Occlum is a single-address-space library OS. Previously, userspace memory are divided for each process.
And all the memory are allocated when the process is created, which leads to a lot of wasted space and
complicated configuration.
In the current implementation, the whole userspace is managed as a memory pool that consists of chunks. There
are two kinds of chunks:
(1) Single VMA chunk: a chunk with only one VMA. Should be owned by exactly one process.
(2) Multi VMA chunk: a chunk with default chunk size and there could be a lot of VMAs in this chunk. Can be used
by different processes.
This design can help to achieve mainly two goals:
(1) Simplify the configuration: Users don't need to configure the process.default_mmap_size anymore. And multiple processes
running in the same Occlum instance can use dramatically different sizes of memory.
(2) Gain better performance: Two-level management(chunks & VMAs) reduces the time for finding, inserting, deleting, and iterating.
This rewrite serves three purposes:
1. Fix some subtle bugs in the old implementation;
2. Implement mremap using mmap and munmap so that mremap can automatically
enjoy new features (e.g., mprotect and memory permissions) once mmap and
munmap support the feature.
3. Write down the invariants hold by VMManager explictly so that the correctness
of the new implementation can be reason more easily.
