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

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Date:	Tue, 1 Jul 2008 11:52:51 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Takashi Sato <t-sato@...jp.nec.com>
Cc:	Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
	mtk.manpages@...glemail.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	viro@...IV.linux.org.uk,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	akpm@...ux-foundation.org,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [dm-devel] Re: [PATCH 3/3] Add timeout feature

On Tue, Jul 01, 2008 at 04:10:26AM -0400, Christoph Hellwig wrote:
> I still disagree with this whole patch.  

Same here - if you want a timeout, what stops you from implementing it in a
userspace process?  If your concern is that the process might die without
thawing the filesystem, take a look at the userspace LVM/multipath code for
ideas - lock into memory, disable OOM killer, run from ramdisk etc.
In practice, those techniques seem to be good enough.

> call can hang and this would be theoretically useful is when the
> filesystem is already frozen by someone else, but this should be fixed
> by refusing to do the second freeze, as suggested in my comment to patch
> 1.

Similarly if a device-mapper device is involved, how should the following
sequence behave - A, B or C?

1. dmsetup suspend (freezes)
2. FIFREEZE
3. FITHAW
4. dmsetup resume (thaws)

A:
  1 succeeds, freezes
  2 succeeds, remains frozen
  3 succeeds, remains frozen
  4 succeeds, thaws

B:
  1 succeeds, freezes
  2 fails, remains frozen
  3 shouldn't be called because 2 failed but if it is: succeeds, thaws
  4 succeeds (already thawed, but still does the device-mapper parts)

C:
  1 succeeds, freezes
  2 fails, remains frozen
  3 fails (because device-mapper owns the freeze/thaw), remains frozen 
  4 succeeds, thaws

Alasdair
-- 
agk@...hat.com
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