It's the Kernel's Fault! Custom Page Fault Handling With bpf_fault

Page faults, which occur when a program accesses a virtual memory page that is not mapped to physical memory, are traditionally handled by the operating system. However, many applications benefit from running custom page fault handling logic. For example, some applications may seek to prefill newly faulted pages with content, or intercept writes in order to make a copy of the original contents. Linux's userfaultfd interface enables some of these use cases by offloading fault handling to userspace. Unfortunately, it suffers from significant limitations, namely high overhead, poor scalability, and a design that precludes its use in libraries.