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Re: [PATCH 3/3] Add timeout feature

To: "Eric Sandeen" <sandeen@xxxxxxxxxxx>, "Christoph Hellwig" <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/3] Add timeout feature
From: "Takashi Sato" <t-sato@xxxxxxxxxxxxx>
Date: Thu, 9 Oct 2008 19:12:17 +0900
Cc: "Ric Wheeler" <rwheeler@xxxxxxxxxx>, "Andrew Morton" <akpm@xxxxxxxxxxxxxxxxxxxx>, "Oleg Nesterov" <oleg@xxxxxxxxxx>, <linux-fsdevel@xxxxxxxxxxxxxxx>, <dm-devel@xxxxxxxxxx>, <viro@xxxxxxxxxxxxxxxxxx>, <linux-ext4@xxxxxxxxxxxxxxx>, <xfs@xxxxxxxxxxx>, <axboe@xxxxxxxxx>, <mtk.manpages@xxxxxxxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>
In-reply-to: <48E0EA0B.7000701@xxxxxxxxxxx>
References: <20080908205337t-sato@xxxxxxxxxxxxxxx> <20080908171119.GB22521@xxxxxxxxxxxxx> <48DBFD42.6030307@xxxxxxxxxx> <D0B0D91F884647D6808626CDFF81E532@xxxxxxxxxxxxxxxx> <20080929141326.GA31781@xxxxxxxxxxxxx> <48E0E7D4.1090409@xxxxxxxxxxx> <20080929143749.GA13286@xxxxxxxxxxxxx> <48E0EA0B.7000701@xxxxxxxxxxx>
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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