Speaker
Description
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.
We present bpf_fault, an experimental framework that allows applications to run page fault handlers directly within the Linux kernel using eBPF. By eliminating the overheads and complexity associated with userfaultfd, bpf_fault reduces fault latency by 2.8-6.1x and eliminates userfaultfd’s scalability bottleneck. We integrate bpf_fault with several applications, including VM live snapshots in Firecracker and QEMU (eliminating tail latency spikes caused by snapshots), JVM garbage collection, and more. We also design a novel lazy dynamic linking mechanism for Linux that defers relocations to fault time using bpf_fault, a use case impossible with userfaultfd, which reduces dirty memory usage of widely-used applications like Chrome and Clang by up to 50%. Together, these results demonstrate that eBPF-based fault handling can improve performance and reduce resource usage in widely-deployed applications.