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

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Date:	Thu, 03 Jul 2008 09:45:48 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Takashi Sato <t-sato@...jp.nec.com>
CC:	Christoph Hellwig <hch@...radead.org>,
	Alasdair G Kergon <agk@...hat.com>,
	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, viro@...IV.linux.org.uk,
	dm-devel@...hat.com, xfs@....sgi.com, linux-kernel@...r.kernel.org,
	mtk.manpages@...glemail.com, axboe@...nel.dk
Subject: Re: [dm-devel] Re: [PATCH 3/3] Add timeout feature

Takashi Sato wrote:
> Hi Christoph and Alasdair,
> 
>> 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.
> 
> If the freezer accesses the frozen filesystem and causes a deadlock,
> the above ideas can't solve it.  The timeout is useful to solve such a deadlock.
> If you don't need the timeout, you can disable it by specifying "0" as the
> timeout period.
> 
>> 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)
> [...]
>> C:
>>  1 succeeds, freezes
>>  2 fails, remains frozen
>>  3 fails (because device-mapper owns the freeze/thaw), remains frozen
>>  4 succeeds, thaws
> 
> I think C is appropriate and the following change makes it possible.
> How do you think?
> 
> 1. Add the new bit flag(BD_FREEZE_DM) in block_device.bd_state.
>    It means that the volume is frozen by the device-mapper.

Will we add a new bit/flag for every possible subysstem that may call
freeze/thaw?  This seems odd to me.

They are different paths to the same underlying mechanism; it should not
matter if it is an existing freeze from DM or via FIFREEZE or via the
xfs ioctl, or any other mechanism should it?  I don't think this generic
interface should use any flag named *_DM, personally.

It seems that nested freeze requests must be handled in a generic way
regardless of what initiates any of the requests?

Refcounting freezes as Alasdair suggests seems to make sense to me, i.e.
freeze, freeze, thaw, thaw leads to:

>> > > 1 (freeze) succeeds, freezes (frozen++)
>> > > 2 (freeze) succeeds, remains frozen (frozen++)
>> > > 3 (thaw) succeeds, remains frozen (frozen--)
>> > > 4 (thaw) succeeds, thaws (frozen--)

that way each caller of freeze is guaranteed that the fs is frozen at
least until they call thaw?

Thanks,
-Eric
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