Takashi Sato wrote:

Hi Christoph and Alasdair,

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On Tue, Jul 01, 2008 at 04:10:26AM -0400, Christoph Hellwig wrote:

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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.

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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)

[...]

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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:

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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