Which statement best describes memory protection keys (MPK) and their role in per-page permissions?

• A. MPK provides a fast hardware mechanism to change access permissions for a group of pages and reduces TLB pressure.
• B. MPK expands the size of the page table to store keys.
• C. MPK does not involve hardware support and is implemented purely in software.
• D. MPK controls network access using keys.

Answer: (A)

Memory protection keys provide a fast, hardware-assisted way to control access to groups of pages. Each page carries a small protection key in its entry, and each thread has a PKRU register that holds the allowed/denied bits for every key. When memory is accessed, the CPU checks the page’s key against the PKRU; if the key is disabled, access is blocked regardless of the page’s normal permissions, and if it’s enabled, the usual read/write/execute rules apply. The power of this design is that you can flip the PKRU bits for a specific key to enable or disable access to all pages using that key in one quick operation, avoiding updates to many page-table entries and reducing TLB pressure. This makes permission changes for a whole group of pages much faster than modifying individual PTEs and issuing TLB reloads. It’s hardware-supported and relies on the PKRU mechanism, not on expanding the page table or on managing network access.