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At present several threads running can corrupt internal virtmem
datastructures (e.g. ram->lru_list, fileh->pagemap, etc).

This can happen even if we have zope instances only with 1 worker thread
- because there are other "system" thread, and python garbage collection
can trigger at any thread, so if a virtmem object, e.g. VMA or FileH was
there sitting at GC queue to be collected, their collection, and thus
e.g. vma_unmap() and fileh_close() will be called from
different-from-worker thread.

Because of that virtmem just has to be aware of threads not to allow
internal datastructure corruption.

On the other hand, the idea of introducing userspace virtual memory
manager turned out to be not so good from performance and complexity
point of view, and thus the plan is to try to move it back into the
kernel. This way it does not make sense to do a well-optimised locking
implementation for userspace version.

So we do just a simple single "protect-all" big lock for virtmem.

O...