NFSD Maintainer Entry Profile¶
A Maintainer Entry Profile supplements the top-level process documents (found in Documentation/process/) with customs that are specific to a subsystem and its maintainers. A contributor may use this document to set their expectations and avoid common mistakes. A maintainer may use these profiles to look across subsystems for opportunities to converge on best common practices.
Overview¶
The Network File System (NFS) is a standardized family of network protocols that enable access to files across a set of network- connected peer hosts. Applications on NFS clients access files that reside on file systems that are shared by NFS servers. A single network peer can act as both an NFS client and an NFS server.
NFSD refers to the NFS server implementation included in the Linux kernel. An in-kernel NFS server has fast access to files stored in file systems local to that server. NFSD can share files stored on most of the file system types native to Linux, including xfs, ext4, btrfs, and tmpfs.
Mailing list¶
The linux-nfs@vger.kernel.org mailing list is a public list. Its purpose is to enable collaboration among developers working on the Linux NFS stack, both client and server. It is not a place for conversations that are not related directly to the Linux NFS stack.
The linux-nfs mailing list is archived on lore.kernel.org.
The Linux NFS community does not have any chat room.
Reporting bugs¶
If you experience an NFSD-related bug on a distribution-built kernel, please start by working with your Linux distributor.
Bug reports against upstream Linux code bases are welcome on the linux-nfs@vger.kernel.org mailing list, where some active triage can be done. NFSD bugs may also be reported in the Linux kernel community’s bugzilla at:
Please file NFSD-related bugs under the “Filesystems/NFSD” component. In general, including as much detail as possible is a good start, including pertinent system log messages from both the client and server.
User space software related to NFSD, such as mountd or the exportfs command, is contained in the nfs-utils package. Report problems with those components to linux-nfs@vger.kernel.org. You might be directed to move the report to a specific bug tracker.
Contributor’s Guide¶
Standards compliance¶
The priority is for NFSD to interoperate fully with the Linux NFS client. We also test against other popular NFS client implementa- tions regularly at NFS bake-a-thon events (also known as plug- fests). Non-Linux NFS clients are not part of upstream NFSD CI/CD.
The NFSD community strives to provide an NFS server implementation that interoperates with all standards-compliant NFS client implementations. This is done by staying as close as is sensible to the normative mandates in the IETF’s published NFS, RPC, and GSS-API standards.
It is always useful to reference an RFC and section number in a code comment where behavior deviates from the standard (and even when the behavior is compliant but the implementation is obfuscatory).
On the rare occasion when a deviation from standard-mandated behavior is needed, brief documentation of the use case or deficiencies in the standard is a required part of in-code documentation.
Care must always be taken to avoid leaking local error codes (ie, errnos) to clients of NFSD. A proper NFS status code is always required in NFS protocol replies.
NFSD administrative interfaces¶
NFSD administrative interfaces include:
an NFSD or SUNRPC module parameter
export options in /etc/exports
files under /proc/fs/nfsd/ or /proc/sys/sunrpc/
the NFSD netlink protocol
Frequently, a request is made to introduce or modify one of NFSD’s traditional administrative interfaces. Certainly it is technically easy to introduce a new administrative setting. However, there are good reasons why the NFSD maintainers prefer to leave that as a last resort:
As with any API, administrative interfaces are difficult to get right.
Once they are documented and have a legacy of use, administrative interfaces become difficult to modify or remove.
Every new administrative setting multiplies the NFSD test matrix.
The cost of one administrative interface is incremental, but costs add up across all of the existing interfaces.
It is often better for everyone if effort is made up front to understanding the underlying requirement of the new setting, and then trying to make it tune itself (or to become otherwise unnecessary).
If a new setting is indeed necessary, first consider adding it to the NFSD netlink protocol. Or if it doesn’t need to be a reliable long term user space feature, it can be added to NFSD’s menagerie of experimental settings which reside under /sys/kernel/debug/nfsd/ .
Field observability¶
NFSD employs several different mechanisms for observing operation, including counters, printks, WARNings, and static trace points. Each have their strengths and weaknesses. Contributors should select the most appropriate tool for their task.
BUG must be avoided if at all possible, as it will frequently result in a full system crash.
WARN is appropriate only when a full stack trace is useful.
printk can show detailed information. These must not be used in code paths where they can be triggered repeatedly by remote users.
dprintk can show detailed information, but can be enabled only in pre-set groups. The overhead of emitting output makes dprintk inappropriate for frequent operations like I/O.
Counters are always on, but provide little information about individual events other than how frequently they occur.
static trace points can be enabled individually or in groups (via a glob). These are generally low overhead, and thus are favored for use in hot paths.
dynamic tracing, such as kprobes or eBPF, are quite flexible but cannot be used in certain environments (eg, full kernel lock- down).
Testing¶
The kdevops project
contains several NFS-specific workflows, as well as the community standard fstests suite. These workflows are based on open source testing tools such as ltp and fio. Contributors are encouraged to use these tools without kdevops, or contributors should install and use kdevops themselves to verify their patches before submission.
Coding style¶
Follow the coding style preferences described in
with the following exceptions:
Add new local variables to a function in reverse Christmas tree order
Use the kdoc comment style for + non-static functions + static inline functions + static functions that are callbacks/virtual functions
All new function names start with “nfsd_” for non-NFS-version- specific functions.
New function names that are specific to NFSv2 or NFSv3, or are used by all minor versions of NFSv4, use “nfsdN_” where N is the version.
New function names specific to an NFSv4 minor version can be named with “nfsd4M_” where M is the minor version.
Patch preparation¶
Read and follow all guidelines in
Use tagging to identify all patch authors. However, reviewers and testers should be added by replying to the email patch submission. Email is extensively used in order to publicly archive review and testing attributions. These tags are automatically inserted into your patches when they are applied.
The code in the body of the diff already shows /what/ is being changed. Thus it is not necessary to repeat that in the patch description. Instead, the description should contain one or more of:
A brief problem statement (“what is this patch trying to fix?”) with a root-cause analysis.
End-user visible symptoms or items that a support engineer might use to search for the patch, like stack traces.
A brief explanation of why the patch is the best way to address the problem.
Any context that reviewers might need to understand the changes made by the patch.
Any relevant benchmarking results, and/or functional test results.
As detailed in Submitting patches: the essential guide to getting your code into the kernel, identify the point in history that the issue being addressed was introduced by using a Fixes: tag.
Mention in the patch description if that point in history cannot be determined -- that is, no Fixes: tag can be provided. In this case, please make it clear to maintainers whether an LTS backport is needed even though there is no Fixes: tag.
The NFSD maintainers prefer to add stable tagging themselves, after public discussion in response to the patch submission. Contributors may suggest stable tagging, but be aware that many version management tools add such stable Cc’s when you post your patches. Don’t add “Cc: stable” unless you are absolutely sure the patch needs to go to stable during the initial submission process.
Patch submission¶
Patches to NFSD are submitted via the kernel’s email-based review process that is common to most other kernel subsystems.
Just before each submission, rebase your patch or series on the nfsd-testing branch at
The NFSD subsystem is maintained separately from the Linux in-kernel NFS client. The NFSD maintainers do not normally take submissions for client changes, nor can they respond authoritatively to bug reports or feature requests for NFS client code.
This means that contributors might be asked to resubmit patches if they were emailed to the incorrect set of maintainers and reviewers. This is not a rejection, but simply a correction of the submission process.
When in doubt, consult the NFSD entry in the MAINTAINERS file to see which files and directories fall under the NFSD subsystem.
The proper set of email addresses for NFSD patches are:
To: the NFSD maintainers and reviewers listed in MAINTAINERS Cc: linux-nfs@vger.kernel.org and optionally linux-kernel@
If there are other subsystems involved in the patches (for example MM or RDMA) their primary mailing list address can be included in the Cc: field. Other contributors and interested parties may be included there as well.
In general we prefer that contributors use common patch email tools such as “git send-email” or “stg email format/send”, which tend to get the details right without a lot of fuss.
A series consisting of a single patch is not required to have a cover letter. However, a cover letter can be included if there is substantial context that is not appropriate to include in the patch description.
Please note that, with an e-mail based submission process, series cover letters are not part of the work that is committed to the kernel source code base or its commit history. Therefore always try to keep pertinent information in the patch descriptions.
Design documentation is welcome, but as cover letters are not preserved, a perhaps better option is to include a patch that adds such documentation under Documentation/filesystems/nfs/.
Reviewers will ask about test coverage and what use cases the patches are expected to address. Please be prepared to answer these questions.
Review comments from maintainers might be politely stated, but in general, these are not optional to address when they are actionable. If necessary, the maintainers retain the right to not apply patches when contributors refuse to address reasonable requests.
Post changes to kernel source code and user space source code as separate series. You can connect the two series with comments in your cover letters.
Generally the NFSD maintainers ask for a reposts even for simple modifications in order to publicly archive the request and the resulting repost before it is pulled into the NFSD trees. This also enables us to rebuild a patch series quickly without missing changes that might have been discussed via email.
Avoid frequently reposting large series with only small changes. As a rule of thumb, posting substantial changes more than once a week will result in reviewer overload.
Remember, there are only a handful of subsystem maintainers and reviewers, but potentially many sources of contributions. The maintainers and reviewers, therefore, are always the less scalable resource. Be kind to your friendly neighborhood maintainer.
Patch Acceptance¶
There isn’t a formal review process for NFSD, but we like to see at least two Reviewed-by: notices for patches that are more than simple clean-ups. Reviews are done in public on linux-nfs@vger.kernel.org and are archived on lore.kernel.org.
Currently the NFSD patch queues are maintained in branches here:
The NFSD maintainers apply patches initially to the nfsd-testing branch, which is always open to new submissions. Patches can be applied while review is ongoing. nfsd-testing is a topic branch, so it can change frequently, it will be rebased, and your patch might get dropped if there is a problem with it.
Generally a script-generated “thank you” email will indicate when your patch has been added to the nfsd-testing branch. You can track the progress of your patch using the linux-nfs patchworks instance:
While your patch is in nfsd-testing, it is exposed to a variety of test environments, including community zero-day bots, static analysis tools, and NFSD continuous integration testing. The soak period is three to four weeks.
Each patch that survives in nfsd-testing for the soak period without changes is moved to the nfsd-next branch.
The nfsd-next branch is automatically merged into linux-next and fs-next on a nightly basis.
Patches that survive in nfsd-next are included in the next NFSD merge window pull request. These windows typically occur once every 63 days (nine weeks).
When the upstream merge window closes, the nfsd-next branch is renamed nfsd-fixes, and a new nfsd-next branch is created, based on the upstream -rc1 tag.
Fixes that are destined for an upstream -rc release also run the nfsd-testing gauntlet, but are then applied to the nfsd-fixes branch. That branch is made available for Linus to pull after a short time. In order to limit the risk of introducing regressions, we limit such fixes to emergency situations or fixes to breakage that occurred during the most recent upstream merge.
Please make it clear when submitting an emergency patch that immediate action (either application to -rc or LTS backport) is needed.
Sensitive patch submissions and bug reports¶
CVEs are generated by specific members of the Linux kernel community and several external entities. The Linux NFS community does not emit or assign CVEs. CVEs are assigned after an issue and its fix are known.
However, the NFSD maintainers sometimes receive sensitive security reports, and at times these are significant enough to need to be embargoed. In such rare cases, fixes can be developed and reviewed out of the public eye.
Please be aware that many version management tools add the stable Cc’s when you post your patches. This is generally a nuisance, but it can result in outing an embargoed security issue accidentally. Don’t add “Cc: stable” unless you are absolutely sure the patch needs to go to stable@ during the initial submission process.
Patches that are merged without ever appearing on any list, and which carry a Reported-by: or Fixes: tag are detected as suspicious by security-focused people. We encourage that, after any private review, security-sensitive patches should be posted to linux-nfs@ for the usual public review, archiving, and test period.
LLM-generated submissions¶
The Linux kernel community as a whole is still exploring the new world of LLM-generated code. The NFSD maintainers will entertain submission of patches that are partially or wholly generated by LLM-based development tools. Such submissions are held to the same standards as submissions created entirely by human authors:
The human contributor identifies themselves via a Signed-off-by: tag. This tag counts as a DoC.
The human contributor is solely responsible for code provenance and any contamination by inadvertently-included code with a conflicting license, as usual.
The human contributor must be able to answer and address review questions. A patch description such as “This fixed my problem but I don’t know why” is not acceptable.
The contribution is subjected to the same test regimen as all other submissions.
An indication (via a Generated-by: tag or otherwise) that the contribution is LLM-generated is not required.
It is easy to address review comments and fix requests in LLM generated code. So easy, in fact, that it becomes tempting to repost refreshed code immediately. Please resist that temptation.
As always, please avoid reposting series revisions more than once every 24 hours.
Clean-up patches¶
The NFSD maintainers discourage patches which perform simple clean- ups, which are not in the context of other work. For example:
Addressing
checkpatch.plwarnings after mergeAddressing Local variable ordering issues
Addressing long-standing whitespace damage
This is because it is felt that the churn that such changes produce comes at a greater cost than the value of such clean-ups.
Conversely, spelling and grammar fixes are encouraged.
Stable and LTS support¶
Upstream NFSD continuous integration testing runs against LTS trees whenever they are updated.
Please indicate when a patch containing a fix needs to be considered for LTS kernels, either via a Fixes: tag or explicit mention.
Feature requests¶
There is no one way to make an official feature request, but discussion about the request should eventually make its way to the linux-nfs@vger.kernel.org mailing list for public review by the community.
Subsystem boundaries¶
NFSD itself is not much more than a protocol engine. This means its primary responsibility is to translate the NFS protocol into API calls in the Linux kernel. For example, NFSD is not responsible for knowing exactly how bytes or file attributes are managed on a block device. It relies on other kernel subsystems for that.
If the subsystems on which NFSD relies do not implement a particular feature, even if the standard NFS protocols do support that feature, that usually means NFSD cannot provide that feature without substantial development work in other areas of the kernel.
Specificity¶
Feature requests can come from anywhere, and thus can often be nebulous. A requester might not understand what a “use case” or “user story” is. These descriptive paradigms are often used by developers and architects to understand what is required of a design, but are terms of art in the software trade, not used in the everyday world.
In order to prevent contributors and maintainers from becoming overwhelmed, we won’t be afraid of saying “no” politely to underspecified requests.