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db_include/utils/freepage.h

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+/*-------------------------------------------------------------------------
+ *
+ * freepage.h
+ *	  Management of page-organized free memory.
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/utils/freepage.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef FREEPAGE_H
+#define FREEPAGE_H
+
+#include "storage/lwlock.h"
+#include "utils/relptr.h"
+
+/* Forward declarations. */
+typedef struct FreePageSpanLeader FreePageSpanLeader;
+typedef struct FreePageBtree FreePageBtree;
+typedef struct FreePageManager FreePageManager;
+
+/*
+ * PostgreSQL normally uses 8kB pages for most things, but many common
+ * architecture/operating system pairings use a 4kB page size for memory
+ * allocation, so we do that here also.
+ */
+#define FPM_PAGE_SIZE			4096
+
+/*
+ * Each freelist except for the last contains only spans of one particular
+ * size.  Everything larger goes on the last one.  In some sense this seems
+ * like a waste since most allocations are in a few common sizes, but it
+ * means that small allocations can simply pop the head of the relevant list
+ * without needing to worry about whether the object we find there is of
+ * precisely the correct size (because we know it must be).
+ */
+#define FPM_NUM_FREELISTS		129
+
+/* Define relative pointer types. */
+relptr_declare(FreePageBtree, RelptrFreePageBtree);
+relptr_declare(FreePageManager, RelptrFreePageManager);
+relptr_declare(FreePageSpanLeader, RelptrFreePageSpanLeader);
+
+/* Everything we need in order to manage free pages (see freepage.c) */
+struct FreePageManager
+{
+	RelptrFreePageManager self;
+	RelptrFreePageBtree btree_root;
+	RelptrFreePageSpanLeader btree_recycle;
+	unsigned	btree_depth;
+	unsigned	btree_recycle_count;
+	Size		singleton_first_page;
+	Size		singleton_npages;
+	Size		contiguous_pages;
+	bool		contiguous_pages_dirty;
+	RelptrFreePageSpanLeader freelist[FPM_NUM_FREELISTS];
+#ifdef FPM_EXTRA_ASSERTS
+	/* For debugging only, pages put minus pages gotten. */
+	Size		free_pages;
+#endif
+};
+
+/* Macros to convert between page numbers (expressed as Size) and pointers. */
+#define fpm_page_to_pointer(base, page) \
+	(AssertVariableIsOfTypeMacro(page, Size), \
+	 (base) + FPM_PAGE_SIZE * (page))
+#define fpm_pointer_to_page(base, ptr)		\
+	(((Size) (((char *) (ptr)) - (base))) / FPM_PAGE_SIZE)
+
+/* Macro to convert an allocation size to a number of pages. */
+#define fpm_size_to_pages(sz) \
+	(((sz) + FPM_PAGE_SIZE - 1) / FPM_PAGE_SIZE)
+
+/* Macros to check alignment of absolute and relative pointers. */
+#define fpm_pointer_is_page_aligned(base, ptr)		\
+	(((Size) (((char *) (ptr)) - (base))) % FPM_PAGE_SIZE == 0)
+#define fpm_relptr_is_page_aligned(base, relptr)		\
+	((relptr).relptr_off % FPM_PAGE_SIZE == 0)
+
+/* Macro to find base address of the segment containing a FreePageManager. */
+#define fpm_segment_base(fpm)	\
+	(((char *) fpm) - fpm->self.relptr_off)
+
+/* Macro to access a FreePageManager's largest consecutive run of pages. */
+#define fpm_largest(fpm) \
+	(fpm->contiguous_pages)
+
+/* Functions to manipulate the free page map. */
+extern void FreePageManagerInitialize(FreePageManager *fpm, char *base);
+extern bool FreePageManagerGet(FreePageManager *fpm, Size npages,
+							   Size *first_page);
+extern void FreePageManagerPut(FreePageManager *fpm, Size first_page,
+							   Size npages);
+extern char *FreePageManagerDump(FreePageManager *fpm);
+
+#endif							/* FREEPAGE_H */