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diff --git a/src/include/storage/fsm_internals.h b/src/include/storage/fsm_internals.h
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+++ b/src/include/storage/fsm_internals.h
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+/*-------------------------------------------------------------------------
+ *
+ * fsm_internals.h
+ *	  internal functions for free space map
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/fsm_internals.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef FSM_INTERNALS_H
+#define FSM_INTERNALS_H
+
+#include "storage/buf.h"
+#include "storage/bufpage.h"
+
+/*
+ * Structure of a FSM page. See src/backend/storage/freespace/README for
+ * details.
+ */
+typedef struct
+{
+	/*
+	 * fsm_search_avail() tries to spread the load of multiple backends by
+	 * returning different pages to different backends in a round-robin
+	 * fashion. fp_next_slot points to the next slot to be returned (assuming
+	 * there's enough space on it for the request). It's defined as an int,
+	 * because it's updated without an exclusive lock. uint16 would be more
+	 * appropriate, but int is more likely to be atomically
+	 * fetchable/storable.
+	 */
+	int			fp_next_slot;
+
+	/*
+	 * fp_nodes contains the binary tree, stored in array. The first
+	 * NonLeafNodesPerPage elements are upper nodes, and the following
+	 * LeafNodesPerPage elements are leaf nodes. Unused nodes are zero.
+	 */
+	uint8		fp_nodes[FLEXIBLE_ARRAY_MEMBER];
+} FSMPageData;
+
+typedef FSMPageData *FSMPage;
+
+/*
+ * Number of non-leaf and leaf nodes, and nodes in total, on an FSM page.
+ * These definitions are internal to fsmpage.c.
+ */
+#define NodesPerPage (BLCKSZ - MAXALIGN(SizeOfPageHeaderData) - \
+					  offsetof(FSMPageData, fp_nodes))
+
+#define NonLeafNodesPerPage (BLCKSZ / 2 - 1)
+#define LeafNodesPerPage (NodesPerPage - NonLeafNodesPerPage)
+
+/*
+ * Number of FSM "slots" on a FSM page. This is what should be used
+ * outside fsmpage.c.
+ */
+#define SlotsPerFSMPage LeafNodesPerPage
+
+/* Prototypes for functions in fsmpage.c */
+extern int	fsm_search_avail(Buffer buf, uint8 min_cat, bool advancenext,
+							 bool exclusive_lock_held);
+extern uint8 fsm_get_avail(Page page, int slot);
+extern uint8 fsm_get_max_avail(Page page);
+extern bool fsm_set_avail(Page page, int slot, uint8 value);
+extern bool fsm_truncate_avail(Page page, int nslots);
+extern bool fsm_rebuild_page(Page page);
+
+#endif							/* FSM_INTERNALS_H */