[RFC]partitions through device-mapper

[christophe.varoqui@free.fr]

Hello, 
 
now that the device mapper is merged into mainline, I would like to open a 
discussion on the possible in-kernel partition handling clean-up. 
 
In-kernel partition handling covers : 
o parsing of the on-disk partition tables 
o partition block devices creation / structs 
 
Along with initramfs will come the possibility to rip off the partition tables 
parsing from the kernel : a userspace parser like partx (part of util-linux 
toolset) can teach the kernel the partition layout. 
 
As driverfs provides elegantly block device add/remove events to hotplug, calls 
to partx can be wrapped into the block.agent 
 
The device-mapper merging could enable the ripping of all kernel partition 
understanding by creating linear device-maps over partitions. 
 
As a proof of concept, I've mutated partx to create those mappings. This tool 
is available for testing and commenting at : 
http://dsit.free.fr/dmpartx.tar.bz2 
 
This tool cannot damage your data : BLKPG_DEL_PARTITION and  
BLKPG_ADD_PARTITION ioctls are removed from the source. 
 
I would like to receive feedback over the following points : 
 
* Is this proposal completely out of the point ? Have I overlooked some 
important implementation details ? 
 
* driverfs send block device-removal-event at the end of the job, but device 
removal cannot happen as there are device-mappings over it. This ordering 
forbids hotplug to trigger the partition-mappings flush before block-device 
removal. Can this ordering be changed, or is there another solution ? 
 
* 2.5.44-ac4 did not notify block-device-add upon scsi disk insertion with 
scsi-add-single-device, which I think is known by scsi subsystem maintainers. 
-> The block.agent provided with dmpartx is not tested. 
 
* Should dmpartx create the 0-lengh partitions ? It does not at the moment. 
Extended partitions are handled the same way as partx : resize to 63 blocks 
 
 
regards, 
cvaroqui