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diff --git a/fs/bcachefs/disk_accounting_format.h b/fs/bcachefs/disk_accounting_format.h
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+++ b/fs/bcachefs/disk_accounting_format.h
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+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _BCACHEFS_DISK_ACCOUNTING_FORMAT_H
+#define _BCACHEFS_DISK_ACCOUNTING_FORMAT_H
+
+#include "replicas_format.h"
+
+/*
+ * Disk accounting - KEY_TYPE_accounting - on disk format:
+ *
+ * Here, the key has considerably more structure than a typical key (bpos); an
+ * accounting key is 'struct disk_accounting_pos', which is a union of bpos.
+ *
+ * More specifically: a key is just a muliword integer (where word endianness
+ * matches native byte order), so we're treating bpos as an opaque 20 byte
+ * integer and mapping bch_accounting_key to that.
+ *
+ * This is a type-tagged union of all our various subtypes; a disk accounting
+ * key can be device counters, replicas counters, et cetera - it's extensible.
+ *
+ * The value is a list of u64s or s64s; the number of counters is specific to a
+ * given accounting type.
+ *
+ * Unlike with other key types, updates are _deltas_, and the deltas are not
+ * resolved until the update to the underlying btree, done by btree write buffer
+ * flush or journal replay.
+ *
+ * Journal replay in particular requires special handling. The journal tracks a
+ * range of entries which may possibly have not yet been applied to the btree
+ * yet - it does not know definitively whether individual entries are dirty and
+ * still need to be applied.
+ *
+ * To handle this, we use the version field of struct bkey, and give every
+ * accounting update a unique version number - a total ordering in time; the
+ * version number is derived from the key's position in the journal. Then
+ * journal replay can compare the version number of the key from the journal
+ * with the version number of the key in the btree to determine if a key needs
+ * to be replayed.
+ *
+ * For this to work, we must maintain this strict time ordering of updates as
+ * they are flushed to the btree, both via write buffer flush and via journal
+ * replay. This has complications for the write buffer code while journal replay
+ * is still in progress; the write buffer cannot flush any accounting keys to
+ * the btree until journal replay has finished replaying its accounting keys, or
+ * the (newer) version number of the keys from the write buffer will cause
+ * updates from journal replay to be lost.
+ */
+
+struct bch_accounting {
+	struct bch_val		v;
+	__u64			d[];
+};
+
+#define BCH_ACCOUNTING_MAX_COUNTERS		3
+
+#define BCH_DATA_TYPES()		\
+	x(free,		0)		\
+	x(sb,		1)		\
+	x(journal,	2)		\
+	x(btree,	3)		\
+	x(user,		4)		\
+	x(cached,	5)		\
+	x(parity,	6)		\
+	x(stripe,	7)		\
+	x(need_gc_gens,	8)		\
+	x(need_discard,	9)		\
+	x(unstriped,	10)
+
+enum bch_data_type {
+#define x(t, n) BCH_DATA_##t,
+	BCH_DATA_TYPES()
+#undef x
+	BCH_DATA_NR
+};
+
+static inline bool data_type_is_empty(enum bch_data_type type)
+{
+	switch (type) {
+	case BCH_DATA_free:
+	case BCH_DATA_need_gc_gens:
+	case BCH_DATA_need_discard:
+		return true;
+	default:
+		return false;
+	}
+}
+
+static inline bool data_type_is_hidden(enum bch_data_type type)
+{
+	switch (type) {
+	case BCH_DATA_sb:
+	case BCH_DATA_journal:
+		return true;
+	default:
+		return false;
+	}
+}
+
+#define BCH_DISK_ACCOUNTING_TYPES()		\
+	x(nr_inodes,		0)		\
+	x(persistent_reserved,	1)		\
+	x(replicas,		2)		\
+	x(dev_data_type,	3)		\
+	x(dev_stripe_buckets,	4)
+
+enum disk_accounting_type {
+#define x(f, nr)	BCH_DISK_ACCOUNTING_##f	= nr,
+	BCH_DISK_ACCOUNTING_TYPES()
+#undef x
+	BCH_DISK_ACCOUNTING_TYPE_NR,
+};
+
+struct bch_nr_inodes {
+};
+
+struct bch_persistent_reserved {
+	__u8			nr_replicas;
+};
+
+struct bch_dev_data_type {
+	__u8			dev;
+	__u8			data_type;
+};
+
+struct bch_dev_stripe_buckets {
+	__u8			dev;
+};
+
+struct disk_accounting_pos {
+	union {
+	struct {
+		__u8				type;
+		union {
+		struct bch_nr_inodes		nr_inodes;
+		struct bch_persistent_reserved	persistent_reserved;
+		struct bch_replicas_entry_v1	replicas;
+		struct bch_dev_data_type	dev_data_type;
+		struct bch_dev_stripe_buckets	dev_stripe_buckets;
+		};
+	};
+		struct bpos			_pad;
+	};
+};
+
+#endif /* _BCACHEFS_DISK_ACCOUNTING_FORMAT_H */