Development notes

From btrfs Wiki
(Difference between revisions)
Jump to: navigation, search
m (Coding style preferences)
m (Coding style preferences)
Line 138: Line 138:
* do not use <tt>spin_is_locked</tt> but <tt>lockdep_assert_held</tt>
* do not use <tt>spin_is_locked</tt> but <tt>lockdep_assert_held</tt>
* default function return value is ''int ret'', temporary return values should be named like ''ret2'' etc
* default function return value is ''int ret'', temporary return values should be named like ''ret2'' etc
* structure initializers should use ''{ 0 }''

Revision as of 15:14, 16 October 2020

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


Misc notes

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, (eg. 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


  "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 happend (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 (eg. 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 everwhere 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.

Handling unexpected conditions

This is different than error handling. An unexpected condtion 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 (eg. memory biflip, memory corrupted by other subystem). 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:

  • ASSERT -- conditionally compiled in and crashes when the condition is false, this is supposed to catch 'must never happen' at the time of development, code must not continue
  • WARN_ON -- light check that is visible in the log and allows the code to continue but the reasons must be investigated
  • BUG_ON -- last resort, checks condition that 'must never happen' and continuing would cause more harm than the instant crash; code should always try to avoid using it, but there are cases when sanity and invariant checks are done in advance

Error injection using eBPF

Functions can be annotated to enable error injection using the eBPF scripts. See eg. 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 (eg. memory allocation), you cau se something like bcc/tools/ kmalloc btrfs_alloc_device() -P 0.001


  • eBPF
  • BCC tools

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 allows focus on the important things.


  • 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

Function declarations:

  • avoid prototypes for static functions, order them in new code in a way that does not need it
  • 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
  * btrfs_do_something - kdoc requires this text to be on one line, but that's making the
  *                      text hard to read for people, so you can do it like this
  * @fs_info:    trivial parameters should be in the list but with some short description
  * @offset:     describe the context of the argument, eg. 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
  • 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
  • default function return value is int ret, temporary return values should be named like ret2 etc
  • structure initializers should use { 0 }


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

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 sideefects
  • use const extensively

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
  • locking
  • sanity checks
    • CONFIG_KASAN -- address sanitizer
    • CONFIG_UBSAN -- undefined behaviour sanitizer
    • CONFIG_KCSAN -- concurrency checker
  • verbose reporting
  • tracing


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_EXT4_FS=m or y

Kernel config options for better bug reports

See the list in the section above for more options.

User space utilities and development library dependencies

  • fio
  • dmsetup (device-mapper)
  • lvm
  • xfsprogs >= 4.3.1
    • xfs_io -c reflink is required.
  • btrfsprogs
  • dbench
  • openssl
  • libacl
  • libattr
  • libaio
  • libuuid
  • libcap-progs
  • duperemove

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 6 such partitions
  • some tests need at least 10G of free space, as determined by df, ie. 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 123456-fsgqa user and group must exist.
Personal tools