timeout stuff is dangerous - it adds significant
> complexity and really does not protect against anything that can't
> be done in userspace.  i.e. If your system is running well enough
> for the timer to fire and unfreeze the filesystem, it's running well
> enough for you to do "freeze X; sleep Y; unfreeze X".  

If the process is terminated at "sleep Y" by an unexpected
accident (e.g. signals),  the filesystem will be left frozen.
So, I think the timeout is needed to unfreeze more definitely.

> FWIW, there is nothing to guarantee that the filesystem has finished
> freezing when the timeout fires (it's not uncommon to see
> freeze_bdev() taking *minutes*) and unfreezing in the middle of a
> freeze operation will cause problems - either for the filesystem
> in the middle of a freeze operation, or for whatever is freezing the
> filesystem to get a consistent image.....

Do you mention the freeze_bdev()'s hang?
The salvage target of my timeout is freeze process's accident as below.
- It is killed before calling the unfreeze ioctl
- It causes a deadlock by accessing the frozen filesystem
So the delayed work for the timeout is set after all of freeze operations
in freeze_bdev() in my patches.
I think the filesystem dependent code (write_super_lockfs operation)
should be implemented not to cause a hang.

Cheers, Takashi
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