Computer Architecture

Question 1)

A page fault occurs when a process accesses a virtual page for which there is no PTE in the page table or whose PTE in some way prohibits the access, e.g., because the page is not present or because the access is in conflict with the access rights of the page. Page faults are triggered by the CPU and handled in the page fault handler.

Because Linux uses demand paging and page-fault-based optimizations such as copy-on-write, page faults occur during the normal course of operation and do not necessarily indicate an error. Thus, when the page fault handler is invoked, it first needs to determine whether the page fault is caused by an access to a valid page. If not, the page fault handler simply sends a segmentation violation signal to the faulting process and returns. Otherwise, it takes one of several possible actions:

If the page is being accessed for the first time, the handler allocates a new page frame and initializes it, e.g., by reading its content from disk. Page faults caused by first-time accesses are called demand page faults.

If the page has been paged out to swap space, the handler reads it back from disk into a newly allocated page frame.

If the page fault occurred because of a page-fault-based optimization (such as copy-on-write), the handler takes the appropriate recovery action (such as performing the delayed page copy).

Each of these actions results either in a new or updated page, and the handler must accordingly either create or update the PTE in the page table. Because the page-table tree is also created on demand, installing a new PTE may require allocating and initializing middle and a PTE directory (the global directory is guaranteed to exist already). The page fault handler can use the pmd_alloc() and pte_alloc() routines from Section 4.3.6 for this purpose. Before the handler installs the PTE, it updates the accessed and dirty bits as well. Since the page fault itself is an indication that the page is being accessed, the handler uses pte_mkyoung() to unconditionally turn on the accessed bit. On the other hand, the dirty bit is turned on by pte_mkdirty() only if the page fault is the result of a write access. After updating the page table, the page fault handler returns and execution resumes in the faulting process. There, the instruction that previously faulted is restarted and, thanks to the updated page table, it can now complete execution.

Question 2)

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Windows Server 2012 RC has a ...