[$] Guard pages for file-backed memory

One of the many new features packed into the 6.13 kernel release was guardpages, a hardening mechanism that makes it possible to inject zero-accesspages into a process's address space in an efficient way. That featureonly supports anonymous (user-space data) pages, though. To make guardpages more widely useful, Lorenzo Stoakes has put together a patchset enabling the feature for file-backed pages as well; in the process,he examined and resolved a long list of potential problems that extendingthe feature could encounter. One potential problem was not on his list,though.