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[dm-devel] Refcounting (and other) questions



I got bcache functioning with dm... it's buggy, but it runs. _Much_ thanks to akg for educating me, I now have a vague idea of how dm is structured.

My most immediate problem is what to do in the target destructor. It looks like when the destructor returns, the dm_target is gone. This an issue if I use dm_get_device() and dm_put_device() - if writeback is going on in the background I can't just close the backing device arbitrarily, I've got refcounting that I need to use.

So I could just not use dm_get_device and instead use open_bdev_exclusive - like I do for caches, but I was hoping to get some input on the subject.

I may have to do that anyways; the way bcache + dm is structed currently is basically a stopgap. I'm planning on starting on volume management + thin provisioning soon; what this means is that it doesn't really make sense for userspace to arbitrarily create and remove targets. Rather, the kernel knows what volumes are available (and their sizes and labels).

It seems to me that there's going to be some moderately painful... model mismatch if I try to do this with device mapper. OTOH, I've heard references of some standard interface for managing volumes that's in the works; I haven't been able to find out anything more about it but it sounds like it might be relevant, if someone wanted to point me at it.

Last question: Right now I'm doing
# dmsetup foo
0 10000 bcache /dev/vdb
^D

for testing. The problem is userspace has no idea how big the target should be; that's not just dependent on the size of the backing device but the start of actual data (for alignment, mainly) is stored in the superblock which the kernel reads. I can't be the first person to run into this issue - how should bcache convey the actual size of the target to dm/somewhere where it can be useful?


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