Backporting patches

Note

This is an advanced topic for developers and maintainers. Readers should familiarize themselves with building and running OVN, with the git tool, and with the OVN patch submission process.

The backporting of patches from one git tree to another takes multiple forms within OVN, but is broadly applied in the following fashion:

  • Contributors submit their proposed changes to the latest development branch
  • Contributors and maintainers provide feedback on the patches
  • When the change is satisfactory, maintainers apply the patch to the development branch.
  • Maintainers backport changes from a development branch to release branches.

With regards to OVN user space code and code that does not comprise the Linux datapath and compat code, the development branch is master in the OVN repository. Patches are applied first to this branch, then to the most recent branch-X.Y, then earlier branch-X.Z, and so on. The most common kind of patch in this category is a bugfix which affects master and other branches.

Changes to userspace components

Patches which are fixing bugs should be considered for backporting from master to release branches. OVN contributors submit their patches targeted to the master branch, using the Fixes tag described in Submitting Patches. The maintainer first applies the patch to master, then backports the patch to each older affected tree, as far back as it goes or at least to all currently supported branches. This is usually each branch back to the most recent LTS release branch.

If the fix only affects a particular branch and not master, contributors should submit the change with the target branch listed in the subject line of the patch. Contributors should list all versions that the bug affects. The git format-patch argument --subject-prefix may be used when posting the patch, for example:

$ git format-patch -1 --subject-prefix="PATCH ovn branch-21.06"

If a maintainer is backporting a change to older branches and the backport is not a trivial cherry-pick, then the maintainer may opt to submit the backport for the older branch on the mailing list for further review. This should be done in the same manner as described above.

Submission

Once the patches are all assembled and working on the OVN tree, they need to be formatted again using git format-patch. The common format for commit messages for Linux backport patches is as follows:

datapath: Remove incorrect WARN_ONCE().

Upstream commit:
    commit c6b2aafffc6934be72d96855c9a1d88970597fbc
    Author: Jarno Rajahalme <jarno@ovn.org>
    Date:   Mon Aug 1 19:08:29 2016 -0700

    openvswitch: Remove incorrect WARN_ONCE().

    ovs_ct_find_existing() issues a warning if an existing conntrack entry
    classified as IP_CT_NEW is found, with the premise that this should
    not happen.  However, a newly confirmed, non-expected conntrack entry
    remains IP_CT_NEW as long as no reply direction traffic is seen.  This
    has resulted into somewhat confusing kernel log messages.  This patch
    removes this check and warning.

    Fixes: 289f2253 ("openvswitch: Find existing conntrack entry after upcall.")
    Suggested-by: Joe Stringer <joe@ovn.org>
    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
    Acked-by: Joe Stringer <joe@ovn.org>

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>

The upstream commit SHA should be the one that appears in Linus’ tree so that reviewers can compare the backported patch with the one upstream. Note that the subject line for the backported patch replaces the original patch’s openvswitch prefix with datapath. Patches which only affect the datapath/linux/compat directory should be prefixed with compat.

The contents of a backport should be equivalent to the changes made by the original patch; explain any variations from the original patch in the commit message - For instance if you rolled in a bugfix. Reviewers will verify that the changes made by the backport patch are the same as the changes made in the original commit which the backport is based upon. Patch submission should otherwise follow the regular steps described in Submitting Patches.