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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
