Block device driver in user space

Hi. I'm thinking about a block device driver which is split between kernel and user space.
But there are some things that give me a headache:

- Memory pressure:
The system runs out of free RAM. So the block buffer is freed by letting all the devices write the data. So the userspace part is asked to process some data. Therefore it might need more RAM, which will increase the memory pressure. Therefore even more memory needs to be written. Therefore the userspace part... I think you get the point. Also, if the userspace part writes it data to the filesystem, it would increase the memory pressure even further by causing new blocks to be added to the cache.

Is there anything one could do to prevent this from happening (besides from praying that there is always enough free memory)?

I'm just starting with kernel stuff, so this might as well a stupid question. I've tried to find something in the archives, but with no result.

Thanks for you answers.

Rolf