Copy-on-write (Extra)

Implement copy-on-write mechanism in Pintos.

Copy-on-write is a resource management technique that allows faster duplication operation by using the same instance of the physical page. If some resource is being used by multiple processes, each process should usually have its own copy of the resource to make sure that no conflict occurs. If the resource is not modified but just being read, however, there is no need to have multiple copies in the physical memory.

For example, suppose a new process is created through fork. The child needs to inherit its parent's resource by duplicating the data into its virtual address space. Usually, adding contents to its virtual memory involves allocating a physical page, writing the data into the frame, and adding a virtual->physical mapping in its page table. These steps can be quite time-consuming.

With copy-on-write technique, however, we do not allocate new physical page for the new copy of resources. This is because, technically, the content already exists in the physical memory. Therefore, we only add the virtual->physical mapping in the child process's page table, where the virtual address is now in the child's memory space. Then, the parent and child are accessing the same data in the same physical page. Yet, they are still isolated through separate virtual address space, and only OS knows that they are referring to the same frame. Only when one of the process tries to modify the content of the shared resource, it will create a separate copy in a new physical page for itself. Therefore, the actual copy operation is deferred to the first write.

That is, OS needs to be able to detect write attempts on copy-on-write pages. To achieve this need, OS uses the "write-protect" mechanism. It is a simple idea: Make a page fault on write accesses. This can be easily implemented with the support of the memory management system, by just marking write-protected page as unwritable at all.

You only need to implement copy-on-write for fork. When a child process inherits the resource from its parent process, the resource can be referencing to the same physical data until the child tries to modify it. All write-protected pages are candidates for eviction.

We only provide basic test cases for copy-on-write. You should consider all the possible cases (Here is a small hint: you should implement sharing of file-backed page). The grading of this extra project will be done with the hidden test cases as well.

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