linux-ext4 - Re: [PATCH 3/3] Add timeout feature

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Date:	Thu, 9 Oct 2008 19:12:17 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Eric Sandeen" <sandeen@...deen.net>,
	"Christoph Hellwig" <hch@...radead.org>
Cc:	"Ric Wheeler" <rwheeler@...hat.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Oleg Nesterov" <oleg@...sign.ru>, <linux-fsdevel@...r.kernel.org>,
	<dm-devel@...hat.com>, <viro@...IV.linux.org.uk>,
	<linux-ext4@...r.kernel.org>, <xfs@....sgi.com>, <axboe@...nel.dk>,
	<mtk.manpages@...glemail.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] Add timeout feature

Hi,

Eric Sandeen wrote:
> Christoph Hellwig wrote:
>> On Mon, Sep 29, 2008 at 09:36:04AM -0500, Eric Sandeen wrote:
>>> Christoph Hellwig wrote:
>>>> On Fri, Sep 26, 2008 at 05:52:35PM +0900, Takashi Sato wrote:
>>>>> I think that your concern is that the freezer cannot recognize the occurrence
>>>>> of a timeout and it continues the backup process and the backup data is
>>>>> corrupted finally.
>>>> What timeout should happen?  the freeze ioctl must not return until the
>>>> filesystem is a clean state and all writes are blocked.
>>> The suggestion was that *UN*freeze would return ETIMEDOUT if the
>>> filesystem had already unfrozen itself, I think.  That way you know that
>>> the snapshot you just took is worthless, at least.
>>
>> But why would the filesystem every unfreeze itself?  That defeats the
>> whole point of freezing it.
>
> I agree.  Was just trying to clarify the above point.
>
> But there have been what, 12 submissions now, with the unfreeze timeout
> in place so it's a persistent theme ;)
>
> Perhaps a demonstration of just how easy (or not easy) it is to deadlock
> a filesystem by freezing the root might be in order, at least.
>
> And even if it is relatively easy, I still maintain that it is the
> administrator's role to not inflict damage on the machine being
> administered.  There are a lot of potentially dangerous tools at root's
> disposal; why this particular one needs a nanny I'm still not quite sure.

I think we need the timeout for the case someone dirties so much data
with mmap, hence the freeze process is swapped out and cannot unfreeze.

Cheers, Takashi
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