Recently I refectored PageManager, which manages mapping between linear and physical address.
A strange bug occured after the refactoring.
Some times tests for mutex crashed and sometimes works as expected.
So my OS became unstable, I could't dogfood(verb).

I finally fixed the bug. It took three days to fix.
The problem is overflow of page table bitmap. On process termination, we should return physical pages to PageManager.
We wrongly returned VRAM also, and this causes strange behaviour.