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>

Documentation/filesystems/bcachefs/errorcodes.rst

+.. 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.

fs/bcachefs/errcode.c

@@ -2,6 +2,7 @@
 
 #include "bcachefs.h"
 #include "errcode.h"
+#include "trace.h"
 
 #include <linux/errname.h>
 
@@ -49,15 +50,17 @@ bool __bch2_err_matches(int err, int class)
 	return err == class;
 }
 
-int __bch2_err_class(int err)
+int __bch2_err_class(int bch_err)
 {
-	err = -err;
-	BUG_ON((unsigned) err >= BCH_ERR_MAX);
+	int std_err = -bch_err;
+	BUG_ON((unsigned) std_err >= BCH_ERR_MAX);
 
-	while (err >= BCH_ERR_START && bch2_errcode_parents[err - BCH_ERR_START])
-		err = bch2_errcode_parents[err - BCH_ERR_START];
+	while (std_err >= BCH_ERR_START && bch2_errcode_parents[std_err - BCH_ERR_START])
+		std_err = bch2_errcode_parents[std_err - BCH_ERR_START];
+
+	trace_error_downcast(bch_err, std_err, _RET_IP_);
 
-	return -err;
+	return -std_err;
 }
 
 const char *bch2_blk_status_to_str(blk_status_t status)

fs/bcachefs/errcode.h

@@ -78,6 +78,7 @@
 	x(ENOMEM,			ENOMEM_fs_name_alloc)			\
 	x(ENOMEM,			ENOMEM_fs_other_alloc)			\
 	x(ENOMEM,			ENOMEM_dev_alloc)			\
+	x(ENOMEM,			ENOMEM_disk_accounting)			\
 	x(ENOSPC,			ENOSPC_disk_reservation)		\
 	x(ENOSPC,			ENOSPC_bucket_alloc)			\
 	x(ENOSPC,			ENOSPC_disk_label_add)			\

fs/bcachefs/trace.h

@@ -1431,6 +1431,25 @@ DEFINE_EVENT(fs_str, data_update,
 	TP_ARGS(c, str)
 );
 
+TRACE_EVENT(error_downcast,
+	TP_PROTO(int bch_err, int std_err, unsigned long ip),
+	TP_ARGS(bch_err, std_err, ip),
+
+	TP_STRUCT__entry(
+		__array(char,		bch_err, 32		)
+		__array(char,		std_err, 32		)
+		__array(char,		ip, 32			)
+	),
+
+	TP_fast_assign(
+		strscpy(__entry->bch_err, bch2_err_str(bch_err), sizeof(__entry->bch_err));
+		strscpy(__entry->std_err, bch2_err_str(std_err), sizeof(__entry->std_err));
+		snprintf(__entry->ip, sizeof(__entry->ip), "%ps", (void *) ip);
+	),
+
+	TP_printk("%s -> %s %s", __entry->bch_err, __entry->std_err, __entry->ip)
+);
+
 #endif /* _TRACE_BCACHEFS_H */
 
 /* This part must be outside protection */