OS News says, according to John Baldwin, The SMPVFS work is a task to add fine-grained locking to the VFS layer of the kernel as well as the UFS and nullfs filesystems. The VFS layer provides the abstractions in the kernel that describe file objects.
Robert N M Watson: One of the other nice benefits to the SMPVFS work is that with our fully preemptive 6.x kernel, not holding the Giant lock over the file system code lets the file system code not only preempt lower precedence kernel threads, such as background crypto operations or file system operations, but be preempted by more timing critical code, such as sound card interrupts, network I/O, and so on. So this isn’t just a win for SMP, but a win for UP also.
Scott Long: SMPVFS also reduces contention for storage drivers that are still under the Giant lock and increases the possible parallelism between these drivers and the filesystems above them. Kris’ tests are a very good example of this; even though the SCSI subsystem and most of the ESP driver are still under the Giant lock, performance still scaled well. Read more.