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diff --git a/c_src/posix_to_bcachefs.h b/c_src/posix_to_bcachefs.h
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+++ b/c_src/posix_to_bcachefs.h
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+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _POSIX_TO_BCACHEFS_H
+#define _POSIX_TO_BCACHEFS_H
+
+/*
+ * This header exports the functionality needed for copying data from existing
+ * posix compliant filesystems to bcachefs. There are two use cases:
+ * 1. Creating a new bcachefs filesystem using `bcachefs format`, we can
+ *    specify a source directory tree which will be copied over the new
+ *    bcachefs filesytem.
+ * 2. Migrating an existing filesystem in place, with `bcachefs migrate`.
+ *    This will allocate space for the bcachefs metadata, but the actual data
+ *    represented by the extents will not be duplicated. The bcachefs metadata
+ *    will simply point to the existing extents.
+ *
+ * To avoid code duplication, `copy_fs` deals with both cases. See the function
+ * documentation for more details.
+ */
+
+#include "libbcachefs.h"
+
+enum bch_migrate_type {
+	BCH_MIGRATE_copy,
+	BCH_MIGRATE_migrate
+};
+
+/*
+ * The migrate action uses all the fields in this struct.
+ * The copy action only uses the `hardlinks` field. Since `hardlinks` is
+ * initialized with zeroes, an empty `copy_fs_state` struct can be passed.
+ */
+struct copy_fs_state {
+	u64			bcachefs_inum;
+	dev_t			dev;
+
+	GENRADIX(u64)		hardlinks;
+	ranges			extents;
+	enum bch_migrate_type	type;
+};
+
+/*
+ * The `copy_fs` function is used for both copying a directory tree to a new
+ * bcachefs filesystem and migrating an existing one, depending on the value
+ * from the `type` field in `copy_fs_state` struct.
+ *
+ * In case of copy, an empty `copy_fs_state` structure is passed to `copy_fs`
+ * (only the `hardlinks` field is used, and that is initialized with zeroes).
+ *
+ * In the migrate case, all the fields from `copy_fs_state` need to be
+ * initialized (`hardlinks` is initialized with zeroes).
+ */
+void copy_fs(struct bch_fs *c, int src_fd, const char *src_path,
+		    struct copy_fs_state *s);
+#endif /* _LIBBCACHE_H */