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

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Message-Id: <9942A69CB65D4A41B39F36AF8EEF6F22@nsl.ad.nec.co.jp>
Date:	Thu, 3 Jul 2008 21:11:05 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Christoph Hellwig" <hch@...radead.org>,
	"Alasdair G Kergon" <agk@...hat.com>
Cc:	<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

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.

2. Operate and check this bit flag as followings.

  - Bit operations in the device-mapper's freeze/thaw
    FREEZE:
      dm_suspend():   set BD_FREEZE_DM
        freeze_bdev():set BD_FREEZE_OP

    THAW:
        thaw_bdev():   clear BD_FREEZE_OP
      dm_resume():    clear BD_FREEZE_DM

  -  Checks in FIFREEZE/FITHAW
    FREEZE:
      ioctl_freeze(): Not need to check BD_FREEZE_DM
        freeze_bdev():set BD_FREEZE_OP

    THAW:
      ioctl_thaw():   If BD_FREEZE_DM is set, fail, otherwise, call thaw_bdev()
        thaw_bdev():   clear BD_FREEZE_OP

Cheers, Takashi
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