From 5b7c4cabbb65f5c469464da6c5f614cbd7f730f2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 21 Feb 2023 18:24:12 -0800 Subject: Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next Pull networking updates from Jakub Kicinski: "Core: - Add dedicated kmem_cache for typical/small skb->head, avoid having to access struct page at kfree time, and improve memory use. - Introduce sysctl to set default RPS configuration for new netdevs. - Define Netlink protocol specification format which can be used to describe messages used by each family and auto-generate parsers. Add tools for generating kernel data structures and uAPI headers. - Expose all net/core sysctls inside netns. - Remove 4s sleep in netpoll if carrier is instantly detected on boot. - Add configurable limit of MDB entries per port, and port-vlan. - Continue populating drop reasons throughout the stack. - Retire a handful of legacy Qdiscs and classifiers. Protocols: - Support IPv4 big TCP (TSO frames larger than 64kB). - Add IP_LOCAL_PORT_RANGE socket option, to control local port range on socket by socket basis. - Track and report in procfs number of MPTCP sockets used. - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path manager. - IPv6: don't check net.ipv6.route.max_size and rely on garbage collection to free memory (similarly to IPv4). - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986). - ICMP: add per-rate limit counters. - Add support for user scanning requests in ieee802154. - Remove static WEP support. - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate reporting. - WiFi 7 EHT channel puncturing support (client & AP). BPF: - Add a rbtree data structure following the "next-gen data structure" precedent set by recently added linked list, that is, by using kfunc + kptr instead of adding a new BPF map type. - Expose XDP hints via kfuncs with initial support for RX hash and timestamp metadata. - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to better support decap on GRE tunnel devices not operating in collect metadata. - Improve x86 JIT's codegen for PROBE_MEM runtime error checks. - Remove the need for trace_printk_lock for bpf_trace_printk and bpf_trace_vprintk helpers. - Extend libbpf's bpf_tracing.h support for tracing arguments of kprobes/uprobes and syscall as a special case. - Significantly reduce the search time for module symbols by livepatch and BPF. - Enable cpumasks to be used as kptrs, which is useful for tracing programs tracking which tasks end up running on which CPUs in different time intervals. - Add support for BPF trampoline on s390x and riscv64. - Add capability to export the XDP features supported by the NIC. - Add __bpf_kfunc tag for marking kernel functions as kfuncs. - Add cgroup.memory=nobpf kernel parameter option to disable BPF memory accounting for container environments. Netfilter: - Remove the CLUSTERIP target. It has been marked as obsolete for years, and we still have WARN splats wrt races of the out-of-band /proc interface installed by this target. - Add 'destroy' commands to nf_tables. They are identical to the existing 'delete' commands, but do not return an error if the referenced object (set, chain, rule...) did not exist. Driver API: - Improve cpumask_local_spread() locality to help NICs set the right IRQ affinity on AMD platforms. - Separate C22 and C45 MDIO bus transactions more clearly. - Introduce new DCB table to control DSCP rewrite on egress. - Support configuration of Physical Layer Collision Avoidance (PLCA) Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of shared medium Ethernet. - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing preemption of low priority frames by high priority frames. - Add support for controlling MACSec offload using netlink SET. - Rework devlink instance refcounts to allow registration and de-registration under the instance lock. Split the code into multiple files, drop some of the unnecessarily granular locks and factor out common parts of netlink operation handling. - Add TX frame aggregation parameters (for USB drivers). - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning messages with notifications for debug. - Allow offloading of UDP NEW connections via act_ct. - Add support for per action HW stats in TC. - Support hardware miss to TC action (continue processing in SW from a specific point in the action chain). - Warn if old Wireless Extension user space interface is used with modern cfg80211/mac80211 drivers. Do not support Wireless Extensions for Wi-Fi 7 devices at all. Everyone should switch to using nl80211 interface instead. - Improve the CAN bit timing configuration. Use extack to return error messages directly to user space, update the SJW handling, including the definition of a new default value that will benefit CAN-FD controllers, by increasing their oscillator tolerance. New hardware / drivers: - Ethernet: - nVidia BlueField-3 support (control traffic driver) - Ethernet support for imx93 SoCs - Motorcomm yt8531 gigabit Ethernet PHY - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA) - Microchip LAN8841 PHY (incl. cable diagnostics and PTP) - Amlogic gxl MDIO mux - WiFi: - RealTek RTL8188EU (rtl8xxxu) - Qualcomm Wi-Fi 7 devices (ath12k) - CAN: - Renesas R-Car V4H Drivers: - Bluetooth: - Set Per Platform Antenna Gain (PPAG) for Intel controllers. - Ethernet NICs: - Intel (1G, igc): - support TSN / Qbv / packet scheduling features of i226 model - Intel (100G, ice): - use GNSS subsystem instead of TTY - multi-buffer XDP support - extend support for GPIO pins to E823 devices - nVidia/Mellanox: - update the shared buffer configuration on PFC commands - implement PTP adjphase function for HW offset control - TC support for Geneve and GRE with VF tunnel offload - more efficient crypto key management method - multi-port eswitch support - Netronome/Corigine: - add DCB IEEE support - support IPsec offloading for NFP3800 - Freescale/NXP (enetc): - support XDP_REDIRECT for XDP non-linear buffers - improve reconfig, avoid link flap and waiting for idle - support MAC Merge layer - Other NICs: - sfc/ef100: add basic devlink support for ef100 - ionic: rx_push mode operation (writing descriptors via MMIO) - bnxt: use the auxiliary bus abstraction for RDMA - r8169: disable ASPM and reset bus in case of tx timeout - cpsw: support QSGMII mode for J721e CPSW9G - cpts: support pulse-per-second output - ngbe: add an mdio bus driver - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing - r8152: handle devices with FW with NCM support - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation - virtio-net: support multi buffer XDP - virtio/vsock: replace virtio_vsock_pkt with sk_buff - tsnep: XDP support - Ethernet high-speed switches: - nVidia/Mellanox (mlxsw): - add support for latency TLV (in FW control messages) - Microchip (sparx5): - separate explicit and implicit traffic forwarding rules, make the implicit rules always active - add support for egress DSCP rewrite - IS0 VCAP support (Ingress Classification) - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS etc.) - ES2 VCAP support (Egress Access Control) - support for Per-Stream Filtering and Policing (802.1Q, 8.6.5.1) - Ethernet embedded switches: - Marvell (mv88e6xxx): - add MAB (port auth) offload support - enable PTP receive for mv88e6390 - NXP (ocelot): - support MAC Merge layer - support for the the vsc7512 internal copper phys - Microchip: - lan9303: convert to PHYLINK - lan966x: support TC flower filter statistics - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x - lan937x: support Credit Based Shaper configuration - ksz9477: support Energy Efficient Ethernet - other: - qca8k: convert to regmap read/write API, use bulk operations - rswitch: Improve TX timestamp accuracy - Intel WiFi (iwlwifi): - EHT (Wi-Fi 7) rate reporting - STEP equalizer support: transfer some STEP (connection to radio on platforms with integrated wifi) related parameters from the BIOS to the firmware. - Qualcomm 802.11ax WiFi (ath11k): - IPQ5018 support - Fine Timing Measurement (FTM) responder role support - channel 177 support - MediaTek WiFi (mt76): - per-PHY LED support - mt7996: EHT (Wi-Fi 7) support - Wireless Ethernet Dispatch (WED) reset support - switch to using page pool allocator - RealTek WiFi (rtw89): - support new version of Bluetooth co-existance - Mobile: - rmnet: support TX aggregation" * tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits) page_pool: add a comment explaining the fragment counter usage net: ethtool: fix __ethtool_dev_mm_supported() implementation ethtool: pse-pd: Fix double word in comments xsk: add linux/vmalloc.h to xsk.c sefltests: netdevsim: wait for devlink instance after netns removal selftest: fib_tests: Always cleanup before exit net/mlx5e: Align IPsec ASO result memory to be as required by hardware net/mlx5e: TC, Set CT miss to the specific ct action instance net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG net/mlx5: Refactor tc miss handling to a single function net/mlx5: Kconfig: Make tc offload depend on tc skb extension net/sched: flower: Support hardware miss to tc action net/sched: flower: Move filter handle initialization earlier net/sched: cls_api: Support hardware miss to tc action net/sched: Rename user cookie and act cookie sfc: fix builds without CONFIG_RTC_LIB sfc: clean up some inconsistent indentings net/mlx4_en: Introduce flexible array to silence overflow warning net: lan966x: Fix possible deadlock inside PTP net/ulp: Remove redundant ->clone() test in inet_clone_ulp(). ... --- Documentation/maintainer/rebasing-and-merging.rst | 226 ++++++++++++++++++++++ 1 file changed, 226 insertions(+) create mode 100644 Documentation/maintainer/rebasing-and-merging.rst (limited to 'Documentation/maintainer/rebasing-and-merging.rst') diff --git a/Documentation/maintainer/rebasing-and-merging.rst b/Documentation/maintainer/rebasing-and-merging.rst new file mode 100644 index 000000000..09f988e7f --- /dev/null +++ b/Documentation/maintainer/rebasing-and-merging.rst @@ -0,0 +1,226 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==================== +Rebasing and merging +==================== + +Maintaining a subsystem, as a general rule, requires a familiarity with the +Git source-code management system. Git is a powerful tool with a lot of +features; as is often the case with such tools, there are right and wrong +ways to use those features. This document looks in particular at the use +of rebasing and merging. Maintainers often get in trouble when they use +those tools incorrectly, but avoiding problems is not actually all that +hard. + +One thing to be aware of in general is that, unlike many other projects, +the kernel community is not scared by seeing merge commits in its +development history. Indeed, given the scale of the project, avoiding +merges would be nearly impossible. Some problems encountered by +maintainers result from a desire to avoid merges, while others come from +merging a little too often. + +Rebasing +======== + +"Rebasing" is the process of changing the history of a series of commits +within a repository. There are two different types of operations that are +referred to as rebasing since both are done with the ``git rebase`` +command, but there are significant differences between them: + + - Changing the parent (starting) commit upon which a series of patches is + built. For example, a rebase operation could take a patch set built on + the previous kernel release and base it, instead, on the current + release. We'll call this operation "reparenting" in the discussion + below. + + - Changing the history of a set of patches by fixing (or deleting) broken + commits, adding patches, adding tags to commit changelogs, or changing + the order in which commits are applied. In the following text, this + type of operation will be referred to as "history modification" + +The term "rebasing" will be used to refer to both of the above operations. +Used properly, rebasing can yield a cleaner and clearer development +history; used improperly, it can obscure that history and introduce bugs. + +There are a few rules of thumb that can help developers to avoid the worst +perils of rebasing: + + - History that has been exposed to the world beyond your private system + should usually not be changed. Others may have pulled a copy of your + tree and built on it; modifying your tree will create pain for them. If + work is in need of rebasing, that is usually a sign that it is not yet + ready to be committed to a public repository. + + That said, there are always exceptions. Some trees (linux-next being + a significant example) are frequently rebased by their nature, and + developers know not to base work on them. Developers will sometimes + expose an unstable branch for others to test with or for automated + testing services. If you do expose a branch that may be unstable in + this way, be sure that prospective users know not to base work on it. + + - Do not rebase a branch that contains history created by others. If you + have pulled changes from another developer's repository, you are now a + custodian of their history. You should not change it. With few + exceptions, for example, a broken commit in a tree like this should be + explicitly reverted rather than disappeared via history modification. + + - Do not reparent a tree without a good reason to do so. Just being on a + newer base or avoiding a merge with an upstream repository is not + generally a good reason. + + - If you must reparent a repository, do not pick some random kernel commit + as the new base. The kernel is often in a relatively unstable state + between release points; basing development on one of those points + increases the chances of running into surprising bugs. When a patch + series must move to a new base, pick a stable point (such as one of + the -rc releases) to move to. + + - Realize that reparenting a patch series (or making significant history + modifications) changes the environment in which it was developed and, + likely, invalidates much of the testing that was done. A reparented + patch series should, as a general rule, be treated like new code and + retested from the beginning. + +A frequent cause of merge-window trouble is when Linus is presented with a +patch series that has clearly been reparented, often to a random commit, +shortly before the pull request was sent. The chances of such a series +having been adequately tested are relatively low - as are the chances of +the pull request being acted upon. + +If, instead, rebasing is limited to private trees, commits are based on a +well-known starting point, and they are well tested, the potential for +trouble is low. + +Merging +======= + +Merging is a common operation in the kernel development process; the 5.1 +development cycle included 1,126 merge commits - nearly 9% of the total. +Kernel work is accumulated in over 100 different subsystem trees, each of +which may contain multiple topic branches; each branch is usually developed +independently of the others. So naturally, at least one merge will be +required before any given branch finds its way into an upstream repository. + +Many projects require that branches in pull requests be based on the +current trunk so that no merge commits appear in the history. The kernel +is not such a project; any rebasing of branches to avoid merges will, most +likely, lead to trouble. + +Subsystem maintainers find themselves having to do two types of merges: +from lower-level subsystem trees and from others, either sibling trees or +the mainline. The best practices to follow differ in those two situations. + +Merging from lower-level trees +------------------------------ + +Larger subsystems tend to have multiple levels of maintainers, with the +lower-level maintainers sending pull requests to the higher levels. Acting +on such a pull request will almost certainly generate a merge commit; that +is as it should be. In fact, subsystem maintainers may want to use +the --no-ff flag to force the addition of a merge commit in the rare cases +where one would not normally be created so that the reasons for the merge +can be recorded. The changelog for the merge should, for any kind of +merge, say *why* the merge is being done. For a lower-level tree, "why" is +usually a summary of the changes that will come with that pull. + +Maintainers at all levels should be using signed tags on their pull +requests, and upstream maintainers should verify the tags when pulling +branches. Failure to do so threatens the security of the development +process as a whole. + +As per the rules outlined above, once you have merged somebody else's +history into your tree, you cannot rebase that branch, even if you +otherwise would be able to. + +Merging from sibling or upstream trees +-------------------------------------- + +While merges from downstream are common and unremarkable, merges from other +trees tend to be a red flag when it comes time to push a branch upstream. +Such merges need to be carefully thought about and well justified, or +there's a good chance that a subsequent pull request will be rejected. + +It is natural to want to merge the master branch into a repository; this +type of merge is often called a "back merge". Back merges can help to make +sure that there are no conflicts with parallel development and generally +gives a warm, fuzzy feeling of being up-to-date. But this temptation +should be avoided almost all of the time. + +Why is that? Back merges will muddy the development history of your own +branch. They will significantly increase your chances of encountering bugs +from elsewhere in the community and make it hard to ensure that the work +you are managing is stable and ready for upstream. Frequent merges can +also obscure problems with the development process in your tree; they can +hide interactions with other trees that should not be happening (often) in +a well-managed branch. + +That said, back merges are occasionally required; when that happens, be +sure to document *why* it was required in the commit message. As always, +merge to a well-known stable point, rather than to some random commit. +Even then, you should not back merge a tree above your immediate upstream +tree; if a higher-level back merge is really required, the upstream tree +should do it first. + +One of the most frequent causes of merge-related trouble is when a +maintainer merges with the upstream in order to resolve merge conflicts +before sending a pull request. Again, this temptation is easy enough to +understand, but it should absolutely be avoided. This is especially true +for the final pull request: Linus is adamant that he would much rather see +merge conflicts than unnecessary back merges. Seeing the conflicts lets +him know where potential problem areas are. He does a lot of merges (382 +in the 5.1 development cycle) and has gotten quite good at conflict +resolution - often better than the developers involved. + +So what should a maintainer do when there is a conflict between their +subsystem branch and the mainline? The most important step is to warn +Linus in the pull request that the conflict will happen; if nothing else, +that demonstrates an awareness of how your branch fits into the whole. For +especially difficult conflicts, create and push a *separate* branch to show +how you would resolve things. Mention that branch in your pull request, +but the pull request itself should be for the unmerged branch. + +Even in the absence of known conflicts, doing a test merge before sending a +pull request is a good idea. It may alert you to problems that you somehow +didn't see from linux-next and helps to understand exactly what you are +asking upstream to do. + +Another reason for doing merges of upstream or another subsystem tree is to +resolve dependencies. These dependency issues do happen at times, and +sometimes a cross-merge with another tree is the best way to resolve them; +as always, in such situations, the merge commit should explain why the +merge has been done. Take a moment to do it right; people will read those +changelogs. + +Often, though, dependency issues indicate that a change of approach is +needed. Merging another subsystem tree to resolve a dependency risks +bringing in other bugs and should almost never be done. If that subsystem +tree fails to be pulled upstream, whatever problems it had will block the +merging of your tree as well. Preferable alternatives include agreeing +with the maintainer to carry both sets of changes in one of the trees or +creating a topic branch dedicated to the prerequisite commits that can be +merged into both trees. If the dependency is related to major +infrastructural changes, the right solution might be to hold the dependent +commits for one development cycle so that those changes have time to +stabilize in the mainline. + +Finally +======= + +It is relatively common to merge with the mainline toward the beginning of +the development cycle in order to pick up changes and fixes done elsewhere +in the tree. As always, such a merge should pick a well-known release +point rather than some random spot. If your upstream-bound branch has +emptied entirely into the mainline during the merge window, you can pull it +forward with a command like:: + + git merge v5.2-rc1^0 + +The "^0" will cause Git to do a fast-forward merge (which should be +possible in this situation), thus avoiding the addition of a spurious merge +commit. + +The guidelines laid out above are just that: guidelines. There will always +be situations that call out for a different solution, and these guidelines +should not prevent developers from doing the right thing when the need +arises. But one should always think about whether the need has truly +arisen and be prepared to explain why something abnormal needs to be done. -- cgit v1.2.3