Linux 3.18 has been released on Sun, 7 Dec 2014

Summary: This release adds support for overlayfs, which allows to combine two filesystem in a single mount point; support for mapping user space memory into the GPU on Radeon devices, a bpf() syscall that allows to upload BPF-like programs that can be attached to events; a TCP congestion algorithm optimized for data centers; the Geneve virtualization encapsulation, support for embedding IP protocols over UDP, improved networking performance thanks to batching the processing of socket buffers, and optional multi-queue SCSI support. There are also new drivers and many other small improvements.

1. Prominent features

1.1. Overlayfs

An overlay filesystem combines two filesystems, an 'upper' filesystem and a 'lower' filesystem, into a single file system namespace, modifications will be done to the upper filesystem. It has many uses, but it is most often used for live CDs, where a read-only OS image is used as lower filesystem and a writeable RAM-backed filesystem is used as the upper one. Any modifications will be done in the upper filesystem, thus allowing users to run the OS image provided normally. Overlayfs differs from other "union filesystem" implementations in that after a file is opened all operations go directly to the underlying, lower or upper, filesystems. This simplifies the implementation and allows native performance in these cases.

It is possible for both directory trees to be in the same filesystem and there is no requirement that the root of a filesystem be given for either upper or lower. The lower filesystem can be any filesystem supported by Linux and does not need to be writable. The lower filesystem can even be another overlayfs. The upper filesystem will normally be writable and if it is it must support the creation of trusted.* extended attributes, and must provide valid d_type in readdir() responses, so NFS is not suitable.

Documentation: commit Code: commit

1.2. Radeon: mapping of user pages into video memory

Linux 3.16 added the ability to map users addresses into the video memory for Intel hardware. In this release, AMD Radeon has also gained support for this feature. Normal application data can be used as a texture source or even as a render target (depending upon the capabilities of the chipset). This has a number of uses, with zero-copy downloads to the GPU and efficient readback making the intermixed streaming of CPU and GPU operations fairly efficient. This ability has many widespread implications from faster rendering of client-side software rasterisers (chromium), mitigation of stalls due to read back (Mozilla Firefox) and to faster pipelining of texture data (such as pixel buffer objects in OpenGL or data blobs in OpenCL).

Code: commit

1.3. bpf() syscall for eBFP virtual machine programs

bpf() syscall is a multiplexor for a range of different operations on eBPF which can be characterized as "universal in-kernel virtual machine". eBPF is similar to original Berkeley Packet Filter used to filter network packets. eBPF "extends" classic BPF in multiple ways including ability to call in-kernel helper functions and access shared data structures like eBPF maps. The programs can be written in a restricted C that is compiled into eBPF bytecode and executed on the eBPF virtual machine or JITed into native instruction set.

eBPF programs are similar to kernel modules. They are loaded by the user process and automatically unloaded when process exits. Each eBPF program is a safe run-to-completion set of instructions. eBPF verifier statically determines that the program terminates and is safe to execute. The programs are attached to different events. These events can be packets, tracepoint events and other types in the future. Beyond storing data the programs may call into in-kernel helper functions which may, for example, dump stack, do trace_printk or other forms of live kernel debugging.

Recommended LWN article: The BPF system call API, version 14

ebfp() man page and design documentation can be read on the merge commit: commit

Code: commit 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

1.4. TCP: Data Center TCP congestion algorithm

This release adds the Data Center TCP (DCTCP) congestion control algorithm. DCTCP is an enhancement to the TCP congestion control algorithm for data center networks. DCTCP has been designed for workloads typical of data center environments to provide/achieve: high burst tolerance, low latency and high throughput.

For more details about DCTCP, see the DCTCP web page

Code: commit

1.5. Networking: Geneve Virtualization Encapsulation

Advent of network virtualization has caused a surge of renewed interest and a corresponding increase in the introduction of new protocols, ranging all the way from VLANs and MPLS through the more recent VXLAN, NVGRE, and STT. Existing tunnel protocols have each attempted to solve different aspects of the new requirements, only to be quickly rendered out of date. This release adds Geneve, a protocol which seeks to avoid these problems by providing a framework for tunneling that provide Layer 2 Networks over Layer 3 Networks.

For more information see http://tools.ietf.org/html/draft-gross-geneve-01

Related VMware blog post: Geneve, VXLAN, and Network Virtualization Encapsulations

Code: commit

1.6. Networking performance optimization: transmission queue batching

This release adds support for deferred flushing of transmission SKBs (socket buffers) to the networking driver. Processing the transmission queue is expensive, so some batching shares that cost with other SKBs. This change allows to achieve 10 Gbit/s full TX wirespeed smallest packet size on a single CPU core (14.8 Mpps). Several drivers already have support for this feature: i40e, igb, ixgbe, mlx4, virtio_net, more will follow in next releases.

Recommended LWN article: Bulk network packet transmission

Recommended blog post Unlocked 10Gbps TX wirespeed smallest packet single core

Code: see the blog post link

1.7. Foo-over-UDP support

This release adds the ability to encapsulate any IP protocol over UDP including tunnels (IPIP, GRE, SIT).

The rationale for this funcionality is that network mechanisms, hardware and optimizations for UDP (such as ECMP and RSS) can be leveraged to provide better service. GRE, IPIP, and SIT have been modified with netlink commands to configure use of FOU on transmit. A new "ip fou" has been added in newer releases of ip to make use of this feature. Details on configuration can be found in the merge link.

Recommended LWN link: Foo over UDP

Merge link: merge

1.8. Optional multiqueue SCSI support

Linux 3.13 added a new design for the block layer that allowed to process multiple IO queues in parallel. This feature, however, wasn't transparent, and required modifications in drivers to support it. In this release, support for the multi-queue layer has been added to the SCSI layer (used by ATA and SATA drivers) as an optional configuration option.

Code: commit

2. Drivers and architectures

All the driver and architecture-specific changes can be found in the Linux_3.18-DriversArch page

3. File systems

F2FS Introduce FITRIM discard support commit Support atomic writes commit Support large sector size commit Support volatile operations for transient data commit

ext4 Support freezing ext2 (nojournal) file systems commit Add sysfs entry showing whether the fs contains errors commit

NFS Implement NFS v4.2 SEEK operation, allowing to use lseek() call with either the SEEK_HOLE or SEEK_DATA flags set commit, commit Allow turning off nfsv3 readdir_plus commit nfsd: add a v4_end_grace file to /proc/fs/nfsd that allows a privileged userland process to end the v4 grace period early commit lockd: add a /proc/fs/lockd/nlm_end_grace file that will allow a (privileged) userland process to end the NLM grace period early commit

SMB Add mfsymlinks support for SMB2.1/SMB3 commit, commit Allow mknod and mkfifo on SMB2/SMB3 mounts commit Allow conversion of characters in Mac remap range commit, commit Remap reserved POSIX characters by default commit

Btrfs Make defragment work with nodatacow option commit Improve free space cache management and space allocation commit Implement repair function when direct read fails commit

XFS Add debug sysfs attribute set commit, commit

NTFS Add FIBMAP ioctl support commit



4. Security

Smack: Add a new access (aka permissive) mode commit

ima: provide 'ima_appraise=log' kernel option that allows logging only mode without fixing it, in order to properly analyze the system commit

5. Block

Implement readpages() to optimize sequential reads of /dev block devices commit

SCSI target userspace backend that uses UIO + shared memory ring to dispatch I/O and control commands into user-space commit

dm raid: add discard support for RAID levels 1 and 10 commit and 4, 5 and 6 commit

zram memory size limitation commit

zram: report maximum used memory commit

6. Memory management

memory-hotplug: add /sys/devices/system/memory/memoryX/valid_zones, shows which zone this memory block can be onlined to commit

balloon_compaction: adds three counters into /proc/vmstat: "balloon_inflate", "balloon_deflate" and "balloon_migrate". They accumulate balloon activity. Under option CONFIG_MEMORY_BALLOON commit

slab merge: If new creating slab has similar size and property with existent slab, this feature reuse it rather than creating new one commit

Support compiling out madvise() and fadvise() commit

Support fadvise without CONFIG_MMU commit

Introduce dump_vma commit

NUMA: Memory zones are allocated by the page allocator in either node or zone order. This release defaults node-ordering on 64-bit NUMA, and to zone-ordering on 32-bit commit

Stress test for memory compaction commit

7. Cryptography

Software asynchronous crypto daemon that provides the kernel thread to assist multi-buffer crypto algorithms for submitting jobs and flushing jobs in multi-buffer crypto algorithms commit, commit, commit, commit, commit

KEYS: Implement binary asymmetric key ID handling commit

8. Virtualization

Xen Xen PV SCSI backend driver commit Add Xen PV SCSI frontend driver commit Remap Xen Identity Mapped RAM commit



9. Tracing & perf

perf tools Add +field argument support for --field option commit Add +field argument support for --sort option commit Add perf-with-kcore script commit Add report.queue-size config file option commit Add support to new style format of kernel PMU event commit Let a user specify a PMU event without any config terms commit Let default config be defined for a PMU commit

top Add -w option for setting column width commit Add a visual cue for toggle zeroing of samples commit Handle 'z' key for toggle zeroing samples in TUI commit

perf bench futex: Support operations for shared futexes commit

perf kvm stat report: Enable the target.system_wide flag commit

10. Networking

ipv4: implement igmp_qrv sysctl to tune IGMP robustness variable commit

ipv6: add sysctl_mld_qrv to configure how many retransmit of unsolicited Multicast Listener Discovery retransmit should happen. Admins might want to tune this on lossy links commit

ipv6: notify userspace when we added or changed an ipv6 token commit

ipvs: Allow heterogeneous pools commit, commit

TCP: Restore RFC5961-compliant behavior for SYN packets commit

TCP: TCP Small Queues and strange attractors. The purpose is to try to keep number of packets in qdisc as small as possible commit

TCP: improve undo on timeout (increases the undo events by 50% on Google servers) commit

TIPC: add name distributor resiliency queue, helps to avoid race conditions in TIPC distributed name table updates commit

UDP: GRO for UDPv6 commit

ip_tunnel: Add GUE support commit

ethtool: adds new ethtool cmd, ETHTOOL_GTUNABLE & ETHTOOL_STUNABLE for getting and setting tunable values commit

pktgen: add flag NO_TIMESTAMP to disable timestamping commit

xfrm: Enable to specify local and remote prefix length thresholds for the policy hash table via a netlink XFRM_MSG_NEWSPDINFO message commit

openvswitch: Add recirc and hash action. commit

openvswitch: Add support for Geneve tunneling. commit

Bluetooth: 6lowpan: Enable multicast support commit

bridge: Add a default_pvid sysfs attribute commit

bridge: Add filtering support for default_pvid commit

Wireless Support DTPC IE (from Cisco Client eXtensions) commit Add Radio Resource Management support to assoc request commit Add WMM traffic stream API commit Re-enable CSA for drivers that support it commit

nftables IPv4 masquerading support commit IPv6 masquerading support commit allow to filter from pre-routing and post-routing commit extend NFT_MSG_DELTABLE to support flushing the ruleset commit nfnetlink_acct: add filter support to nfacct counter list/reset commit export rule-set generation ID commit nft_meta: Add CPU attribute support, allows to match packets with CPU number commit nft_meta: add pkttype support for ip, ipv6 and inet families of tables. Allows to match packets based on the link layer information commit nft_meta: add devgroup support in meta expression, lets match device group of a packets incoming or outgoing interface commit

netfilter ipset: hash:mac type added to ipset, by which one can store MAC (Ethernet address) elements in a set. commit conntrack: disable generic tracking for known protocols commit



11. Core (various)