Commit Comments

Commit message content

Writing good commit comments is critical to ensuring that changes are easily understood, even years after they were originally written. The commit comment should contain enough information about the change to allow the reader to understand the motivation for the change, what parts of the code it is affecting, and any interesting, unusual, or complex parts of the change to draw attention to.

The reason for a change may be manyfold: bug, enhancement, feature, code style, etc. so providing information about this sets the stage for understanding the change. If it is a bug, include information about what usage triggers the bug and how it manifests (error messages, LBUG, etc). If it is a feature, include information about what improvement is being made, and how it will affect usage.

Providing some high-level information about the code path that is being modified is useful for the reader, since the files and patch fragments are not necessarily going to be listed in a sensible order in the patch. Including the important functions being modified provides a starting point for the reader to follow the logic of the change, and makes it easier to search for such changes in the future.

If the patch is based on some earlier patch, then including the git commit hash of the original patch, Jira ticket number, etc. is useful for tracking the chain of dependencies. This can be very useful if a patch is landed separately to different maintenance branches, if it is fixing a problem in a previously landed patch, or if it is being imported from an upstream kernel commit.

Having long commit comments that describe the change well is a good thing. The commit comments will be tightly associated with the code for a long time into the future, even many of the original commit comments from years earlier are still available through changes of the source code repository. In contrast, bug tracking systems come and go, and cannot be relied upon to track information about a change for extended periods of time.

Commit message format

Unlike the content of the commit message, the format is relatively easy to verify for correctness. Having the same standard format allows Git tools like git shortlog to extract information from the patches more easily.

The first line of the commit comment is the commit summary of the change. Changes submitted to the fs/lustre-release branch require a Lustre Jira ticket number at the beginning of the commit summary. A Lustre Jira ticket is one that begins with LU and is therefore part of the Lustre project within Jira. If the patch is submitted for the fs/lustre-release repository without a Lustre Jira ID in the first line, then it will automatically receive a -2 review which will prevent the patch from being submitted to a release branch. You would then need to fix the summary line and resubmit the patch.

The commit summary should also have a component: tag immediately following the Jira ticket number that indicates which Lustre subsystem that the commit is related to. Example Lustre subsystems relate to modules like: llite, lov, osc, mdc, lmv, ldlm, ptlrpc, mds, oss, mdd, osd-ldiskfs, ldiskfs, lnet, libcfs, socklnd, o2iblnd; functional components like recovery, quota, grant; or auxiliary components like build, tests, iokit, docs. This subsystem list is not exhaustive, but provides a good guideline for consistency.

The commit summary line must be 62 characters or less, including the Jira ticket number and component tag, so that git shortlog and git format-patch can fit the summary onto a single line. The summary must be followed by a blank like. The rest of the comments should be wrapped to 70 columns or less. This allows for the first line to be used a subject in emails, and also for the entire body to be displayed using tools like git log or git shortlog in an 80 column window.

LU-nnn component: short description of change under 62 columns

The "component:" should be a lower-case single-word subsystem of the
Lustre code that best encompasses the change being made.  Examples of
components include modules: llite, lov, lmv, osc, mdc, ldlm, lnet,
ptlrpc, mds, oss, osd-ldiskfs, ldiskfs, libcfs, socklnd, o2iblnd;
functional subsystems: recovery, quota, grant; and auxilliary areas:
build, tests, docs.  This list is not exhaustive, but is a guideline.

The commit comment should contain a detailed explanation of changes
being made.  This can be as long as you'd like.  Please give details
of what problem was solved (including error messages or problems that
were seen), a good high-level description of how it was solved, and
which parts of the code were changed (including important functions
that were changed, if this is useful to understand the patch, and
for easier searching).  Wrap lines at/under 70 columns.

Signed-off-by: Your Real Name <your_email@domain.name>
Change-Id: Ixxxx(added automatically if missing)xxxx

The Change-Id: line and Code Style

Gerrit needs to automatically identify updates to existing patches and update the existing change instead of creating a new one for each patch submitted. This preserves the history of patch comments, and allows comparing old and new versions of a patch for more efficient inspections. For this to work properly the changes you create locally need to have a unique commit id in them.

Please make sure the Change-Id: line and Signed-off-by: lines are at the bottom of the comment. This is required by the fs/lustre-release project in order for the patch to be approved. If your comments are missing either of these lines the patch will be automatically be rejected at submission time.

The Commit-Id: field is inserted automatically into the commit message by installing the build/commit-msg hook into each Git repository's .git/hooks/ directory. The build/prepare-commit-msg script should be installed as well. From the top-level Lustre tree checkout:

ln -sf ../../build/commit-msg .git/hooks/
ln -sf ../../build/prepare-commit-msg .git/hooks/

At git commit} time this will run the {{build/prepare-commit-msg script from the currently Lustre tree to check the code changes for style errors using build/checkpatch.pl, and add comments at the bottom of the commit message with any warnings or errors. After the commit message has been saved, the build/commit-msg script will verify that the commit message format matches the format specified above, and insert the Commit-Id: field into the commit message, if one isn't already present.

Additional commit tags

A number of additional commit tags can be used to further explain who has contributed to the patch, and for tracking purposes. These tags are commonly used with Linux kernel patches. These tags should appear before the Signed-off-by: tag.

Acked-by: User Name <user@domain.com>
Tested-by: User Name <user@domain.com>
Reported-by: User Name <user@domain.com>
Reviewed-by: User Name <user@domain.com>
CC: User Name <user@domain.com>
{Organization}-bug-id: arbitrary bug tracking identifier
Original-commit: {git commit hash of original patch}
Test-Parameters: additional testing parameters

The {Organization}-bug-id: tag (e.g. Whamcloud-bug-id: ORI-123, Xyratex-bug-id: MRP-123, or Oracle-bug-id: b=12345) can be used to track this patch in other bug databases (e.g. bugzilla or internal bug databases).

  Original-commit: is used to reference the original version of a patch that is being ported to another branch.  Test-Parameters: is used to specify additional testing parameters for this patch, see Changing Test Parameters with Gerrit Commit Messages.

Examples of good commit comments

LU-477 ldiskfs: allocate s_group_desc/s_group_info by vmalloc()

Add the patch to the RHEL6 ldiskfs patch series.

Large kmalloc() for sbi->s_group_desc and sbi->s_group_info can fail
for large filesystems (typically over 16TB), which will cause the
"not enough memory" error while mounting.  This patch makes allocation
fall back to vmalloc() if the kmalloc() failed, as what was done for
sbi->s_flex_groups.

To avoid colliding with an valid on-disk inode number, EXT4_BAD_INO
is used as the number of the buddy cache inode.

The patch also incorporates the following upstream kernel fix:

commit 32a9bb57d7c1fd04ae0f72b8f671501f000a0e9f
ext4: fix missing iput() of root inode for some mount error paths
https://bugzilla.kernel.org/show_bug.cgi?id=26752

Signed-off-by: Yu Jian <yujian whamcloud.com>
Change-Id: I3950425835ea7f2968ceb2edbc622e3ff3ed8545

LU-723 build: Enhance lustre/ldiskfs build system


Enhance the lustre/ldiskfs build system so it is more robust, flexible,
and consistent with lustre/zfs build system. This change is being made
in the interest of standardizing the infrastructure around backend
filesystems.

This change does not effect the current behavior of the --with-ldiskfs,
--enable-ext4, or --with-ldiskfsprogs configure options. However, it
does remove the obsolete --with-ldiskfs-inkernel configure option which
was only used by LLNL. It also adds the --with-ldiskfs-obj configure
option which improves flexibility. And the --enable-ldiskfs-build
configure option to support building against the lustre-ldiskfs-devel
package. The behavior of these options is consistent with their ZFS
counterparts, see commit 8c7266c for further details.

--enable-ext4 enable ldiskfs build using ext4
--enable-ldiskfs-build enable ldiskfs configure/make
--with-ldiskfs=path set path to ldiskfs source
--with-ldiskfs-obj=path set path to ldiskfs objects
--with-ldiskfsprogs use alternate names for ldiskfs-enabled e2fsprogs

Sample ./configure results when building lustre and ldiskfs using
the kernel-devel and kernel-debuginfo-common packages.

checking whether to enable ldiskfs... yes
checking ldiskfs source directory... /home/behlendo/src/git/lustre/ldiskfs
checking ldiskfs object directory... /home/behlendo/src/git/lustre/ldiskfs
checking ldiskfs module symbols... Module.symvers
checking ldiskfs source release... 3.3.0
checking whether to use ext3 or ext4 source... ext4
checking ext4 source directory... /usr/src/debug/.../fs/ext4
checking whether to build ldiskfs... yes
checking for /home/behlendo/src/git/lustre/ldiskfs/configure... yes
checking for /usr/src/debug/.../fs/ext4/dir.c... yes
checking for /usr/src/debug/.../fs/ext4/file.c... yes
checking for /usr/src/debug/.../fs/ext4/inode.c... yes
checking for /usr/src/debug/.../fs/ext4/super.c... yes
checking if ext4_ext_walk_space() takes i_data_sem... yes
checking if LDISKFS_SINGLEDATA_TRANS_BLOCKS takes sb as argument... yes
checking if ldiskfs_discard_preallocations defined... yes
checking if ldiskfs_ext_insert_extent needs 5 arguments... yes

In the context of this change additional cleanup has been done and
all of the ldiskfs specific code relocated to lustre-build-ldiskfs.m4.

Note that this change moves us closer to supporting patchless Lustre
servers with ldiskfs. Once the remaining kernel patches for Lustre
are dropped you will be able to build Lustre using the distribution
provided kernel-devel and kernel-debuginfo-common packages.

This change also incorporates ORI-340 commit f604951, which ensures
that the Module.symvers file will cleanly include the symbols for all
enabled Lustre backends. While the only backend supported by master
right now is ldiskfs this brings the master and orion branches into
sync in this regard.

Change-Id: I6f13f266944ec6967f4d7705a30b83ab8e577b15
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <surya1@llnl.gov>