Crash consistency using persistent memory programming libraries requires programmers to use complex transactions and manual annotations. In contrast, the failure-atomic msync() (FAMS) interface is much simpler as it transparently tracks updates and guarantees that modified data is atomically durable on a call to the failure-atomic variant of msync(). However, FAMS suffers from several drawbacks, like the overhead of msync() and the write amplification from page-level dirty data tracking. To address these drawbacks while preserving the advantages of FAMS, we propose Snapshot, an efficient userspace implementation of FAMS. Snapshot uses compiler-based annotation to transparently track updates in userspace and syncs them with the backing byte-addressable storage copy on a call to msync(). By keeping a copy of application data in DRAM, Snapshot improves access latency. Moreover, with automatic tracking and syncing changes only on a call to msync(), Snapshot provides crash-consistency guarantees, unlike the POSIX msync() system call. For a KV-Store backed by Intel Optane running the YCSB benchmark, Snapshot achieves at least 1.2$\times$ speedup over PMDK while significantly outperforming conventional (non-crash-consistent) msync(). On an emulated CXL memory semantic SSD, Snapshot outperforms PMDK by up to 10.9$\times$ on all but one YCSB workload, where PMDK is 1.2$\times$ faster than Snapshot. Further, Kyoto Cabinet commits perform up to 8.0$\times$ faster with Snapshot than its built-in, msync()-based crash-consistency mechanism.