Page Faults

userfaultfd Write Protection Turns Memory Writes into Userspace Events

A thread can reach a valid, resident page and still stop before modifying it. With Linux userfaultfd write-protect mode, a registered page can be marked so that a write generates a userfaultfd page-fault event. A userspace handler receives that event, performs its bookkeeping, removes the protection, and lets the blocked thread continue. The mechanism sits between ordinary page permissions and application-level memory accounting. The page remains part of the process address space; the kernel redirects the write fault into a file-descriptor protocol instead of forcing the application to build the same control path around mprotect() and SIGSEGV.