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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