Management of free memory pages.

This is intended as infrastructure for a full-fledged allocator for
dynamic shared memory.  The interface looks a bit like a real
allocator, but only supports allocating and freeing memory in
multiples of the 4kB page size.  Further, to free memory, you must
know the size of the span you wish to free, in pages.  While these are
make it unsuitable as an allocator in and of itself, it still serves
as very useful scaffolding for a full-fledged allocator.

Robert Haas and Thomas Munro.  This code is mostly the same as my 2014
submission, but Thomas fixed quite a few bugs and made some changes to
the interface.

Discussion: CA+TgmobkeWptGwiNa+SGFWsTLzTzD-CeLz0KcE-y6LFgoUus4A@mail.gmail.com
Discussion: CAEepm=1z5WLuNoJ80PaCvz6EtG9dN0j-KuHcHtU6QEfcPP5-qA@mail.gmail.com

src/backend/utils/mmgr/Makefile

@@ -12,6 +12,6 @@ subdir = src/backend/utils/mmgr
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = aset.o mcxt.o portalmem.o
+OBJS = aset.o freepage.o mcxt.o portalmem.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/utils/mmgr/freepage.c

+/*-------------------------------------------------------------------------
+ *
+ * freepage.c
+ *	  Management of free memory pages.
+ *
+ * The intention of this code is to provide infrastructure for memory
+ * allocators written specifically for PostgreSQL.  At least in the case
+ * of dynamic shared memory, we can't simply use malloc() or even
+ * relatively thin wrappers like palloc() which sit on top of it, because
+ * no allocator built into the operating system will deal with relative
+ * pointers.  In the future, we may find other cases in which greater
+ * control over our own memory management seems desirable.
+ *
+ * A FreePageManager keeps track of which 4kB pages of memory are currently
+ * unused from the point of view of some higher-level memory allocator.
+ * Unlike a user-facing allocator such as palloc(), a FreePageManager can
+ * only allocate and free in units of whole pages, and freeing an
+ * allocation can only be done given knowledge of its length in pages.
+ *
+ * Since a free page manager has only a fixed amount of dedicated memory,
+ * and since there is no underlying allocator, it uses the free pages
+ * it is given to manage to store its bookkeeping data.  It keeps multiple
+ * freelists of runs of pages, sorted by the size of the run; the head of
+ * each freelist is stored in the FreePageManager itself, and the first
+ * page of each run contains a relative pointer to the next run. See
+ * FreePageManagerGetInternal for more details on how the freelists are
+ * managed.
+ *
+ * To avoid memory fragmentation, it's important to consolidate adjacent
+ * spans of pages whenever possible; otherwise, large allocation requests
+ * might not be satisfied even when sufficient contiguous space is
+ * available.  Therefore, in addition to the freelists, we maintain an
+ * in-memory btree of free page ranges ordered by page number.  If a
+ * range being freed precedes or follows a range that is already free,
+ * the existing range is extended; if it exactly bridges the gap between
+ * free ranges, then the two existing ranges are consolidated with the
+ * newly-freed range to form one great big range of free pages.
+ *
+ * When there is only one range of free pages, the btree is trivial and
+ * is stored within the FreePageManager proper; otherwise, pages are
+ * allocated from the area under management as needed.  Even in cases
+ * where memory fragmentation is very severe, only a tiny fraction of
+ * the pages under management are consumed by this btree.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/mmgr/freepage.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+#include "lib/stringinfo.h"
+#include "miscadmin.h"
+
+#include "utils/freepage.h"
+#include "utils/relptr.h"
+
+
+/* Magic numbers to identify various page types */
+#define FREE_PAGE_SPAN_LEADER_MAGIC		0xea4020f0
+#define FREE_PAGE_LEAF_MAGIC			0x98eae728
+#define FREE_PAGE_INTERNAL_MAGIC		0x19aa32c9
+
+/* Doubly linked list of spans of free pages; stored in first page of span. */
+struct FreePageSpanLeader
+{
+	int			magic;			/* always FREE_PAGE_SPAN_LEADER_MAGIC */
+	Size		npages;			/* number of pages in span */
+	RelptrFreePageSpanLeader prev;
+	RelptrFreePageSpanLeader next;
+};
+
+/* Common header for btree leaf and internal pages. */
+typedef struct FreePageBtreeHeader
+{
+	int			magic;			/* FREE_PAGE_LEAF_MAGIC or
+								 * FREE_PAGE_INTERNAL_MAGIC */
+	Size		nused;			/* number of items used */
+	RelptrFreePageBtree parent; /* uplink */
+} FreePageBtreeHeader;
+
+/* Internal key; points to next level of btree. */
+typedef struct FreePageBtreeInternalKey
+{
+	Size		first_page;		/* low bound for keys on child page */
+	RelptrFreePageBtree child;	/* downlink */
+} FreePageBtreeInternalKey;
+
+/* Leaf key; no payload data. */
+typedef struct FreePageBtreeLeafKey
+{
+	Size		first_page;		/* first page in span */
+	Size		npages;			/* number of pages in span */
+} FreePageBtreeLeafKey;
+
+/* Work out how many keys will fit on a page. */
+#define FPM_ITEMS_PER_INTERNAL_PAGE \
+	((FPM_PAGE_SIZE - sizeof(FreePageBtreeHeader)) / \
+		sizeof(FreePageBtreeInternalKey))
+#define FPM_ITEMS_PER_LEAF_PAGE \
+	((FPM_PAGE_SIZE - sizeof(FreePageBtreeHeader)) / \
+		sizeof(FreePageBtreeLeafKey))
+
+/* A btree page of either sort */
+struct FreePageBtree
+{
+	FreePageBtreeHeader hdr;
+	union
+	{
+		FreePageBtreeInternalKey internal_key[FPM_ITEMS_PER_INTERNAL_PAGE];
+		FreePageBtreeLeafKey leaf_key[FPM_ITEMS_PER_LEAF_PAGE];
+	}			u;
+};
+
+/* Results of a btree search */
+typedef struct FreePageBtreeSearchResult
+{
+	FreePageBtree *page;
+	Size		index;
+	bool		found;
+	unsigned	split_pages;
+} FreePageBtreeSearchResult;
+
+/* Helper functions */
+static void FreePageBtreeAdjustAncestorKeys(FreePageManager *fpm,
+								FreePageBtree *btp);
+static Size FreePageBtreeCleanup(FreePageManager *fpm);
+static FreePageBtree *FreePageBtreeFindLeftSibling(char *base,
+							 FreePageBtree *btp);
+static FreePageBtree *FreePageBtreeFindRightSibling(char *base,
+							  FreePageBtree *btp);
+static Size FreePageBtreeFirstKey(FreePageBtree *btp);
+static FreePageBtree *FreePageBtreeGetRecycled(FreePageManager *fpm);
+static void FreePageBtreeInsertInternal(char *base, FreePageBtree *btp,
+						  Size index, Size first_page, FreePageBtree *child);
+static void FreePageBtreeInsertLeaf(FreePageBtree *btp, Size index,
+						Size first_page, Size npages);
+static void FreePageBtreeRecycle(FreePageManager *fpm, Size pageno);
+static void FreePageBtreeRemove(FreePageManager *fpm, FreePageBtree *btp,
+					Size index);
+static void FreePageBtreeRemovePage(FreePageManager *fpm, FreePageBtree *btp);
+static void FreePageBtreeSearch(FreePageManager *fpm, Size first_page,
+					FreePageBtreeSearchResult *result);
+static Size FreePageBtreeSearchInternal(FreePageBtree *btp, Size first_page);
+static Size FreePageBtreeSearchLeaf(FreePageBtree *btp, Size first_page);
+static FreePageBtree *FreePageBtreeSplitPage(FreePageManager *fpm,
+					   FreePageBtree *btp);
+static void FreePageBtreeUpdateParentPointers(char *base, FreePageBtree *btp);
+static void FreePageManagerDumpBtree(FreePageManager *fpm, FreePageBtree *btp,
+						 FreePageBtree *parent, int level, StringInfo buf);
+static void FreePageManagerDumpSpans(FreePageManager *fpm,
+						 FreePageSpanLeader *span, Size expected_pages,
+						 StringInfo buf);
+static bool FreePageManagerGetInternal(FreePageManager *fpm, Size npages,
+						   Size *first_page);
+static Size FreePageManagerPutInternal(FreePageManager *fpm, Size first_page,
+						   Size npages, bool soft);
+static void FreePagePopSpanLeader(FreePageManager *fpm, Size pageno);
+static void FreePagePushSpanLeader(FreePageManager *fpm, Size first_page,
+					   Size npages);
+static Size FreePageManagerLargestContiguous(FreePageManager *fpm);
+static void FreePageManagerUpdateLargest(FreePageManager *fpm);
+
+#if FPM_EXTRA_ASSERTS
+static Size sum_free_pages(FreePageManager *fpm);
+#endif
+
+/*
+ * Initialize a new, empty free page manager.
+ *
+ * 'fpm' should reference caller-provided memory large enough to contain a
+ * FreePageManager.  We'll initialize it here.
+ *
+ * 'base' is the address to which all pointers are relative.  When managing
+ * a dynamic shared memory segment, it should normally be the base of the
+ * segment.  When managing backend-private memory, it can be either NULL or,
+ * if managing a single contiguous extent of memory, the start of that extent.
+ */
+void
+FreePageManagerInitialize(FreePageManager *fpm, char *base)
+{
+	Size		f;
+
+	relptr_store(base, fpm->self, fpm);
+	relptr_store(base, fpm->btree_root, (FreePageBtree *) NULL);
+	relptr_store(base, fpm->btree_recycle, (FreePageSpanLeader *) NULL);
+	fpm->btree_depth = 0;
+	fpm->btree_recycle_count = 0;
+	fpm->singleton_first_page = 0;
+	fpm->singleton_npages = 0;
+	fpm->contiguous_pages = 0;
+	fpm->contiguous_pages_dirty = true;
+#ifdef FPM_EXTRA_ASSERTS
+	fpm->free_pages = 0;
+#endif
+
+	for (f = 0; f < FPM_NUM_FREELISTS; f++)
+		relptr_store(base, fpm->freelist[f], (FreePageSpanLeader *) NULL);
+}
+
+/*
+ * Allocate a run of pages of the given length from the free page manager.
+ * The return value indicates whether we were able to satisfy the request;
+ * if true, the first page of the allocation is stored in *first_page.
+ */
+bool
+FreePageManagerGet(FreePageManager *fpm, Size npages, Size *first_page)
+{
+	bool		result;
+	Size		contiguous_pages;
+
+	result = FreePageManagerGetInternal(fpm, npages, first_page);
+
+	/*
+	 * It's a bit counterintuitive, but allocating pages can actually create
+	 * opportunities for cleanup that create larger ranges.  We might pull a
+	 * key out of the btree that enables the item at the head of the btree
+	 * recycle list to be inserted; and then if there are more items behind it
+	 * one of those might cause two currently-separated ranges to merge,
+	 * creating a single range of contiguous pages larger than any that
+	 * existed previously.  It might be worth trying to improve the cleanup
+	 * algorithm to avoid such corner cases, but for now we just notice the
+	 * condition and do the appropriate reporting.
+	 */
+	contiguous_pages = FreePageBtreeCleanup(fpm);
+	if (fpm->contiguous_pages < contiguous_pages)
+		fpm->contiguous_pages = contiguous_pages;
+
+	/*
+	 * FreePageManagerGetInternal may have set contiguous_pages_dirty.
+	 * Recompute contigous_pages if so.
+	 */
+	FreePageManagerUpdateLargest(fpm);
+
+#ifdef FPM_EXTRA_ASSERTS
+	if (result)
+	{
+		Assert(fpm->free_pages >= npages);
+		fpm->free_pages -= npages;
+	}
+	Assert(fpm->free_pages == sum_free_pages(fpm));
+	Assert(fpm->contiguous_pages == FreePageManagerLargestContiguous(fpm));
+#endif
+	return result;
+}
+
+#ifdef FPM_EXTRA_ASSERTS
+static void
+sum_free_pages_recurse(FreePageManager *fpm, FreePageBtree *btp, Size *sum)
+{
+	char	   *base = fpm_segment_base(fpm);
+
+	Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC ||
+		   btp->hdr.magic == FREE_PAGE_LEAF_MAGIC);
+	++*sum;
+	if (btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC)
+	{
+		Size		index;
+
+
+		for (index = 0; index < btp->hdr.nused; ++index)
+		{
+			FreePageBtree *child;
+
+			child = relptr_access(base, btp->u.internal_key[index].child);
+			sum_free_pages_recurse(fpm, child, sum);
+		}
+	}
+}
+static Size
+sum_free_pages(FreePageManager *fpm)
+{
+	FreePageSpanLeader *recycle;
+	char	   *base = fpm_segment_base(fpm);
+	Size		sum = 0;
+	int			list;
+
+	/* Count the spans by scanning the freelists. */
+	for (list = 0; list < FPM_NUM_FREELISTS; ++list)
+	{
+
+		if (!relptr_is_null(fpm->freelist[list]))
+		{
+			FreePageSpanLeader *candidate =
+			relptr_access(base, fpm->freelist[list]);
+
+			do
+			{
+				sum += candidate->npages;
+				candidate = relptr_access(base, candidate->next);
+			} while (candidate != NULL);
+		}
+	}
+
+	/* Count btree internal pages. */
+	if (fpm->btree_depth > 0)
+	{
+		FreePageBtree *root = relptr_access(base, fpm->btree_root);
+
+		sum_free_pages_recurse(fpm, root, &sum);
+	}
+
+	/* Count the recycle list. */
+	for (recycle = relptr_access(base, fpm->btree_recycle);
+		 recycle != NULL;
+		 recycle = relptr_access(base, recycle->next))
+	{
+		Assert(recycle->npages == 1);
+		++sum;
+	}
+
+	return sum;
+}
+#endif
+
+/*
+ * Compute the size of the largest run of pages that the user could
+ * succesfully get.
+ */
+static Size
+FreePageManagerLargestContiguous(FreePageManager *fpm)
+{
+	char	   *base;
+	Size		largest;
+
+	base = fpm_segment_base(fpm);
+	largest = 0;
+	if (!relptr_is_null(fpm->freelist[FPM_NUM_FREELISTS - 1]))
+	{
+		FreePageSpanLeader *candidate;
+
+		candidate = relptr_access(base, fpm->freelist[FPM_NUM_FREELISTS - 1]);
+		do
+		{
+			if (candidate->npages > largest)
+				largest = candidate->npages;
+			candidate = relptr_access(base, candidate->next);
+		} while (candidate != NULL);
+	}
+	else
+	{
+		Size		f = FPM_NUM_FREELISTS - 1;
+
+		do
+		{
+			--f;
+			if (!relptr_is_null(fpm->freelist[f]))
+			{
+				largest = f + 1;
+				break;
+			}
+		} while (f > 0);
+	}
+
+	return largest;
+}
+
+/*
+ * Recompute the size of the largest run of pages that the user could
+ * succesfully get, if it has been marked dirty.
+ */
+static void
+FreePageManagerUpdateLargest(FreePageManager *fpm)
+{
+	if (!fpm->contiguous_pages_dirty)
+		return;
+
+	fpm->contiguous_pages = FreePageManagerLargestContiguous(fpm);
+	fpm->contiguous_pages_dirty = false;
+}
+
+/*
+ * Transfer a run of pages to the free page manager.
+ */
+void
+FreePageManagerPut(FreePageManager *fpm, Size first_page, Size npages)
+{
+	Size		contiguous_pages;
+
+	Assert(npages > 0);
+
+	/* Record the new pages. */
+	contiguous_pages =
+		FreePageManagerPutInternal(fpm, first_page, npages, false);
+
+	/*
+	 * If the new range we inserted into the page manager was contiguous with
+	 * an existing range, it may have opened up cleanup opportunities.
+	 */
+	if (contiguous_pages > npages)
+	{
+		Size		cleanup_contiguous_pages;
+
+		cleanup_contiguous_pages = FreePageBtreeCleanup(fpm);
+		if (cleanup_contiguous_pages > contiguous_pages)
+			contiguous_pages = cleanup_contiguous_pages;
+	}
+
+	/* See if we now have a new largest chunk. */
+	if (fpm->contiguous_pages < contiguous_pages)
+		fpm->contiguous_pages = contiguous_pages;
+
+	/*
+	 * The earlier call to FreePageManagerPutInternal may have set
+	 * contiguous_pages_dirty if it needed to allocate internal pages, so
+	 * recompute contiguous_pages if necessary.
+	 */
+	FreePageManagerUpdateLargest(fpm);
+
+#ifdef FPM_EXTRA_ASSERTS
+	fpm->free_pages += npages;
+	Assert(fpm->free_pages == sum_free_pages(fpm));
+	Assert(fpm->contiguous_pages == FreePageManagerLargestContiguous(fpm));
+#endif
+}
+
+/*
+ * Produce a debugging dump of the state of a free page manager.
+ */
+char *
+FreePageManagerDump(FreePageManager *fpm)
+{
+	char	   *base = fpm_segment_base(fpm);
+	StringInfoData buf;
+	FreePageSpanLeader *recycle;
+	bool		dumped_any_freelist = false;
+	Size		f;
+
+	/* Initialize output buffer. */
+	initStringInfo(&buf);
+
+	/* Dump general stuff. */
+	appendStringInfo(&buf, "metadata: self %zu max contiguous pages = %zu\n",
+					 fpm->self.relptr_off, fpm->contiguous_pages);
+
+	/* Dump btree. */
+	if (fpm->btree_depth > 0)
+	{
+		FreePageBtree *root;
+
+		appendStringInfo(&buf, "btree depth %u:\n", fpm->btree_depth);
+		root = relptr_access(base, fpm->btree_root);
+		FreePageManagerDumpBtree(fpm, root, NULL, 0, &buf);
+	}
+	else if (fpm->singleton_npages > 0)
+	{
+		appendStringInfo(&buf, "singleton: %zu(%zu)\n",
+						 fpm->singleton_first_page, fpm->singleton_npages);
+	}
+
+	/* Dump btree recycle list. */
+	recycle = relptr_access(base, fpm->btree_recycle);
+	if (recycle != NULL)
+	{
+		appendStringInfo(&buf, "btree recycle:");
+		FreePageManagerDumpSpans(fpm, recycle, 1, &buf);
+	}
+
+	/* Dump free lists. */
+	for (f = 0; f < FPM_NUM_FREELISTS; ++f)
+	{
+		FreePageSpanLeader *span;
+
+		if (relptr_is_null(fpm->freelist[f]))
+			continue;
+		if (!dumped_any_freelist)
+		{
+			appendStringInfo(&buf, "freelists:\n");
+			dumped_any_freelist = true;
+		}
+		appendStringInfo(&buf, "  %zu:", f + 1);
+		span = relptr_access(base, fpm->freelist[f]);
+		FreePageManagerDumpSpans(fpm, span, f + 1, &buf);
+	}
+
+	/* And return result to caller. */
+	return buf.data;
+}
+
+
+/*
+ * The first_page value stored at index zero in any non-root page must match
+ * the first_page value stored in its parent at the index which points to that
+ * page.  So when the value stored at index zero in a btree page changes, we've
+ * got to walk up the tree adjusting ancestor keys until we reach an ancestor
+ * where that key isn't index zero.  This function should be called after
+ * updating the first key on the target page; it will propagate the change
+ * upward as far as needed.
+ *
+ * We assume here that the first key on the page has not changed enough to
+ * require changes in the ordering of keys on its ancestor pages.  Thus,
+ * if we search the parent page for the first key greater than or equal to
+ * the first key on the current page, the downlink to this page will be either
+ * the exact index returned by the search (if the first key decreased)
+ * or one less (if the first key increased).
+ */
+static void
+FreePageBtreeAdjustAncestorKeys(FreePageManager *fpm, FreePageBtree *btp)
+{
+	char	   *base = fpm_segment_base(fpm);
+	Size		first_page;
+	FreePageBtree *parent;
+	FreePageBtree *child;
+
+	/* This might be either a leaf or an internal page. */
+	Assert(btp->hdr.nused > 0);
+	if (btp->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+	{
+		Assert(btp->hdr.nused <= FPM_ITEMS_PER_LEAF_PAGE);
+		first_page = btp->u.leaf_key[0].first_page;
+	}
+	else
+	{
+		Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+		Assert(btp->hdr.nused <= FPM_ITEMS_PER_INTERNAL_PAGE);
+		first_page = btp->u.internal_key[0].first_page;
+	}
+	child = btp;
+
+	/* Loop until we find an ancestor that does not require adjustment. */
+	for (;;)
+	{
+		Size		s;
+
+		parent = relptr_access(base, child->hdr.parent);
+		if (parent == NULL)
+			break;
+		s = FreePageBtreeSearchInternal(parent, first_page);
+
+		/* Key is either at index s or index s-1; figure out which. */
+		if (s >= parent->hdr.nused)
+		{
+			Assert(s == parent->hdr.nused);
+			--s;
+		}
+		else
+		{
+			FreePageBtree *check;
+
+			check = relptr_access(base, parent->u.internal_key[s].child);
+			if (check != child)
+			{
+				Assert(s > 0);
+				--s;
+			}
+		}
+
+#ifdef USE_ASSERT_CHECKING
+		/* Debugging double-check. */
+		{
+			FreePageBtree *check;
+
+			check = relptr_access(base, parent->u.internal_key[s].child);
+			Assert(s < parent->hdr.nused);
+			Assert(child == check);
+		}
+#endif
+
+		/* Update the parent key. */
+		parent->u.internal_key[s].first_page = first_page;
+
+		/*
+		 * If this is the first key in the parent, go up another level; else
+		 * done.
+		 */
+		if (s > 0)
+			break;
+		child = parent;
+	}
+}
+
+/*
+ * Attempt to reclaim space from the free-page btree.  The return value is
+ * the largest range of contiguous pages created by the cleanup operation.
+ */
+static Size
+FreePageBtreeCleanup(FreePageManager *fpm)
+{
+	char	   *base = fpm_segment_base(fpm);
+	Size		max_contiguous_pages = 0;
+
+	/* Attempt to shrink the depth of the btree. */
+	while (!relptr_is_null(fpm->btree_root))
+	{
+		FreePageBtree *root = relptr_access(base, fpm->btree_root);
+
+		/* If the root contains only one key, reduce depth by one. */
+		if (root->hdr.nused == 1)
+		{
+			/* Shrink depth of tree by one. */
+			Assert(fpm->btree_depth > 0);
+			--fpm->btree_depth;
+			if (root->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+			{
+				/* If root is a leaf, convert only entry to singleton range. */
+				relptr_store(base, fpm->btree_root, (FreePageBtree *) NULL);
+				fpm->singleton_first_page = root->u.leaf_key[0].first_page;
+				fpm->singleton_npages = root->u.leaf_key[0].npages;
+			}
+			else
+			{
+				FreePageBtree *newroot;
+
+				/* If root is an internal page, make only child the root. */
+				Assert(root->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+				relptr_copy(fpm->btree_root, root->u.internal_key[0].child);
+				newroot = relptr_access(base, fpm->btree_root);
+				relptr_store(base, newroot->hdr.parent, (FreePageBtree *) NULL);
+			}
+			FreePageBtreeRecycle(fpm, fpm_pointer_to_page(base, root));
+		}
+		else if (root->hdr.nused == 2 &&
+				 root->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+		{
+			Size		end_of_first;
+			Size		start_of_second;
+
+			end_of_first = root->u.leaf_key[0].first_page +
+				root->u.leaf_key[0].npages;
+			start_of_second = root->u.leaf_key[1].first_page;
+
+			if (end_of_first + 1 == start_of_second)
+			{
+				Size		root_page = fpm_pointer_to_page(base, root);
+
+				if (end_of_first == root_page)
+				{
+					FreePagePopSpanLeader(fpm, root->u.leaf_key[0].first_page);
+					FreePagePopSpanLeader(fpm, root->u.leaf_key[1].first_page);
+					fpm->singleton_first_page = root->u.leaf_key[0].first_page;
+					fpm->singleton_npages = root->u.leaf_key[0].npages +
+						root->u.leaf_key[1].npages + 1;
+					fpm->btree_depth = 0;
+					relptr_store(base, fpm->btree_root,
+								 (FreePageBtree *) NULL);
+					FreePagePushSpanLeader(fpm, fpm->singleton_first_page,
+										   fpm->singleton_npages);
+					Assert(max_contiguous_pages == 0);
+					max_contiguous_pages = fpm->singleton_npages;
+				}
+			}
+
+			/* Whether it worked or not, it's time to stop. */
+			break;
+		}
+		else
+		{
+			/* Nothing more to do.  Stop. */
+			break;
+		}
+	}
+
+	/*
+	 * Attempt to free recycled btree pages.  We skip this if releasing the
+	 * recycled page would require a btree page split, because the page we're
+	 * trying to recycle would be consumed by the split, which would be
+	 * counterproductive.
+	 *
+	 * We also currently only ever attempt to recycle the first page on the
+	 * list; that could be made more aggressive, but it's not clear that the
+	 * complexity would be worthwhile.
+	 */
+	while (fpm->btree_recycle_count > 0)
+	{
+		FreePageBtree *btp;
+		Size		first_page;
+		Size		contiguous_pages;
+
+		btp = FreePageBtreeGetRecycled(fpm);
+		first_page = fpm_pointer_to_page(base, btp);
+		contiguous_pages = FreePageManagerPutInternal(fpm, first_page, 1, true);
+		if (contiguous_pages == 0)
+		{
+			FreePageBtreeRecycle(fpm, first_page);
+			break;
+		}
+		else
+		{
+			if (contiguous_pages > max_contiguous_pages)
+				max_contiguous_pages = contiguous_pages;
+		}
+	}
+
+	return max_contiguous_pages;
+}
+
+/*
+ * Consider consolidating the given page with its left or right sibling,
+ * if it's fairly empty.
+ */
+static void
+FreePageBtreeConsolidate(FreePageManager *fpm, FreePageBtree *btp)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageBtree *np;
+	Size		max;
+
+	/*
+	 * We only try to consolidate pages that are less than a third full. We
+	 * could be more aggressive about this, but that might risk performing
+	 * consolidation only to end up splitting again shortly thereafter.  Since
+	 * the btree should be very small compared to the space under management,
+	 * our goal isn't so much to ensure that it always occupies the absolutely
+	 * smallest possible number of pages as to reclaim pages before things get
+	 * too egregiously out of hand.
+	 */
+	if (btp->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+		max = FPM_ITEMS_PER_LEAF_PAGE;
+	else
+	{
+		Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+		max = FPM_ITEMS_PER_INTERNAL_PAGE;
+	}
+	if (btp->hdr.nused >= max / 3)
+		return;
+
+	/*
+	 * If we can fit our right sibling's keys onto this page, consolidate.
+	 */
+	np = FreePageBtreeFindRightSibling(base, btp);
+	if (np != NULL && btp->hdr.nused + np->hdr.nused <= max)
+	{
+		if (btp->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+		{
+			memcpy(&btp->u.leaf_key[btp->hdr.nused], &np->u.leaf_key[0],
+				   sizeof(FreePageBtreeLeafKey) * np->hdr.nused);
+			btp->hdr.nused += np->hdr.nused;
+		}
+		else
+		{
+			memcpy(&btp->u.internal_key[btp->hdr.nused], &np->u.internal_key[0],
+				   sizeof(FreePageBtreeInternalKey) * np->hdr.nused);
+			btp->hdr.nused += np->hdr.nused;
+			FreePageBtreeUpdateParentPointers(base, btp);
+		}
+		FreePageBtreeRemovePage(fpm, np);
+		return;
+	}
+
+	/*
+	 * If we can fit our keys onto our left sibling's page, consolidate. In
+	 * this case, we move our keys onto the other page rather than visca
+	 * versa, to avoid having to adjust ancestor keys.
+	 */
+	np = FreePageBtreeFindLeftSibling(base, btp);
+	if (np != NULL && btp->hdr.nused + np->hdr.nused <= max)
+	{
+		if (btp->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+		{
+			memcpy(&np->u.leaf_key[np->hdr.nused], &btp->u.leaf_key[0],
+				   sizeof(FreePageBtreeLeafKey) * btp->hdr.nused);
+			np->hdr.nused += btp->hdr.nused;
+		}
+		else
+		{
+			memcpy(&np->u.internal_key[np->hdr.nused], &btp->u.internal_key[0],
+				   sizeof(FreePageBtreeInternalKey) * btp->hdr.nused);
+			np->hdr.nused += btp->hdr.nused;
+			FreePageBtreeUpdateParentPointers(base, np);
+		}
+		FreePageBtreeRemovePage(fpm, btp);
+		return;
+	}
+}
+
+/*
+ * Find the passed page's left sibling; that is, the page at the same level
+ * of the tree whose keyspace immediately precedes ours.
+ */
+static FreePageBtree *
+FreePageBtreeFindLeftSibling(char *base, FreePageBtree *btp)
+{
+	FreePageBtree *p = btp;
+	int			levels = 0;
+
+	/* Move up until we can move left. */
+	for (;;)
+	{
+		Size		first_page;
+		Size		index;
+
+		first_page = FreePageBtreeFirstKey(p);
+		p = relptr_access(base, p->hdr.parent);
+
+		if (p == NULL)
+			return NULL;		/* we were passed the rightmost page */
+
+		index = FreePageBtreeSearchInternal(p, first_page);
+		if (index > 0)
+		{
+			Assert(p->u.internal_key[index].first_page == first_page);
+			p = relptr_access(base, p->u.internal_key[index - 1].child);
+			break;
+		}
+		Assert(index == 0);
+		++levels;
+	}
+
+	/* Descend left. */
+	while (levels > 0)
+	{
+		Assert(p->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+		p = relptr_access(base, p->u.internal_key[p->hdr.nused - 1].child);
+		--levels;
+	}
+	Assert(p->hdr.magic == btp->hdr.magic);
+
+	return p;
+}
+
+/*
+ * Find the passed page's right sibling; that is, the page at the same level
+ * of the tree whose keyspace immediately follows ours.
+ */
+static FreePageBtree *
+FreePageBtreeFindRightSibling(char *base, FreePageBtree *btp)
+{
+	FreePageBtree *p = btp;
+	int			levels = 0;
+
+	/* Move up until we can move right. */
+	for (;;)
+	{
+		Size		first_page;
+		Size		index;
+
+		first_page = FreePageBtreeFirstKey(p);
+		p = relptr_access(base, p->hdr.parent);
+
+		if (p == NULL)
+			return NULL;		/* we were passed the rightmost page */
+
+		index = FreePageBtreeSearchInternal(p, first_page);
+		if (index < p->hdr.nused - 1)
+		{
+			Assert(p->u.internal_key[index].first_page == first_page);
+			p = relptr_access(base, p->u.internal_key[index + 1].child);
+			break;
+		}
+		Assert(index == p->hdr.nused - 1);
+		++levels;
+	}
+
+	/* Descend left. */
+	while (levels > 0)
+	{
+		Assert(p->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+		p = relptr_access(base, p->u.internal_key[0].child);
+		--levels;
+	}
+	Assert(p->hdr.magic == btp->hdr.magic);
+
+	return p;
+}
+
+/*
+ * Get the first key on a btree page.
+ */
+static Size
+FreePageBtreeFirstKey(FreePageBtree *btp)
+{
+	Assert(btp->hdr.nused > 0);
+
+	if (btp->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+		return btp->u.leaf_key[0].first_page;
+	else
+	{
+		Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+		return btp->u.internal_key[0].first_page;
+	}
+}
+
+/*
+ * Get a page from the btree recycle list for use as a btree page.
+ */
+static FreePageBtree *
+FreePageBtreeGetRecycled(FreePageManager *fpm)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageSpanLeader *victim = relptr_access(base, fpm->btree_recycle);
+	FreePageSpanLeader *newhead;
+
+	Assert(victim != NULL);
+	newhead = relptr_access(base, victim->next);
+	if (newhead != NULL)
+		relptr_copy(newhead->prev, victim->prev);
+	relptr_store(base, fpm->btree_recycle, newhead);
+	Assert(fpm_pointer_is_page_aligned(base, victim));
+	fpm->btree_recycle_count--;
+	return (FreePageBtree *) victim;
+}
+
+/*
+ * Insert an item into an internal page.
+ */
+static void
+FreePageBtreeInsertInternal(char *base, FreePageBtree *btp, Size index,
+							Size first_page, FreePageBtree *child)
+{
+	Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+	Assert(btp->hdr.nused <= FPM_ITEMS_PER_INTERNAL_PAGE);
+	Assert(index <= btp->hdr.nused);
+	memmove(&btp->u.internal_key[index + 1], &btp->u.internal_key[index],
+			sizeof(FreePageBtreeInternalKey) * (btp->hdr.nused - index));
+	btp->u.internal_key[index].first_page = first_page;
+	relptr_store(base, btp->u.internal_key[index].child, child);
+	++btp->hdr.nused;
+}
+
+/*
+ * Insert an item into a leaf page.
+ */
+static void
+FreePageBtreeInsertLeaf(FreePageBtree *btp, Size index, Size first_page,
+						Size npages)
+{
+	Assert(btp->hdr.magic == FREE_PAGE_LEAF_MAGIC);
+	Assert(btp->hdr.nused <= FPM_ITEMS_PER_LEAF_PAGE);
+	Assert(index <= btp->hdr.nused);
+	memmove(&btp->u.leaf_key[index + 1], &btp->u.leaf_key[index],
+			sizeof(FreePageBtreeLeafKey) * (btp->hdr.nused - index));
+	btp->u.leaf_key[index].first_page = first_page;
+	btp->u.leaf_key[index].npages = npages;
+	++btp->hdr.nused;
+}
+
+/*
+ * Put a page on the btree recycle list.
+ */
+static void
+FreePageBtreeRecycle(FreePageManager *fpm, Size pageno)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageSpanLeader *head = relptr_access(base, fpm->btree_recycle);
+	FreePageSpanLeader *span;
+
+	span = (FreePageSpanLeader *) fpm_page_to_pointer(base, pageno);
+	span->magic = FREE_PAGE_SPAN_LEADER_MAGIC;
+	span->npages = 1;
+	relptr_store(base, span->next, head);
+	relptr_store(base, span->prev, (FreePageSpanLeader *) NULL);
+	if (head != NULL)
+		relptr_store(base, head->prev, span);
+	relptr_store(base, fpm->btree_recycle, span);
+	fpm->btree_recycle_count++;
+}
+
+/*
+ * Remove an item from the btree at the given position on the given page.
+ */
+static void
+FreePageBtreeRemove(FreePageManager *fpm, FreePageBtree *btp, Size index)
+{
+	Assert(btp->hdr.magic == FREE_PAGE_LEAF_MAGIC);
+	Assert(index < btp->hdr.nused);
+
+	/* When last item is removed, extirpate entire page from btree. */
+	if (btp->hdr.nused == 1)
+	{
+		FreePageBtreeRemovePage(fpm, btp);
+		return;
+	}
+
+	/* Physically remove the key from the page. */
+	--btp->hdr.nused;
+	if (index < btp->hdr.nused)
+		memmove(&btp->u.leaf_key[index], &btp->u.leaf_key[index + 1],
+				sizeof(FreePageBtreeLeafKey) * (btp->hdr.nused - index));
+
+	/* If we just removed the first key, adjust ancestor keys. */
+	if (index == 0)
+		FreePageBtreeAdjustAncestorKeys(fpm, btp);
+
+	/* Consider whether to consolidate this page with a sibling. */
+	FreePageBtreeConsolidate(fpm, btp);
+}
+
+/*
+ * Remove a page from the btree.  Caller is responsible for having relocated
+ * any keys from this page that are still wanted.  The page is placed on the
+ * recycled list.
+ */
+static void
+FreePageBtreeRemovePage(FreePageManager *fpm, FreePageBtree *btp)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageBtree *parent;
+	Size		index;
+	Size		first_page;
+
+	for (;;)
+	{
+		/* Find parent page. */
+		parent = relptr_access(base, btp->hdr.parent);
+		if (parent == NULL)
+		{
+			/* We are removing the root page. */
+			relptr_store(base, fpm->btree_root, (FreePageBtree *) NULL);
+			fpm->btree_depth = 0;
+			Assert(fpm->singleton_first_page == 0);
+			Assert(fpm->singleton_npages == 0);
+			return;
+		}
+
+		/*
+		 * If the parent contains only one item, we need to remove it as well.
+		 */
+		if (parent->hdr.nused > 1)
+			break;
+		FreePageBtreeRecycle(fpm, fpm_pointer_to_page(base, btp));
+		btp = parent;
+	}
+
+	/* Find and remove the downlink. */
+	first_page = FreePageBtreeFirstKey(btp);
+	if (parent->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+	{
+		index = FreePageBtreeSearchLeaf(parent, first_page);
+		Assert(index < parent->hdr.nused);
+		if (index < parent->hdr.nused - 1)
+			memmove(&parent->u.leaf_key[index],
+					&parent->u.leaf_key[index + 1],
+					sizeof(FreePageBtreeLeafKey)
+					* (parent->hdr.nused - index - 1));
+	}
+	else
+	{
+		index = FreePageBtreeSearchInternal(parent, first_page);
+		Assert(index < parent->hdr.nused);
+		if (index < parent->hdr.nused - 1)
+			memmove(&parent->u.internal_key[index],
+					&parent->u.internal_key[index + 1],
+					sizeof(FreePageBtreeInternalKey)
+					* (parent->hdr.nused - index - 1));
+	}
+	parent->hdr.nused--;
+	Assert(parent->hdr.nused > 0);
+
+	/* Recycle the page. */
+	FreePageBtreeRecycle(fpm, fpm_pointer_to_page(base, btp));
+
+	/* Adjust ancestor keys if needed. */
+	if (index == 0)
+		FreePageBtreeAdjustAncestorKeys(fpm, parent);
+
+	/* Consider whether to consolidate the parent with a sibling. */
+	FreePageBtreeConsolidate(fpm, parent);
+}
+
+/*
+ * Search the btree for an entry for the given first page and initialize
+ * *result with the results of the search.  result->page and result->index
+ * indicate either the position of an exact match or the position at which
+ * the new key should be inserted.  result->found is true for an exact match,
+ * otherwise false.  result->split_pages will contain the number of additional
+ * btree pages that will be needed when performing a split to insert a key.
+ * Except as described above, the contents of fields in the result object are
+ * undefined on return.
+ */
+static void
+FreePageBtreeSearch(FreePageManager *fpm, Size first_page,
+					FreePageBtreeSearchResult *result)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageBtree *btp = relptr_access(base, fpm->btree_root);
+	Size		index;
+
+	result->split_pages = 1;
+
+	/* If the btree is empty, there's nothing to find. */
+	if (btp == NULL)
+	{
+		result->page = NULL;
+		result->found = false;
+		return;
+	}
+
+	/* Descend until we hit a leaf. */
+	while (btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC)
+	{
+		FreePageBtree *child;
+		bool		found_exact;
+
+		index = FreePageBtreeSearchInternal(btp, first_page);
+		found_exact = index < btp->hdr.nused &&
+			btp->u.internal_key[index].first_page == first_page;
+
+		/*
+		 * If we found an exact match we descend directly.  Otherwise, we
+		 * descend into the child to the left if possible so that we can find
+		 * the insertion point at that child's high end.
+		 */
+		if (!found_exact && index > 0)
+			--index;
+
+		/* Track required split depth for leaf insert. */
+		if (btp->hdr.nused >= FPM_ITEMS_PER_INTERNAL_PAGE)
+		{
+			Assert(btp->hdr.nused == FPM_ITEMS_PER_INTERNAL_PAGE);
+			result->split_pages++;
+		}
+		else
+			result->split_pages = 0;
+
+		/* Descend to appropriate child page. */
+		Assert(index < btp->hdr.nused);
+		child = relptr_access(base, btp->u.internal_key[index].child);
+		Assert(relptr_access(base, child->hdr.parent) == btp);
+		btp = child;
+	}
+
+	/* Track required split depth for leaf insert. */
+	if (btp->hdr.nused >= FPM_ITEMS_PER_LEAF_PAGE)
+	{
+		Assert(btp->hdr.nused == FPM_ITEMS_PER_INTERNAL_PAGE);
+		result->split_pages++;
+	}
+	else
+		result->split_pages = 0;
+
+	/* Search leaf page. */
+	index = FreePageBtreeSearchLeaf(btp, first_page);
+
+	/* Assemble results. */
+	result->page = btp;
+	result->index = index;
+	result->found = index < btp->hdr.nused &&
+		first_page == btp->u.leaf_key[index].first_page;
+}
+
+/*
+ * Search an internal page for the first key greater than or equal to a given
+ * page number.  Returns the index of that key, or one greater than the number
+ * of keys on the page if none.
+ */
+static Size
+FreePageBtreeSearchInternal(FreePageBtree *btp, Size first_page)
+{
+	Size		low = 0;
+	Size		high = btp->hdr.nused;
+
+	Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+	Assert(high > 0 && high <= FPM_ITEMS_PER_INTERNAL_PAGE);
+
+	while (low < high)
+	{
+		Size		mid = (low + high) / 2;
+		Size		val = btp->u.internal_key[mid].first_page;
+
+		if (first_page == val)
+			return mid;
+		else if (first_page < val)
+			high = mid;
+		else
+			low = mid + 1;
+	}
+
+	return low;
+}
+
+/*
+ * Search a leaf page for the first key greater than or equal to a given
+ * page number.  Returns the index of that key, or one greater than the number
+ * of keys on the page if none.
+ */
+static Size
+FreePageBtreeSearchLeaf(FreePageBtree *btp, Size first_page)
+{
+	Size		low = 0;
+	Size		high = btp->hdr.nused;
+
+	Assert(btp->hdr.magic == FREE_PAGE_LEAF_MAGIC);
+	Assert(high > 0 && high <= FPM_ITEMS_PER_LEAF_PAGE);
+
+	while (low < high)
+	{
+		Size		mid = (low + high) / 2;
+		Size		val = btp->u.leaf_key[mid].first_page;
+
+		if (first_page == val)
+			return mid;
+		else if (first_page < val)
+			high = mid;
+		else
+			low = mid + 1;
+	}
+
+	return low;
+}
+
+/*
+ * Allocate a new btree page and move half the keys from the provided page
+ * to the new page.  Caller is responsible for making sure that there's a
+ * page available from fpm->btree_recycle.  Returns a pointer to the new page,
+ * to which caller must add a downlink.
+ */
+static FreePageBtree *
+FreePageBtreeSplitPage(FreePageManager *fpm, FreePageBtree *btp)
+{
+	FreePageBtree *newsibling;
+
+	newsibling = FreePageBtreeGetRecycled(fpm);
+	newsibling->hdr.magic = btp->hdr.magic;
+	newsibling->hdr.nused = btp->hdr.nused / 2;
+	relptr_copy(newsibling->hdr.parent, btp->hdr.parent);
+	btp->hdr.nused -= newsibling->hdr.nused;
+
+	if (btp->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+		memcpy(&newsibling->u.leaf_key,
+			   &btp->u.leaf_key[btp->hdr.nused],
+			   sizeof(FreePageBtreeLeafKey) * newsibling->hdr.nused);
+	else
+	{
+		Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+		memcpy(&newsibling->u.internal_key,
+			   &btp->u.internal_key[btp->hdr.nused],
+			   sizeof(FreePageBtreeInternalKey) * newsibling->hdr.nused);
+		FreePageBtreeUpdateParentPointers(fpm_segment_base(fpm), newsibling);
+	}
+
+	return newsibling;
+}
+
+/*
+ * When internal pages are split or merged, the parent pointers of their
+ * children must be updated.
+ */
+static void
+FreePageBtreeUpdateParentPointers(char *base, FreePageBtree *btp)
+{
+	Size		i;
+
+	Assert(btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+	for (i = 0; i < btp->hdr.nused; ++i)
+	{
+		FreePageBtree *child;
+
+		child = relptr_access(base, btp->u.internal_key[i].child);
+		relptr_store(base, child->hdr.parent, btp);
+	}
+}
+
+/*
+ * Debugging dump of btree data.
+ */
+static void
+FreePageManagerDumpBtree(FreePageManager *fpm, FreePageBtree *btp,
+						 FreePageBtree *parent, int level, StringInfo buf)
+{
+	char	   *base = fpm_segment_base(fpm);
+	Size		pageno = fpm_pointer_to_page(base, btp);
+	Size		index;
+	FreePageBtree *check_parent;
+
+	check_stack_depth();
+	check_parent = relptr_access(base, btp->hdr.parent);
+	appendStringInfo(buf, "  %zu@%d %c", pageno, level,
+					 btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC ? 'i' : 'l');
+	if (parent != check_parent)
+		appendStringInfo(buf, " [actual parent %zu, expected %zu]",
+						 fpm_pointer_to_page(base, check_parent),
+						 fpm_pointer_to_page(base, parent));
+	appendStringInfoChar(buf, ':');
+	for (index = 0; index < btp->hdr.nused; ++index)
+	{
+		if (btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC)
+			appendStringInfo(buf, " %zu->%zu",
+							 btp->u.internal_key[index].first_page,
+				btp->u.internal_key[index].child.relptr_off / FPM_PAGE_SIZE);
+		else
+			appendStringInfo(buf, " %zu(%zu)",
+							 btp->u.leaf_key[index].first_page,
+							 btp->u.leaf_key[index].npages);
+	}
+	appendStringInfo(buf, "\n");
+
+	if (btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC)
+	{
+		for (index = 0; index < btp->hdr.nused; ++index)
+		{
+			FreePageBtree *child;
+
+			child = relptr_access(base, btp->u.internal_key[index].child);
+			FreePageManagerDumpBtree(fpm, child, btp, level + 1, buf);
+		}
+	}
+}
+
+/*
+ * Debugging dump of free-span data.
+ */
+static void
+FreePageManagerDumpSpans(FreePageManager *fpm, FreePageSpanLeader *span,
+						 Size expected_pages, StringInfo buf)
+{
+	char	   *base = fpm_segment_base(fpm);
+
+	while (span != NULL)
+	{
+		if (span->npages != expected_pages)
+			appendStringInfo(buf, " %zu(%zu)", fpm_pointer_to_page(base, span),
+							 span->npages);
+		else
+			appendStringInfo(buf, " %zu", fpm_pointer_to_page(base, span));
+		span = relptr_access(base, span->next);
+	}
+
+	appendStringInfo(buf, "\n");
+}
+
+/*
+ * This function allocates a run of pages of the given length from the free
+ * page manager.
+ */
+static bool
+FreePageManagerGetInternal(FreePageManager *fpm, Size npages, Size *first_page)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageSpanLeader *victim = NULL;
+	FreePageSpanLeader *prev;
+	FreePageSpanLeader *next;
+	FreePageBtreeSearchResult result;
+	Size		victim_page = 0;	/* placate compiler */
+	Size		f;
+
+	/*
+	 * Search for a free span.
+	 *
+	 * Right now, we use a simple best-fit policy here, but it's possible for
+	 * this to result in memory fragmentation if we're repeatedly asked to
+	 * allocate chunks just a little smaller than what we have available.
+	 * Hopefully, this is unlikely, because we expect most requests to be
+	 * single pages or superblock-sized chunks -- but no policy can be optimal
+	 * under all circumstances unless it has knowledge of future allocation
+	 * patterns.
+	 */
+	for (f = Min(npages, FPM_NUM_FREELISTS) - 1; f < FPM_NUM_FREELISTS; ++f)
+	{
+		/* Skip empty freelists. */
+		if (relptr_is_null(fpm->freelist[f]))
+			continue;
+
+		/*
+		 * All of the freelists except the last one contain only items of a
+		 * single size, so we just take the first one.  But the final free
+		 * list contains everything too big for any of the other lists, so we
+		 * need to search the list.
+		 */
+		if (f < FPM_NUM_FREELISTS - 1)
+			victim = relptr_access(base, fpm->freelist[f]);
+		else
+		{
+			FreePageSpanLeader *candidate;
+
+			candidate = relptr_access(base, fpm->freelist[f]);
+			do
+			{
+				if (candidate->npages >= npages && (victim == NULL ||
+										 victim->npages > candidate->npages))
+				{
+					victim = candidate;
+					if (victim->npages == npages)
+						break;
+				}
+				candidate = relptr_access(base, candidate->next);
+			} while (candidate != NULL);
+		}
+		break;
+	}
+
+	/* If we didn't find an allocatable span, return failure. */
+	if (victim == NULL)
+		return false;
+
+	/* Remove span from free list. */
+	Assert(victim->magic == FREE_PAGE_SPAN_LEADER_MAGIC);
+	prev = relptr_access(base, victim->prev);
+	next = relptr_access(base, victim->next);
+	if (prev != NULL)
+		relptr_copy(prev->next, victim->next);
+	else
+		relptr_copy(fpm->freelist[f], victim->next);
+	if (next != NULL)
+		relptr_copy(next->prev, victim->prev);
+	victim_page = fpm_pointer_to_page(base, victim);
+
+	/* Decide whether we might be invalidating contiguous_pages. */
+	if (f == FPM_NUM_FREELISTS - 1 &&
+		victim->npages == fpm->contiguous_pages)
+	{
+		/*
+		 * The victim span came from the oversized freelist, and had the same
+		 * size as the longest span.  There may or may not be another one of
+		 * the same size, so contiguous_pages must be recomputed just to be
+		 * safe.
+		 */
+		fpm->contiguous_pages_dirty = true;
+	}
+	else if (f + 1 == fpm->contiguous_pages &&
+			 relptr_is_null(fpm->freelist[f]))
+	{
+		/*
+		 * The victim span came from a fixed sized freelist, and it was the
+		 * list for spans of the same size as the current longest span, and
+		 * the list is now empty after removing the victim.  So
+		 * contiguous_pages must be recomputed without a doubt.
+		 */
+		fpm->contiguous_pages_dirty = true;
+	}
+
+	/*
+	 * If we haven't initialized the btree yet, the victim must be the single
+	 * span stored within the FreePageManager itself.  Otherwise, we need to
+	 * update the btree.
+	 */
+	if (relptr_is_null(fpm->btree_root))
+	{
+		Assert(victim_page == fpm->singleton_first_page);
+		Assert(victim->npages == fpm->singleton_npages);
+		Assert(victim->npages >= npages);
+		fpm->singleton_first_page += npages;
+		fpm->singleton_npages -= npages;
+		if (fpm->singleton_npages > 0)
+			FreePagePushSpanLeader(fpm, fpm->singleton_first_page,
+								   fpm->singleton_npages);
+	}
+	else
+	{
+		/*
+		 * If the span we found is exactly the right size, remove it from the
+		 * btree completely.  Otherwise, adjust the btree entry to reflect the
+		 * still-unallocated portion of the span, and put that portion on the
+		 * appropriate free list.
+		 */
+		FreePageBtreeSearch(fpm, victim_page, &result);
+		Assert(result.found);
+		if (victim->npages == npages)
+			FreePageBtreeRemove(fpm, result.page, result.index);
+		else
+		{
+			FreePageBtreeLeafKey *key;
+
+			/* Adjust btree to reflect remaining pages. */
+			Assert(victim->npages > npages);
+			key = &result.page->u.leaf_key[result.index];
+			Assert(key->npages == victim->npages);
+			key->first_page += npages;
+			key->npages -= npages;
+			if (result.index == 0)
+				FreePageBtreeAdjustAncestorKeys(fpm, result.page);
+
+			/* Put the unallocated pages back on the appropriate free list. */
+			FreePagePushSpanLeader(fpm, victim_page + npages,
+								   victim->npages - npages);
+		}
+	}
+
+	/* Return results to caller. */
+	*first_page = fpm_pointer_to_page(base, victim);
+	return true;
+}
+
+/*
+ * Put a range of pages into the btree and freelists, consolidating it with
+ * existing free spans just before and/or after it.  If 'soft' is true,
+ * only perform the insertion if it can be done without allocating new btree
+ * pages; if false, do it always.  Returns 0 if the soft flag caused the
+ * insertion to be skipped, or otherwise the size of the contiguous span
+ * created by the insertion.  This may be larger than npages if we're able
+ * to consolidate with an adjacent range.  *internal_pages_used is set to
+ * true if the btree allocated pages for internal purposes, which might
+ * invalidate the current largest run requiring it to be recomputed.
+ */
+static Size
+FreePageManagerPutInternal(FreePageManager *fpm, Size first_page, Size npages,
+						   bool soft)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageBtreeSearchResult result;
+	FreePageBtreeLeafKey *prevkey = NULL;
+	FreePageBtreeLeafKey *nextkey = NULL;
+	FreePageBtree *np;
+	Size		nindex;
+
+	Assert(npages > 0);
+
+	/* We can store a single free span without initializing the btree. */
+	if (fpm->btree_depth == 0)
+	{
+		if (fpm->singleton_npages == 0)
+		{
+			/* Don't have a span yet; store this one. */
+			fpm->singleton_first_page = first_page;
+			fpm->singleton_npages = npages;
+			FreePagePushSpanLeader(fpm, first_page, npages);
+			return fpm->singleton_npages;
+		}
+		else if (fpm->singleton_first_page + fpm->singleton_npages ==
+				 first_page)
+		{
+			/* New span immediately follows sole existing span. */
+			fpm->singleton_npages += npages;
+			FreePagePopSpanLeader(fpm, fpm->singleton_first_page);
+			FreePagePushSpanLeader(fpm, fpm->singleton_first_page,
+								   fpm->singleton_npages);
+			return fpm->singleton_npages;
+		}
+		else if (first_page + npages == fpm->singleton_first_page)
+		{
+			/* New span immediately precedes sole existing span. */
+			FreePagePopSpanLeader(fpm, fpm->singleton_first_page);
+			fpm->singleton_first_page = first_page;
+			fpm->singleton_npages += npages;
+			FreePagePushSpanLeader(fpm, fpm->singleton_first_page,
+								   fpm->singleton_npages);
+			return fpm->singleton_npages;
+		}
+		else
+		{
+			/* Not contiguous; we need to initialize the btree. */
+			Size		root_page;
+			FreePageBtree *root;
+
+			if (!relptr_is_null(fpm->btree_recycle))
+				root = FreePageBtreeGetRecycled(fpm);
+			else if (FreePageManagerGetInternal(fpm, 1, &root_page))
+				root = (FreePageBtree *) fpm_page_to_pointer(base, root_page);
+			else
+			{
+				/* We'd better be able to get a page from the existing range. */
+				elog(FATAL, "free page manager btree is corrupt");
+			}
+
+			/* Create the btree and move the preexisting range into it. */
+			root->hdr.magic = FREE_PAGE_LEAF_MAGIC;
+			root->hdr.nused = 1;
+			relptr_store(base, root->hdr.parent, (FreePageBtree *) NULL);
+			root->u.leaf_key[0].first_page = fpm->singleton_first_page;
+			root->u.leaf_key[0].npages = fpm->singleton_npages;
+			relptr_store(base, fpm->btree_root, root);
+			fpm->singleton_first_page = 0;
+			fpm->singleton_npages = 0;
+			fpm->btree_depth = 1;
+
+			/*
+			 * Corner case: it may be that the btree root took the very last
+			 * free page.  In that case, the sole btree entry covers a zero
+			 * page run, which is invalid.  Overwrite it with the entry we're
+			 * trying to insert and get out.
+			 */
+			if (root->u.leaf_key[0].npages == 0)
+			{
+				root->u.leaf_key[0].first_page = first_page;
+				root->u.leaf_key[0].npages = npages;
+				FreePagePushSpanLeader(fpm, first_page, npages);
+				return npages;
+			}
+
+			/* Fall through to insert the new key. */
+		}
+	}
+
+	/* Search the btree. */
+	FreePageBtreeSearch(fpm, first_page, &result);
+	Assert(!result.found);
+	if (result.index > 0)
+		prevkey = &result.page->u.leaf_key[result.index - 1];
+	if (result.index < result.page->hdr.nused)
+	{
+		np = result.page;
+		nindex = result.index;
+		nextkey = &result.page->u.leaf_key[result.index];
+	}
+	else
+	{
+		np = FreePageBtreeFindRightSibling(base, result.page);
+		nindex = 0;
+		if (np != NULL)
+			nextkey = &np->u.leaf_key[0];
+	}
+
+	/* Consolidate with the previous entry if possible. */
+	if (prevkey != NULL && prevkey->first_page + prevkey->npages >= first_page)
+	{
+		bool		remove_next = false;
+		Size		result;
+
+		Assert(prevkey->first_page + prevkey->npages == first_page);
+		prevkey->npages = (first_page - prevkey->first_page) + npages;
+
+		/* Check whether we can *also* consolidate with the following entry. */
+		if (nextkey != NULL &&
+			prevkey->first_page + prevkey->npages >= nextkey->first_page)
+		{
+			Assert(prevkey->first_page + prevkey->npages ==
+				   nextkey->first_page);
+			prevkey->npages = (nextkey->first_page - prevkey->first_page)
+				+ nextkey->npages;
+			FreePagePopSpanLeader(fpm, nextkey->first_page);
+			remove_next = true;
+		}
+
+		/* Put the span on the correct freelist and save size. */
+		FreePagePopSpanLeader(fpm, prevkey->first_page);
+		FreePagePushSpanLeader(fpm, prevkey->first_page, prevkey->npages);
+		result = prevkey->npages;
+
+		/*
+		 * If we consolidated with both the preceding and following entries,
+		 * we must remove the following entry.  We do this last, because
+		 * removing an element from the btree may invalidate pointers we hold
+		 * into the current data structure.
+		 *
+		 * NB: The btree is technically in an invalid state a this point
+		 * because we've already updated prevkey to cover the same key space
+		 * as nextkey.  FreePageBtreeRemove() shouldn't notice that, though.
+		 */
+		if (remove_next)
+			FreePageBtreeRemove(fpm, np, nindex);
+
+		return result;
+	}
+
+	/* Consolidate with the next entry if possible. */
+	if (nextkey != NULL && first_page + npages >= nextkey->first_page)
+	{
+		Size		newpages;
+
+		/* Compute new size for span. */
+		Assert(first_page + npages == nextkey->first_page);
+		newpages = (nextkey->first_page - first_page) + nextkey->npages;
+
+		/* Put span on correct free list. */
+		FreePagePopSpanLeader(fpm, nextkey->first_page);
+		FreePagePushSpanLeader(fpm, first_page, newpages);
+
+		/* Update key in place. */
+		nextkey->first_page = first_page;
+		nextkey->npages = newpages;
+
+		/* If reducing first key on page, ancestors might need adjustment. */
+		if (nindex == 0)
+			FreePageBtreeAdjustAncestorKeys(fpm, np);
+
+		return nextkey->npages;
+	}
+
+	/* Split leaf page and as many of its ancestors as necessary. */
+	if (result.split_pages > 0)
+	{
+		/*
+		 * NB: We could consider various coping strategies here to avoid a
+		 * split; most obviously, if np != result.page, we could target that
+		 * page instead.   More complicated shuffling strategies could be
+		 * possible as well; basically, unless every single leaf page is 100%
+		 * full, we can jam this key in there if we try hard enough.  It's
+		 * unlikely that trying that hard is worthwhile, but it's possible we
+		 * might need to make more than no effort.  For now, we just do the
+		 * easy thing, which is nothing.
+		 */
+
+		/* If this is a soft insert, it's time to give up. */
+		if (soft)
+			return 0;
+
+		/* Check whether we need to allocate more btree pages to split. */
+		if (result.split_pages > fpm->btree_recycle_count)
+		{
+			Size		pages_needed;
+			Size		recycle_page;
+			Size		i;
+
+			/*
+			 * Allocate the required number of pages and split each one in
+			 * turn.  This should never fail, because if we've got enough
+			 * spans of free pages kicking around that we need additional
+			 * storage space just to remember them all, then we should
+			 * certainly have enough to expand the btree, which should only
+			 * ever use a tiny number of pages compared to the number under
+			 * management.  If it does, something's badly screwed up.
+			 */
+			pages_needed = result.split_pages - fpm->btree_recycle_count;
+			for (i = 0; i < pages_needed; ++i)
+			{
+				if (!FreePageManagerGetInternal(fpm, 1, &recycle_page))
+					elog(FATAL, "free page manager btree is corrupt");
+				FreePageBtreeRecycle(fpm, recycle_page);
+			}
+
+			/*
+			 * The act of allocating pages to recycle may have invalidated the
+			 * results of our previous btree reserch, so repeat it. (We could
+			 * recheck whether any of our split-avoidance strategies that were
+			 * not viable before now are, but it hardly seems worthwhile, so
+			 * we don't bother. Consolidation can't be possible now if it
+			 * wasn't previously.)
+			 */
+			FreePageBtreeSearch(fpm, first_page, &result);
+
+			/*
+			 * The act of allocating pages for use in constructing our btree
+			 * should never cause any page to become more full, so the new
+			 * split depth should be no greater than the old one, and perhaps
+			 * less if we fortutiously allocated a chunk that freed up a slot
+			 * on the page we need to update.
+			 */
+			Assert(result.split_pages <= fpm->btree_recycle_count);
+		}
+
+		/* If we still need to perform a split, do it. */
+		if (result.split_pages > 0)
+		{
+			FreePageBtree *split_target = result.page;
+			FreePageBtree *child = NULL;
+			Size		key = first_page;
+
+			for (;;)
+			{
+				FreePageBtree *newsibling;
+				FreePageBtree *parent;
+
+				/* Identify parent page, which must receive downlink. */
+				parent = relptr_access(base, split_target->hdr.parent);
+
+				/* Split the page - downlink not added yet. */
+				newsibling = FreePageBtreeSplitPage(fpm, split_target);
+
+				/*
+				 * At this point in the loop, we're always carrying a pending
+				 * insertion.  On the first pass, it's the actual key we're
+				 * trying to insert; on subsequent passes, it's the downlink
+				 * that needs to be added as a result of the split performed
+				 * during the previous loop iteration.  Since we've just split
+				 * the page, there's definitely room on one of the two
+				 * resulting pages.
+				 */
+				if (child == NULL)
+				{
+					Size		index;
+					FreePageBtree *insert_into;
+
+					insert_into = key < newsibling->u.leaf_key[0].first_page ?
+						split_target : newsibling;
+					index = FreePageBtreeSearchLeaf(insert_into, key);
+					FreePageBtreeInsertLeaf(insert_into, index, key, npages);
+					if (index == 0 && insert_into == split_target)
+						FreePageBtreeAdjustAncestorKeys(fpm, split_target);
+				}
+				else
+				{
+					Size		index;
+					FreePageBtree *insert_into;
+
+					insert_into =
+						key < newsibling->u.internal_key[0].first_page ?
+						split_target : newsibling;
+					index = FreePageBtreeSearchInternal(insert_into, key);
+					FreePageBtreeInsertInternal(base, insert_into, index,
+												key, child);
+					relptr_store(base, child->hdr.parent, insert_into);
+					if (index == 0 && insert_into == split_target)
+						FreePageBtreeAdjustAncestorKeys(fpm, split_target);
+				}
+
+				/* If the page we just split has no parent, split the root. */
+				if (parent == NULL)
+				{
+					FreePageBtree *newroot;
+
+					newroot = FreePageBtreeGetRecycled(fpm);
+					newroot->hdr.magic = FREE_PAGE_INTERNAL_MAGIC;
+					newroot->hdr.nused = 2;
+					relptr_store(base, newroot->hdr.parent,
+								 (FreePageBtree *) NULL);
+					newroot->u.internal_key[0].first_page =
+						FreePageBtreeFirstKey(split_target);
+					relptr_store(base, newroot->u.internal_key[0].child,
+								 split_target);
+					relptr_store(base, split_target->hdr.parent, newroot);
+					newroot->u.internal_key[1].first_page =
+						FreePageBtreeFirstKey(newsibling);
+					relptr_store(base, newroot->u.internal_key[1].child,
+								 newsibling);
+					relptr_store(base, newsibling->hdr.parent, newroot);
+					relptr_store(base, fpm->btree_root, newroot);
+					fpm->btree_depth++;
+
+					break;
+				}
+
+				/* If the parent page isn't full, insert the downlink. */
+				key = newsibling->u.internal_key[0].first_page;
+				if (parent->hdr.nused < FPM_ITEMS_PER_INTERNAL_PAGE)
+				{
+					Size		index;
+
+					index = FreePageBtreeSearchInternal(parent, key);
+					FreePageBtreeInsertInternal(base, parent, index,
+												key, newsibling);
+					relptr_store(base, newsibling->hdr.parent, parent);
+					if (index == 0)
+						FreePageBtreeAdjustAncestorKeys(fpm, parent);
+					break;
+				}
+
+				/* The parent also needs to be split, so loop around. */
+				child = newsibling;
+				split_target = parent;
+			}
+
+			/*
+			 * The loop above did the insert, so just need to update the free
+			 * list, and we're done.
+			 */
+			FreePagePushSpanLeader(fpm, first_page, npages);
+
+			return npages;
+		}
+	}
+
+	/* Physically add the key to the page. */
+	Assert(result.page->hdr.nused < FPM_ITEMS_PER_LEAF_PAGE);
+	FreePageBtreeInsertLeaf(result.page, result.index, first_page, npages);
+
+	/* If new first key on page, ancestors might need adjustment. */
+	if (result.index == 0)
+		FreePageBtreeAdjustAncestorKeys(fpm, result.page);
+
+	/* Put it on the free list. */
+	FreePagePushSpanLeader(fpm, first_page, npages);
+
+	return npages;
+}
+
+/*
+ * Remove a FreePageSpanLeader from the linked-list that contains it, either
+ * because we're changing the size of the span, or because we're allocating it.
+ */
+static void
+FreePagePopSpanLeader(FreePageManager *fpm, Size pageno)
+{
+	char	   *base = fpm_segment_base(fpm);
+	FreePageSpanLeader *span;
+	FreePageSpanLeader *next;
+	FreePageSpanLeader *prev;
+
+	span = (FreePageSpanLeader *) fpm_page_to_pointer(base, pageno);
+
+	next = relptr_access(base, span->next);
+	prev = relptr_access(base, span->prev);
+	if (next != NULL)
+		relptr_copy(next->prev, span->prev);
+	if (prev != NULL)
+		relptr_copy(prev->next, span->next);
+	else
+	{
+		Size		f = Min(span->npages, FPM_NUM_FREELISTS) - 1;
+
+		Assert(fpm->freelist[f].relptr_off == pageno * FPM_PAGE_SIZE);
+		relptr_copy(fpm->freelist[f], span->next);
+	}
+}
+
+/*
+ * Initialize a new FreePageSpanLeader and put it on the appropriate free list.
+ */
+static void
+FreePagePushSpanLeader(FreePageManager *fpm, Size first_page, Size npages)
+{
+	char	   *base = fpm_segment_base(fpm);
+	Size		f = Min(npages, FPM_NUM_FREELISTS) - 1;
+	FreePageSpanLeader *head = relptr_access(base, fpm->freelist[f]);
+	FreePageSpanLeader *span;
+
+	span = (FreePageSpanLeader *) fpm_page_to_pointer(base, first_page);
+	span->magic = FREE_PAGE_SPAN_LEADER_MAGIC;
+	span->npages = npages;
+	relptr_store(base, span->next, head);
+	relptr_store(base, span->prev, (FreePageSpanLeader *) NULL);
+	if (head != NULL)
+		relptr_store(base, head->prev, span);
+	relptr_store(base, fpm->freelist[f], span);
+}

src/include/utils/freepage.h

+/*-------------------------------------------------------------------------
+ *
+ * freepage.h
+ *	  Management of page-organized free memory.
+ *
+ * Portions Copyright (c) 1996-2016, 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 */

src/tools/pgindent/typedefs.list

@@ -726,6 +726,13 @@ Form_pg_user_mapping
 FormatNode
 FreeBlockNumberArray
 FreeListData
+FreePageBtree
+FreePageBtreeHeader
+FreePageBtreeInternalKey
+FreePageBtreeLeafKey
+FreePageBtreeSearchResult
+FreePageManager
+FreePageSpanLeader
 FromCharDateMode
 FromExpr
 FuncCall