bcachefs: Errcode tracepoint, documentation

Add a tracepoint for downcasting private errors to standard errors, so
they can be recovered even when not logged; also, add some
documentation.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/filesystems/bcachefs/errorcodes.rst b/Documentation/filesystems/bcachefs/errorcodes.rst
new file mode 100644
index 000000000000..2cccaa0ba7cd
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+++ b/Documentation/filesystems/bcachefs/errorcodes.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+bcachefs private error codes
+----------------------------
+
+In bcachefs, as a hard rule we do not throw or directly use standard error
+codes (-EINVAL, -EBUSY, etc.). Instead, we define private error codes as needed
+in fs/bcachefs/errcode.h.
+
+This gives us much better error messages and makes debugging much easier. Any
+direct uses of standard error codes you see in the source code are simply old
+code that has yet to be converted - feel free to clean it up!
+
+Private error codes may subtype another error code, this allows for grouping of
+related errors that should be handled similarly (e.g. transaction restart
+errors), as well as specifying which standard error code should be returned at
+the bcachefs module boundary.
+
+At the module boundary, we use bch2_err_class() to convert to a standard error
+code; this also emits a trace event so that the original error code be
+recovered even if it wasn't logged.
+
+Do not reuse error codes! Generally speaking, a private error code should only
+be thrown in one place. That means that when we see it in a log message we can
+see, unambiguously, exactly which file and line number it was returned from.
+
+Try to give error codes names that are as reasonably descriptive of the error
+as possible. Frequently, the error will be logged at a place far removed from
+where the error was generated; good names for error codes mean much more
+descriptive and useful error messages.