Development notes

Collection of various notes about development practices, how-to’s or checklists.

Adding a new ioctl, extending an existing one

  • add code to strace so the ioctl calls are parsed into a human readable form. Most of the ioctls are already implemented and can be used a reference.


The tracepoint message format should be compact and consistent, so please stick to the following format:

  • key=value no spaces around =

  • separated by spaces, not commas

  • named values: print value and string, like “%llu(%s)”, no space between, string in parens

  • avoid abbreviating key values too much, (e.g. use ‘size’ not ‘sz’)

  • hexadecimal values are always preceded by 0x (use “0x%llx”)

  • use struct btrfs_inode for inode types, not plain struct inode

  • inode number type is u64, use btrfs_ino if needed

  • key can be printed as [%llu,%u,%llu]

  • enum types need to be exported as TRACE_DEFINE_ENUM


event: btrfs__chunk

string: "root=%llu(%s) offset=%llu size=%llu num_stripes=%d sub_stripes=%d type=%s"

Error messages, verbosity

  • use btrfs_* helpers (btrfs_err, btrfs_info, …), they print a filesystem identification like BTRFS info (device sdb): ...

  • first letter in the string is lowercase

  • message contents

    • be descriptive

    • keep the text length reasonable (fits one line without wrapping)

    • no typos

    • print values that refer to what happened (inode number, subvolume id, path, offset, …)

    • print error value from the last call

    • no “\n” at the end of the string

    • no “.’’ at the end of text

    • un-indent the string so it fits under 80 columns

    • don’t split long strings, overflow 80 is ok in this case (we want to be able to search for the error messages in the sources easily)

  • value representation

    • decimal: offsets, length, ordinary numbers

    • hexadecimal: checksums

    • hexadecimal + string: bitmasks (e.g. raid profiles, flags)

    • intervals of integers:

      • closed interval (end values inclusive): [0, 4096]

      • half-open (right value excluded): [0, 4096)

      • half-open (left value excluded): (0, 4096] -- that one may look strange but is used in several places

Message level

  • btrfs_err -- such messages have high visibility so use them for serious problems that need user attention

  • btrfs_warn -- conditions that are not too serious but can point to potential problems, the system should be still in a good state

  • btrfs_info -- use for informative messages that are useful to see what’s happening in the filesystem or might help debugging problems in the future and are worth keeping in the logs

Error handling and transaction abort

Please keep all transaction abort exactly at the place where they happen and do not merge them to one. This pattern should be used everywhere and is important when debugging because we can pinpoint the line in the code from the syslog message and do not have to guess which way it got to the merged call.

Error handling and return values

Functions are supposed to handle all errors of the callees and clean up the local context before returning. If a function does not need to pass errors to the caller it can be switched to return void. Before doing so make sure that:

  • the function does not call any BUG/BUG_ON

  • all callees properly handle errors and do not call BUG/BUG_ON in place of error handling

  • the whole call chain starting from the function satisfies the above

Handling unexpected conditions

This is different than error handling. An unexpected condition happens when the code invariants/assumptions do not hold and there’s no way to recover from the situation. This means that returning an error to the caller can’t be done and continuing would only propagate the logic error further. The reasons for that bug can be two fold: internal (a genuine bug) or external (e.g. memory bitflip, memory corrupted by other subsystem). In this case it is allowed to use the nuclear option and do BUG_ON, that is otherwise highly discouraged.

There are several ways how to react to the unexpected conditions:

  • btrfs_abort_transaction()

    The recommended way if and only if we can not recover from the situation and have a transaction handle.

    This would cause the filesystem to be flipped read-only to prevent further corruption.

    Additionally call trace would be dumpped for the first btrfs_abort_transaction() call site.

  • ASSERT()

    Conditionally compiled in and crashes when the condition is false.

    This should only be utilized for debugging purposes, acts as a fail-fast option for developers, thus should not be utilized for error handling.

    It’s recommended only for very basic (thus sometimes unnecessary) requirements. Such usage should be easy to locate, have no complex call chain. E.g. to rule out invalid function parameter combinations.

    Should not be utilized on any data/metadata reads from disks, as they can be invalid. For sanity check examples of on-disk metadata, please refer to Tree checker.


    Unconditional and noisy checks, but still allow the code to continue.

    Should only be utilized if a call trace is critical for debugging.

    Not recommended if:

    • The call site is unique or can be easily located

      In that case, an error message is recommended.

    • The call site would eventually lead to a btrfs_abort_transaction() call

      btrfs_abort_transaction() call would dump the stack anyway. If the call trace is critical, it’s recommended to move the btrfs_abort_transaction() call closer to the place where the error happens.

  • BUG_ON

    Should not be utilized, and is incrementally removed or replaced in the code.

Error injection using eBPF

Functions can be annotated to enable error injection using the eBPF scripts. See e.g. disk-io.c:open_ctree. For btrfs-specific injection, the annotation is ALLOW_ERROR_INJECTION, but beware that this only overrides the return value and this can leak memory or other resources. For error injection to generic helpers (e.g. memory allocation), you can use something like bcc/tools/ kmalloc btrfs_alloc_device() -P 0.001


Warnings and issues found by static checkers and similar tools

There are tools to automatically identify known issues in code and report them as problems to be fixed, but not all such reports are valid or relevant in the context of the code base. The fix should really fix the code, not just the tool’s warning. Such patches will be rejected with explanation first time and ignored when sent repeatedly. Patches fixing real problems with a good explanation are welcome. If you’re not sure about sending such patch, please ask the for help.

Do not blindly report issues caught by:

  • -- the script is good for catching some coding style but this whole wiki page exists to be explicit what we want, not necessarily what checkpatch wants

  • clang static analyzer -- lots of the reports are not real problems and may depend on a condition that’s not recognized by the checker

  • gcc -Wunused -- any of the -Wunused-* options can report a valid issue but it must be viewed in wider context and not just removed to get rid of the warning

  • codespell -- fixing typos is fine but should be done in batches and over whole code base


  • if you find an issue, look in the whole code base if there are more instances same or following a similar pattern

  • look into git history of the changed code, why it got there and when, it may help to understand if it’s a bug or e.g. a stale code

Coding style preferences

Before applying recommendations from this sections, please make sure you’re familiar with the kernel coding style guide.

The purpose of coding style is to maintain unified and consistent look & feel of the patches and code, keeping distractions to minimum which decreases cognitive load and allows focus on the important things. Coding style is not only where to put white space or curly brackets but also coding patterns with meaning that is established and understood in the developer group. The code in linux kernel is maintained for a long period of time and maintainability is of crucial importance. This means it does take time to write good code, with attention to detail. Once written the code could stay unchanged for years but will be read many times. Read more.


  • for patch subject use “btrfs:” followed by a lowercase

  • read the patch again and fix all typos and grammar

  • size units should use short form K/M/G/T or IEC form KiB/MiB/GiB

  • don’t write references to parameters to subject (like removing @pointer)

  • do not end subject with a dot ‘.’

  • parts of btrfs that could have a subject prefix to point to a specific subsystem

    • scrub, tests, integrity-checker, tree-checker, discard, locking, sysfs, raid56, qgroup, compression, send, ioctl

  • additional information

    • if there’s a stack trace relevant for the patch, add it ther (lockdep, crash, warning)

    • steps to reproduce a bug (that will also get turned to a proper fstests case)

    • sample output before/after if it could have impact on userspace

    • pahole output if structure is being reorganized and optimized

Function declarations

  • avoid prototypes for static functions, order them in new code in a way that does not need it

    • but don’t move static functions just to get rid of the prototype

  • exported functions have btrfs_ prefix

  • do not use functions with double underscore, there’s only a few valid uses of that, namely when __function is doing the core of the work with looser checks, no locks or more parameters than function

  • function type and name are on the same line, if this is too long, the parameters continue on the next line (indented)

  • ‘static inline’ functions should be small (measured by their resulting binary size)

  • conditional definitions should follow the style below, where the full function body is in .c

void btrfs_assert_everything_is_fine(void *ptr);
void btrfs_assert_everything_is_fine(void *ptr) { }


  • leave one newline before #endif in headers


  • function comment goes to the .c file, not the header

    • kdoc style is recommended but the exact syntax is not mandated and we’re using only subset of the formatting

    • the first line (mandatory): contains a brief description of what the function does and should provide a summary information

      • do write in the imperative style “Iterate all pages and clear some bits”

      • don’t write “This function is a helper to …”, “This is used to …”

    • parameter description (optional):

      • each line describes the parameter

      • the list needs to be in the same order as for the function

      • the list needs to be complete

      • trivial parameters don’t need to be explained, e.g. fs_info is clear so the description could be ‘the filesystem’

      • context of the parameters matters a lot in some cases and cannot be inferred from the name, then it should be documented

 * Look for blocks in the given offset.
 * @fs_info:    trivial parameters should be in the list but with some short description
 * @offset:     describe the context of the argument, e.g. offset to page or inode ...
 * Long description comes here if necessary.
 * Return value semantics if it's not obvious
  • comment new enum/define values, brief description or pointers to the code that uses them

  • comment new data structures, their purpose and context of use

  • do not put struct member comments on the same line, put it on the line before and do not trim/abbreviate the text

  • comment text that fits on one line can use the /* text */ format, slight overflow of 80 chars is OK


  • fix spelling, grammar and formatting of comments that get moved or changed

  • fix coding style in code that’s only moved

  • one newline between functions


  • do not use spin_is_locked but lockdep_assert_held

  • do not use assert_spin_locked without reading it’s semantics (it does not check if the caller hold the lock)

  • use lockdep_assert_held and its friends for lock assertions

  • add lock assertions to functions called deep in the call chain


  • default function return value is int ret, temporary return values should be named like ret2 etc

  • structure initializers should use { 0 }

  • do not use short type if possible, if it fits to char/u8 use it instead, or plain int


  • when dumping a lot of data after an error, consider what will remain visible last

    • in case of btrfs_print_leaf, print the specific error message after that

Expressions, operators

  • spaces around binary operators, no spaces for unary operators

  • extra braces around expressions that might be harder to understand wrt precedence are fine (logical and/or, shifts with other operations)

    • a * b + c, (a << b) + c, (a % b) + c

  • 64bit division is not allowed, either avoid it completely, or use bit shifts or use div_u64 helpers

  • do not use chained assignments: no a = b = c;

Variable declarations, parameters

  • declaration block in a function should do only simple initializations (pointers, constants); nothing that would require error handling or has non-trivial side effects

  • use const extensively

  • add temporary variable to store a value if it’s used multiple times in the function, or if reading the value needs to chase a long pointer chain

Kernel config options

Compile-time config options for kernel that can help debugging, testing. They usually take a hit on performance or resources (memory) so they should be selected wisely. The options in bold should be safe to use by default for debugging builds.

Please refer to the option documentation for further details.

  • devices for testing

    • CONFIG_BLK_DEV_LOOP - enable loop device


    • CONFIG_SCSI_DEBUG - fake scsi block device

  • memory

    • CONFIG_SLUB_DEBUG - boot with slub_debug





    • CONFIG_FAILSLAB -- fault injection to kmalloc



  • btrfs



    • CONFIG_BTRFS_FS_RUN_SANITY_TESTS -- basic tests on module load

    • CONFIG_BTRFS_FS_CHECK_INTEGRITY -- block integrity checker enabled by mount options

    • CONFIG_BTRFS_FS_REF_VERIFY -- additional checks for block references

  • locking







  • sanity checks



    • CONFIG_KASAN -- address sanitizer

    • CONFIG_UBSAN -- undefined behaviour sanitizer

    • CONFIG_KCSAN -- concurrency checker

  • verbose reporting


  • tracing



Not a bug. The lockdep structures can get in some cases full and cannot properly track locks anymore. There’s only a workaround to increase the kernel config value of CONFIG_LOCKDEP_CHAINS_BITS, default is 16, 18 tends to work, increase if needed.

fstests setup

The fstests suite has very few “hard” requirements and will succeed without actually running many tests. In order to ensure full test coverage, your test environment should provide the settings from the following sections. Please note that newly added tests silently add new dependencies, so you should always review results after an update.

Kernel config options for complete test coverage







  • CONFIG_DM_DELAY=m or y

  • CONFIG_DM_ERROR=m or y


  • CONFIG_DM_DUST=m or y

  • CONFIG_DM_ZERO=m or y


  • CONFIG_EXT4_FS=m or y


  • CONFIG_BLK_DEV_ZONED=y for zoned mode test coverage


Kernel config options for better bug reports

See the list in the section above for more options.

User space utilities and development library dependencies

  • acl

  • attr

  • btrfsprogs

  • dbench

  • dmsetup (device-mapper)

  • duperemove

  • psmisc (killall)

  • e2fsprogs

  • fio

  • fsverity-utils

  • libacl

  • libaio

  • libattr

  • libcap-progs

  • libuuid

  • lvm2

  • openssl

  • xfsprogs >= 4.3.1 (xfs_io -c reflink is required)

Note: This list may be incomplete.

Storage environment

  • At least 4 identically sized partitions/disks/virtual disks, specified using $SCRATCH_DEV_POOL, some tests may require 8 such partitions

  • some tests need at least 10G of free space, as determined by df, i.e. the size of the device may need to be larger

  • some tests require $LOGWRITES_DEV, yet another separate block device, for power fail testing

  • for testing trim and discard, the devices must be capable of that (physical or virtual)

Other requirements

  • An fsgqa user and group must exist.

  • An fsgqa2 user and group must exist.

  • The user nobody must exist.

  • An 123456-fsgqa user and group must exist.