https://nalar.dev/tags/page-faults/ Recent content in Page Faults on Nalar Hugo en-us Sat, 19 Sep 2026 00:00:00 +0000 https://nalar.dev/userfaultfd-write-protection-turns-memory-writes-into-userspace-events/ Sat, 19 Sep 2026 00:00:00 +0000 https://nalar.dev/userfaultfd-write-protection-turns-memory-writes-into-userspace-events/ <p>A thread can reach a valid, resident page and still stop before modifying it. With Linux <code>userfaultfd</code> 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.</p> <p>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 <code>mprotect()</code> and <code>SIGSEGV</code>.</p>