The Kernel Column with Jon Masters: Developing Linux Kernel 3.5

Linus Torvalds announced the latest 3.5 Linux kernel following a quiet Release Candidate (RC) 7 that had only trivial fixes and some MIPS architecture cleanups (which Linus called out as demonstrating a “horrible track record” of late breaking code churn). The 3.5 kernel includes new support for Android-style “wake locks”, userspace probes (“Uprobes”), a new system call level security filter (“seccomp” – “secure computing”), metadata checksum support on ext4 filesystems similar to btrfs, and support for Dan Magenheimer’s “frontswap”. Of the new features, the first and last are particularly contentious, having taken many attempts to merge.

Frontswap allows Linux virtual machines to leverage the availability of “transient” or “transcendent” memory – excess memory within a hypervisor that is not directly addressable by guest Operating Systems (such as Linux) and might disappear in the future, but is still faster than disk storage – to store additional copies of data that will (also) be written to slow disks. This allows a virtual machine to avoid many expensive reads from (virtual) backing storage in the common case that the transcendent memory is not pulled from under the virtual machine due to a more pressing need by the hypervisor.