Fix integer underflow in shared memory debugging

[pgsql.git] / src / backend / utils / mmgr / dsa.c

/*-------------------------------------------------------------------------
 *
 * dsa.c
 *        Dynamic shared memory areas.
 *
 * This module provides dynamic shared memory areas which are built on top of
 * DSM segments.  While dsm.c allows segments of memory of shared memory to be
 * created and shared between backends, it isn't designed to deal with small
 * objects.  A DSA area is a shared memory heap usually backed by one or more
 * DSM segments which can allocate memory using dsa_allocate() and dsa_free().
 * Alternatively, it can be created in pre-existing shared memory, including a
 * DSM segment, and then create extra DSM segments as required.  Unlike the
 * regular system heap, it deals in pseudo-pointers which must be converted to
 * backend-local pointers before they are dereferenced.  These pseudo-pointers
 * can however be shared with other backends, and can be used to construct
 * shared data structures.
 *
 * Each DSA area manages a set of DSM segments, adding new segments as
 * required and detaching them when they are no longer needed.  Each segment
 * contains a number of 4KB pages, a free page manager for tracking
 * consecutive runs of free pages, and a page map for tracking the source of
 * objects allocated on each page.  Allocation requests above 8KB are handled
 * by choosing a segment and finding consecutive free pages in its free page
 * manager.  Allocation requests for smaller sizes are handled using pools of
 * objects of a selection of sizes.  Each pool consists of a number of 16 page
 * (64KB) superblocks allocated in the same way as large objects.  Allocation
 * of large objects and new superblocks is serialized by a single LWLock, but
 * allocation of small objects from pre-existing superblocks uses one LWLock
 * per pool.  Currently there is one pool, and therefore one lock, per size
 * class.  Per-core pools to increase concurrency and strategies for reducing
 * the resulting fragmentation are areas for future research.  Each superblock
 * is managed with a 'span', which tracks the superblock's freelist.  Free
 * requests are handled by looking in the page map to find which span an
 * address was allocated from, so that small objects can be returned to the
 * appropriate free list, and large object pages can be returned directly to
 * the free page map.  When allocating, simple heuristics for selecting
 * segments and superblocks try to encourage occupied memory to be
 * concentrated, increasing the likelihood that whole superblocks can become
 * empty and be returned to the free page manager, and whole segments can
 * become empty and be returned to the operating system.
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        src/backend/utils/mmgr/dsa.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "port/atomics.h"
#include "port/pg_bitutils.h"
#include "storage/dsm.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "utils/dsa.h"
#include "utils/freepage.h"
#include "utils/memutils.h"
#include "utils/resowner.h"

/*
 * The size of the initial DSM segment that backs a dsa_area created by
 * dsa_create.  After creating some number of segments of this size we'll
 * double this size, and so on.  Larger segments may be created if necessary
 * to satisfy large requests.
 */
#define DSA_INITIAL_SEGMENT_SIZE ((size_t) (1 * 1024 * 1024))

/*
 * How many segments to create before we double the segment size.  If this is
 * low, then there is likely to be a lot of wasted space in the largest
 * segment.  If it is high, then we risk running out of segment slots (see
 * dsm.c's limits on total number of segments), or limiting the total size
 * an area can manage when using small pointers.
 */
#define DSA_NUM_SEGMENTS_AT_EACH_SIZE 2

/*
 * The number of bits used to represent the offset part of a dsa_pointer.
 * This controls the maximum size of a segment, the maximum possible
 * allocation size and also the maximum number of segments per area.
 */
#if SIZEOF_DSA_POINTER == 4
#define DSA_OFFSET_WIDTH 27             /* 32 segments of size up to 128MB */
#else
#define DSA_OFFSET_WIDTH 40             /* 1024 segments of size up to 1TB */
#endif

/*
 * The maximum number of DSM segments that an area can own, determined by
 * the number of bits remaining (but capped at 1024).
 */
#define DSA_MAX_SEGMENTS \
        Min(1024, (1 << ((SIZEOF_DSA_POINTER * 8) - DSA_OFFSET_WIDTH)))

/* The bitmask for extracting the offset from a dsa_pointer. */
#define DSA_OFFSET_BITMASK (((dsa_pointer) 1 << DSA_OFFSET_WIDTH) - 1)

/* The maximum size of a DSM segment. */
#define DSA_MAX_SEGMENT_SIZE ((size_t) 1 << DSA_OFFSET_WIDTH)

/* Number of pages (see FPM_PAGE_SIZE) per regular superblock. */
#define DSA_PAGES_PER_SUPERBLOCK                16

/*
 * A magic number used as a sanity check for following DSM segments belonging
 * to a DSA area (this number will be XORed with the area handle and
 * the segment index).
 */
#define DSA_SEGMENT_HEADER_MAGIC 0x0ce26608

/* Build a dsa_pointer given a segment number and offset. */
#define DSA_MAKE_POINTER(segment_number, offset) \
        (((dsa_pointer) (segment_number) << DSA_OFFSET_WIDTH) | (offset))

/* Extract the segment number from a dsa_pointer. */
#define DSA_EXTRACT_SEGMENT_NUMBER(dp) ((dp) >> DSA_OFFSET_WIDTH)

/* Extract the offset from a dsa_pointer. */
#define DSA_EXTRACT_OFFSET(dp) ((dp) & DSA_OFFSET_BITMASK)

/* The type used for index segment indexes (zero based). */
typedef size_t dsa_segment_index;

/* Sentinel value for dsa_segment_index indicating 'none' or 'end'. */
#define DSA_SEGMENT_INDEX_NONE (~(dsa_segment_index)0)

/*
 * How many bins of segments do we have?  The bins are used to categorize
 * segments by their largest contiguous run of free pages.
 */
#define DSA_NUM_SEGMENT_BINS 16

/*
 * What is the lowest bin that holds segments that *might* have n contiguous
 * free pages?  There is no point in looking in segments in lower bins; they
 * definitely can't service a request for n free pages.
 */
static inline size_t
contiguous_pages_to_segment_bin(size_t n)
{
        size_t          bin;

        if (n == 0)
                bin = 0;
        else
                bin = pg_leftmost_one_pos_size_t(n) + 1;

        return Min(bin, DSA_NUM_SEGMENT_BINS - 1);
}

/* Macros for access to locks. */
#define DSA_AREA_LOCK(area) (&area->control->lock)
#define DSA_SCLASS_LOCK(area, sclass) (&area->control->pools[sclass].lock)

/*
 * The header for an individual segment.  This lives at the start of each DSM
 * segment owned by a DSA area including the first segment (where it appears
 * as part of the dsa_area_control struct).
 */
typedef struct
{
        /* Sanity check magic value. */
        uint32          magic;
        /* Total number of pages in this segment (excluding metadata area). */
        size_t          usable_pages;
        /* Total size of this segment in bytes. */
        size_t          size;

        /*
         * Index of the segment that precedes this one in the same segment bin, or
         * DSA_SEGMENT_INDEX_NONE if this is the first one.
         */
        dsa_segment_index prev;

        /*
         * Index of the segment that follows this one in the same segment bin, or
         * DSA_SEGMENT_INDEX_NONE if this is the last one.
         */
        dsa_segment_index next;
        /* The index of the bin that contains this segment. */
        size_t          bin;

        /*
         * A flag raised to indicate that this segment is being returned to the
         * operating system and has been unpinned.
         */
        bool            freed;
} dsa_segment_header;

/*
 * Metadata for one superblock.
 *
 * For most blocks, span objects are stored out-of-line; that is, the span
 * object is not stored within the block itself.  But, as an exception, for a
 * "span of spans", the span object is stored "inline".  The allocation is
 * always exactly one page, and the dsa_area_span object is located at
 * the beginning of that page.  The size class is DSA_SCLASS_BLOCK_OF_SPANS,
 * and the remaining fields are used just as they would be in an ordinary
 * block.  We can't allocate spans out of ordinary superblocks because
 * creating an ordinary superblock requires us to be able to allocate a span
 * *first*.  Doing it this way avoids that circularity.
 */
typedef struct
{
        dsa_pointer pool;                       /* Containing pool. */
        dsa_pointer prevspan;           /* Previous span. */
        dsa_pointer nextspan;           /* Next span. */
        dsa_pointer start;                      /* Starting address. */
        size_t          npages;                 /* Length of span in pages. */
        uint16          size_class;             /* Size class. */
        uint16          ninitialized;   /* Maximum number of objects ever allocated. */
        uint16          nallocatable;   /* Number of objects currently allocatable. */
        uint16          firstfree;              /* First object on free list. */
        uint16          nmax;                   /* Maximum number of objects ever possible. */
        uint16          fclass;                 /* Current fullness class. */
} dsa_area_span;

/*
 * Given a pointer to an object in a span, access the index of the next free
 * object in the same span (ie in the span's freelist) as an L-value.
 */
#define NextFreeObjectIndex(object) (* (uint16 *) (object))

/*
 * Small allocations are handled by dividing a single block of memory into
 * many small objects of equal size.  The possible allocation sizes are
 * defined by the following array.  Larger size classes are spaced more widely
 * than smaller size classes.  We fudge the spacing for size classes >1kB to
 * avoid space wastage: based on the knowledge that we plan to allocate 64kB
 * blocks, we bump the maximum object size up to the largest multiple of
 * 8 bytes that still lets us fit the same number of objects into one block.
 *
 * NB: Because of this fudging, if we were ever to use differently-sized blocks
 * for small allocations, these size classes would need to be reworked to be
 * optimal for the new size.
 *
 * NB: The optimal spacing for size classes, as well as the size of the blocks
 * out of which small objects are allocated, is not a question that has one
 * right answer.  Some allocators (such as tcmalloc) use more closely-spaced
 * size classes than we do here, while others (like aset.c) use more
 * widely-spaced classes.  Spacing the classes more closely avoids wasting
 * memory within individual chunks, but also means a larger number of
 * potentially-unfilled blocks.
 */
static const uint16 dsa_size_classes[] = {
        sizeof(dsa_area_span), 0,       /* special size classes */
        8, 16, 24, 32, 40, 48, 56, 64,  /* 8 classes separated by 8 bytes */
        80, 96, 112, 128,                       /* 4 classes separated by 16 bytes */
        160, 192, 224, 256,                     /* 4 classes separated by 32 bytes */
        320, 384, 448, 512,                     /* 4 classes separated by 64 bytes */
        640, 768, 896, 1024,            /* 4 classes separated by 128 bytes */
        1280, 1560, 1816, 2048,         /* 4 classes separated by ~256 bytes */
        2616, 3120, 3640, 4096,         /* 4 classes separated by ~512 bytes */
        5456, 6552, 7280, 8192          /* 4 classes separated by ~1024 bytes */
};
#define DSA_NUM_SIZE_CLASSES                            lengthof(dsa_size_classes)

/* Special size classes. */
#define DSA_SCLASS_BLOCK_OF_SPANS               0
#define DSA_SCLASS_SPAN_LARGE                   1

/*
 * The following lookup table is used to map the size of small objects
 * (less than 1kB) onto the corresponding size class.  To use this table,
 * round the size of the object up to the next multiple of 8 bytes, and then
 * index into this array.
 */
static const uint8 dsa_size_class_map[] = {
        2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 11, 11, 12, 12, 13, 13,
        14, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 17, 17,
        18, 18, 18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19, 19,
        20, 20, 20, 20, 20, 20, 20, 20, 21, 21, 21, 21, 21, 21, 21, 21,
        22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
        23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
        24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
        25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25
};
#define DSA_SIZE_CLASS_MAP_QUANTUM      8

/*
 * Superblocks are binned by how full they are.  Generally, each fullness
 * class corresponds to one quartile, but the block being used for
 * allocations is always at the head of the list for fullness class 1,
 * regardless of how full it really is.
 */
#define DSA_FULLNESS_CLASSES            4

/*
 * A dsa_area_pool represents a set of objects of a given size class.
 *
 * Perhaps there should be multiple pools for the same size class for
 * contention avoidance, but for now there is just one!
 */
typedef struct
{
        /* A lock protecting access to this pool. */
        LWLock          lock;
        /* A set of linked lists of spans, arranged by fullness. */
        dsa_pointer spans[DSA_FULLNESS_CLASSES];
        /* Should we pad this out to a cacheline boundary? */
} dsa_area_pool;

/*
 * The control block for an area.  This lives in shared memory, at the start of
 * the first DSM segment controlled by this area.
 */
typedef struct
{
        /* The segment header for the first segment. */
        dsa_segment_header segment_header;
        /* The handle for this area. */
        dsa_handle      handle;
        /* The handles of the segments owned by this area. */
        dsm_handle      segment_handles[DSA_MAX_SEGMENTS];
        /* Lists of segments, binned by maximum contiguous run of free pages. */
        dsa_segment_index segment_bins[DSA_NUM_SEGMENT_BINS];
        /* The object pools for each size class. */
        dsa_area_pool pools[DSA_NUM_SIZE_CLASSES];
        /* The total size of all active segments. */
        size_t          total_segment_size;
        /* The maximum total size of backing storage we are allowed. */
        size_t          max_total_segment_size;
        /* Highest used segment index in the history of this area. */
        dsa_segment_index high_segment_index;
        /* The reference count for this area. */
        int                     refcnt;
        /* A flag indicating that this area has been pinned. */
        bool            pinned;
        /* The number of times that segments have been freed. */
        size_t          freed_segment_counter;
        /* The LWLock tranche ID. */
        int                     lwlock_tranche_id;
        /* The general lock (protects everything except object pools). */
        LWLock          lock;
} dsa_area_control;

/* Given a pointer to a pool, find a dsa_pointer. */
#define DsaAreaPoolToDsaPointer(area, p)        \
        DSA_MAKE_POINTER(0, (char *) p - (char *) area->control)

/*
 * A dsa_segment_map is stored within the backend-private memory of each
 * individual backend.  It holds the base address of the segment within that
 * backend, plus the addresses of key objects within the segment.  Those
 * could instead be derived from the base address but it's handy to have them
 * around.
 */
typedef struct
{
        dsm_segment *segment;           /* DSM segment */
        char       *mapped_address; /* Address at which segment is mapped */
        dsa_segment_header *header; /* Header (same as mapped_address) */
        FreePageManager *fpm;           /* Free page manager within segment. */
        dsa_pointer *pagemap;           /* Page map within segment. */
} dsa_segment_map;

/*
 * Per-backend state for a storage area.  Backends obtain one of these by
 * creating an area or attaching to an existing one using a handle.  Each
 * process that needs to use an area uses its own object to track where the
 * segments are mapped.
 */
struct dsa_area
{
        /* Pointer to the control object in shared memory. */
        dsa_area_control *control;

        /*
         * All the mappings are owned by this.  The dsa_area itself is not
         * directly tracked by the ResourceOwner, but the effect is the same. NULL
         * if the attachment has session lifespan, i.e if dsa_pin_mapping() has
         * been called.
         */
        ResourceOwner resowner;

        /*
         * This backend's array of segment maps, ordered by segment index
         * corresponding to control->segment_handles.  Some of the area's segments
         * may not be mapped in this backend yet, and some slots may have been
         * freed and need to be detached; these operations happen on demand.
         */
        dsa_segment_map segment_maps[DSA_MAX_SEGMENTS];

        /* The highest segment index this backend has ever mapped. */
        dsa_segment_index high_segment_index;

        /* The last observed freed_segment_counter. */
        size_t          freed_segment_counter;
};

#define DSA_SPAN_NOTHING_FREE   ((uint16) -1)
#define DSA_SUPERBLOCK_SIZE (DSA_PAGES_PER_SUPERBLOCK * FPM_PAGE_SIZE)

/* Given a pointer to a segment_map, obtain a segment index number. */
#define get_segment_index(area, segment_map_ptr) \
        (segment_map_ptr - &area->segment_maps[0])

static void init_span(dsa_area *area, dsa_pointer span_pointer,
                                          dsa_area_pool *pool, dsa_pointer start, size_t npages,
                                          uint16 size_class);
static bool transfer_first_span(dsa_area *area, dsa_area_pool *pool,
                                                                int fromclass, int toclass);
static inline dsa_pointer alloc_object(dsa_area *area, int size_class);
static bool ensure_active_superblock(dsa_area *area, dsa_area_pool *pool,
                                                                         int size_class);
static dsa_segment_map *get_segment_by_index(dsa_area *area,
                                                                                         dsa_segment_index index);
static void destroy_superblock(dsa_area *area, dsa_pointer span_pointer);
static void unlink_span(dsa_area *area, dsa_area_span *span);
static void add_span_to_fullness_class(dsa_area *area, dsa_area_span *span,
                                                                           dsa_pointer span_pointer, int fclass);
static void unlink_segment(dsa_area *area, dsa_segment_map *segment_map);
static dsa_segment_map *get_best_segment(dsa_area *area, size_t npages);
static dsa_segment_map *make_new_segment(dsa_area *area, size_t requested_pages);
static dsa_area *create_internal(void *place, size_t size,
                                                                 int tranche_id,
                                                                 dsm_handle control_handle,
                                                                 dsm_segment *control_segment);
static dsa_area *attach_internal(void *place, dsm_segment *segment,
                                                                 dsa_handle handle);
static void check_for_freed_segments(dsa_area *area);
static void check_for_freed_segments_locked(dsa_area *area);
static void rebin_segment(dsa_area *area, dsa_segment_map *segment_map);

/*
 * Create a new shared area in a new DSM segment.  Further DSM segments will
 * be allocated as required to extend the available space.
 *
 * We can't allocate a LWLock tranche_id within this function, because tranche
 * IDs are a scarce resource; there are only 64k available, using low numbers
 * when possible matters, and we have no provision for recycling them.  So,
 * we require the caller to provide one.
 */
dsa_area *
dsa_create(int tranche_id)
{
        dsm_segment *segment;
        dsa_area   *area;

        /*
         * Create the DSM segment that will hold the shared control object and the
         * first segment of usable space.
         */
        segment = dsm_create(DSA_INITIAL_SEGMENT_SIZE, 0);

        /*
         * All segments backing this area are pinned, so that DSA can explicitly
         * control their lifetime (otherwise a newly created segment belonging to
         * this area might be freed when the only backend that happens to have it
         * mapped in ends, corrupting the area).
         */
        dsm_pin_segment(segment);

        /* Create a new DSA area with the control object in this segment. */
        area = create_internal(dsm_segment_address(segment),
                                                   DSA_INITIAL_SEGMENT_SIZE,
                                                   tranche_id,
                                                   dsm_segment_handle(segment), segment);

        /* Clean up when the control segment detaches. */
        on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                                  PointerGetDatum(dsm_segment_address(segment)));

        return area;
}

/*
 * Create a new shared area in an existing shared memory space, which may be
 * either DSM or Postmaster-initialized memory.  DSM segments will be
 * allocated as required to extend the available space, though that can be
 * prevented with dsa_set_size_limit(area, size) using the same size provided
 * to dsa_create_in_place.
 *
 * Areas created in-place must eventually be released by the backend that
 * created them and all backends that attach to them.  This can be done
 * explicitly with dsa_release_in_place, or, in the special case that 'place'
 * happens to be in a pre-existing DSM segment, by passing in a pointer to the
 * segment so that a detach hook can be registered with the containing DSM
 * segment.
 *
 * See dsa_create() for a note about the tranche arguments.
 */
dsa_area *
dsa_create_in_place(void *place, size_t size,
                                        int tranche_id, dsm_segment *segment)
{
        dsa_area   *area;

        area = create_internal(place, size, tranche_id,
                                                   DSM_HANDLE_INVALID, NULL);

        /*
         * Clean up when the control segment detaches, if a containing DSM segment
         * was provided.
         */
        if (segment != NULL)
                on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                                          PointerGetDatum(place));

        return area;
}

/*
 * Obtain a handle that can be passed to other processes so that they can
 * attach to the given area.  Cannot be called for areas created with
 * dsa_create_in_place.
 */
dsa_handle
dsa_get_handle(dsa_area *area)
{
        Assert(area->control->handle != DSA_HANDLE_INVALID);
        return area->control->handle;
}

/*
 * Attach to an area given a handle generated (possibly in another process) by
 * dsa_get_handle.  The area must have been created with dsa_create (not
 * dsa_create_in_place).
 */
dsa_area *
dsa_attach(dsa_handle handle)
{
        dsm_segment *segment;
        dsa_area   *area;

        /*
         * An area handle is really a DSM segment handle for the first segment, so
         * we go ahead and attach to that.
         */
        segment = dsm_attach(handle);
        if (segment == NULL)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("could not attach to dynamic shared area")));

        area = attach_internal(dsm_segment_address(segment), segment, handle);

        /* Clean up when the control segment detaches. */
        on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                                  PointerGetDatum(dsm_segment_address(segment)));

        return area;
}

/*
 * Attach to an area that was created with dsa_create_in_place.  The caller
 * must somehow know the location in memory that was used when the area was
 * created, though it may be mapped at a different virtual address in this
 * process.
 *
 * See dsa_create_in_place for note about releasing in-place areas, and the
 * optional 'segment' argument which can be provided to allow automatic
 * release if the containing memory happens to be a DSM segment.
 */
dsa_area *
dsa_attach_in_place(void *place, dsm_segment *segment)
{
        dsa_area   *area;

        area = attach_internal(place, NULL, DSA_HANDLE_INVALID);

        /*
         * Clean up when the control segment detaches, if a containing DSM segment
         * was provided.
         */
        if (segment != NULL)
                on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                                          PointerGetDatum(place));

        return area;
}

/*
 * Release a DSA area that was produced by dsa_create_in_place or
 * dsa_attach_in_place.  The 'segment' argument is ignored but provides an
 * interface suitable for on_dsm_detach, for the convenience of users who want
 * to create a DSA segment inside an existing DSM segment and have it
 * automatically released when the containing DSM segment is detached.
 * 'place' should be the address of the place where the area was created.
 *
 * This callback is automatically registered for the DSM segment containing
 * the control object of in-place areas when a segment is provided to
 * dsa_create_in_place or dsa_attach_in_place, and also for all areas created
 * with dsa_create.
 */
void
dsa_on_dsm_detach_release_in_place(dsm_segment *segment, Datum place)
{
        dsa_release_in_place(DatumGetPointer(place));
}

/*
 * Release a DSA area that was produced by dsa_create_in_place or
 * dsa_attach_in_place.  The 'code' argument is ignored but provides an
 * interface suitable for on_shmem_exit or before_shmem_exit, for the
 * convenience of users who want to create a DSA segment inside shared memory
 * other than a DSM segment and have it automatically release at backend exit.
 * 'place' should be the address of the place where the area was created.
 */
void
dsa_on_shmem_exit_release_in_place(int code, Datum place)
{
        dsa_release_in_place(DatumGetPointer(place));
}

/*
 * Release a DSA area that was produced by dsa_create_in_place or
 * dsa_attach_in_place.  It is preferable to use one of the 'dsa_on_XXX'
 * callbacks so that this is managed automatically, because failure to release
 * an area created in-place leaks its segments permanently.
 *
 * This is also called automatically for areas produced by dsa_create or
 * dsa_attach as an implementation detail.
 */
void
dsa_release_in_place(void *place)
{
        dsa_area_control *control = (dsa_area_control *) place;
        int                     i;

        LWLockAcquire(&control->lock, LW_EXCLUSIVE);
        Assert(control->segment_header.magic ==
                   (DSA_SEGMENT_HEADER_MAGIC ^ control->handle ^ 0));
        Assert(control->refcnt > 0);
        if (--control->refcnt == 0)
        {
                for (i = 0; i <= control->high_segment_index; ++i)
                {
                        dsm_handle      handle;

                        handle = control->segment_handles[i];
                        if (handle != DSM_HANDLE_INVALID)
                                dsm_unpin_segment(handle);
                }
        }
        LWLockRelease(&control->lock);
}

/*
 * Keep a DSA area attached until end of session or explicit detach.
 *
 * By default, areas are owned by the current resource owner, which means they
 * are detached automatically when that scope ends.
 */
void
dsa_pin_mapping(dsa_area *area)
{
        int                     i;

        if (area->resowner != NULL)
        {
                area->resowner = NULL;

                for (i = 0; i <= area->high_segment_index; ++i)
                        if (area->segment_maps[i].segment != NULL)
                                dsm_pin_mapping(area->segment_maps[i].segment);
        }
}

/*
 * Allocate memory in this storage area.  The return value is a dsa_pointer
 * that can be passed to other processes, and converted to a local pointer
 * with dsa_get_address.  'flags' is a bitmap which should be constructed
 * from the following values:
 *
 * DSA_ALLOC_HUGE allows allocations >= 1GB.  Otherwise, such allocations
 * will result in an ERROR.
 *
 * DSA_ALLOC_NO_OOM causes this function to return InvalidDsaPointer when
 * no memory is available or a size limit established by dsa_set_size_limit
 * would be exceeded.  Otherwise, such allocations will result in an ERROR.
 *
 * DSA_ALLOC_ZERO causes the allocated memory to be zeroed.  Otherwise, the
 * contents of newly-allocated memory are indeterminate.
 *
 * These flags correspond to similarly named flags used by
 * MemoryContextAllocExtended().  See also the macros dsa_allocate and
 * dsa_allocate0 which expand to a call to this function with commonly used
 * flags.
 */
dsa_pointer
dsa_allocate_extended(dsa_area *area, size_t size, int flags)
{
        uint16          size_class;
        dsa_pointer start_pointer;
        dsa_segment_map *segment_map;
        dsa_pointer result;

        Assert(size > 0);

        /* Sanity check on huge individual allocation size. */
        if (((flags & DSA_ALLOC_HUGE) != 0 && !AllocHugeSizeIsValid(size)) ||
                ((flags & DSA_ALLOC_HUGE) == 0 && !AllocSizeIsValid(size)))
                elog(ERROR, "invalid DSA memory alloc request size %zu", size);

        /*
         * If bigger than the largest size class, just grab a run of pages from
         * the free page manager, instead of allocating an object from a pool.
         * There will still be a span, but it's a special class of span that
         * manages this whole allocation and simply gives all pages back to the
         * free page manager when dsa_free is called.
         */
        if (size > dsa_size_classes[lengthof(dsa_size_classes) - 1])
        {
                size_t          npages = fpm_size_to_pages(size);
                size_t          first_page;
                dsa_pointer span_pointer;
                dsa_area_pool *pool = &area->control->pools[DSA_SCLASS_SPAN_LARGE];

                /* Obtain a span object. */
                span_pointer = alloc_object(area, DSA_SCLASS_BLOCK_OF_SPANS);
                if (!DsaPointerIsValid(span_pointer))
                {
                        /* Raise error unless asked not to. */
                        if ((flags & DSA_ALLOC_NO_OOM) == 0)
                                ereport(ERROR,
                                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                                 errmsg("out of memory"),
                                                 errdetail("Failed on DSA request of size %zu.",
                                                                   size)));
                        return InvalidDsaPointer;
                }

                LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);

                /* Find a segment from which to allocate. */
                segment_map = get_best_segment(area, npages);
                if (segment_map == NULL)
                        segment_map = make_new_segment(area, npages);
                if (segment_map == NULL)
                {
                        /* Can't make any more segments: game over. */
                        LWLockRelease(DSA_AREA_LOCK(area));
                        dsa_free(area, span_pointer);

                        /* Raise error unless asked not to. */
                        if ((flags & DSA_ALLOC_NO_OOM) == 0)
                                ereport(ERROR,
                                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                                 errmsg("out of memory"),
                                                 errdetail("Failed on DSA request of size %zu.",
                                                                   size)));
                        return InvalidDsaPointer;
                }

                /*
                 * Ask the free page manager for a run of pages.  This should always
                 * succeed, since both get_best_segment and make_new_segment should
                 * only return a non-NULL pointer if it actually contains enough
                 * contiguous freespace.  If it does fail, something in our backend
                 * private state is out of whack, so use FATAL to kill the process.
                 */
                if (!FreePageManagerGet(segment_map->fpm, npages, &first_page))
                        elog(FATAL,
                                 "dsa_allocate could not find %zu free pages", npages);
                LWLockRelease(DSA_AREA_LOCK(area));

                start_pointer = DSA_MAKE_POINTER(get_segment_index(area, segment_map),
                                                                                 first_page * FPM_PAGE_SIZE);

                /* Initialize span and pagemap. */
                LWLockAcquire(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE),
                                          LW_EXCLUSIVE);
                init_span(area, span_pointer, pool, start_pointer, npages,
                                  DSA_SCLASS_SPAN_LARGE);
                segment_map->pagemap[first_page] = span_pointer;
                LWLockRelease(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE));

                /* Zero-initialize the memory if requested. */
                if ((flags & DSA_ALLOC_ZERO) != 0)
                        memset(dsa_get_address(area, start_pointer), 0, size);

                return start_pointer;
        }

        /* Map allocation to a size class. */
        if (size < lengthof(dsa_size_class_map) * DSA_SIZE_CLASS_MAP_QUANTUM)
        {
                int                     mapidx;

                /* For smaller sizes we have a lookup table... */
                mapidx = ((size + DSA_SIZE_CLASS_MAP_QUANTUM - 1) /
                                  DSA_SIZE_CLASS_MAP_QUANTUM) - 1;
                size_class = dsa_size_class_map[mapidx];
        }
        else
        {
                uint16          min;
                uint16          max;

                /* ... and for the rest we search by binary chop. */
                min = dsa_size_class_map[lengthof(dsa_size_class_map) - 1];
                max = lengthof(dsa_size_classes) - 1;

                while (min < max)
                {
                        uint16          mid = (min + max) / 2;
                        uint16          class_size = dsa_size_classes[mid];

                        if (class_size < size)
                                min = mid + 1;
                        else
                                max = mid;
                }

                size_class = min;
        }
        Assert(size <= dsa_size_classes[size_class]);
        Assert(size_class == 0 || size > dsa_size_classes[size_class - 1]);

        /* Attempt to allocate an object from the appropriate pool. */
        result = alloc_object(area, size_class);

        /* Check for failure to allocate. */
        if (!DsaPointerIsValid(result))
        {
                /* Raise error unless asked not to. */
                if ((flags & DSA_ALLOC_NO_OOM) == 0)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OUT_OF_MEMORY),
                                         errmsg("out of memory"),
                                         errdetail("Failed on DSA request of size %zu.", size)));
                return InvalidDsaPointer;
        }

        /* Zero-initialize the memory if requested. */
        if ((flags & DSA_ALLOC_ZERO) != 0)
                memset(dsa_get_address(area, result), 0, size);

        return result;
}

/*
 * Free memory obtained with dsa_allocate.
 */
void
dsa_free(dsa_area *area, dsa_pointer dp)
{
        dsa_segment_map *segment_map;
        int                     pageno;
        dsa_pointer span_pointer;
        dsa_area_span *span;
        char       *superblock;
        char       *object;
        size_t          size;
        int                     size_class;

        /* Make sure we don't have a stale segment in the slot 'dp' refers to. */
        check_for_freed_segments(area);

        /* Locate the object, span and pool. */
        segment_map = get_segment_by_index(area, DSA_EXTRACT_SEGMENT_NUMBER(dp));
        pageno = DSA_EXTRACT_OFFSET(dp) / FPM_PAGE_SIZE;
        span_pointer = segment_map->pagemap[pageno];
        span = dsa_get_address(area, span_pointer);
        superblock = dsa_get_address(area, span->start);
        object = dsa_get_address(area, dp);
        size_class = span->size_class;
        size = dsa_size_classes[size_class];

        /*
         * Special case for large objects that live in a special span: we return
         * those pages directly to the free page manager and free the span.
         */
        if (span->size_class == DSA_SCLASS_SPAN_LARGE)
        {

#ifdef CLOBBER_FREED_MEMORY
                memset(object, 0x7f, span->npages * FPM_PAGE_SIZE);
#endif

                /* Give pages back to free page manager. */
                LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
                FreePageManagerPut(segment_map->fpm,
                                                   DSA_EXTRACT_OFFSET(span->start) / FPM_PAGE_SIZE,
                                                   span->npages);

                /* Move segment to appropriate bin if necessary. */
                rebin_segment(area, segment_map);
                LWLockRelease(DSA_AREA_LOCK(area));

                /* Unlink span. */
                LWLockAcquire(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE),
                                          LW_EXCLUSIVE);
                unlink_span(area, span);
                LWLockRelease(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE));
                /* Free the span object so it can be reused. */
                dsa_free(area, span_pointer);
                return;
        }

#ifdef CLOBBER_FREED_MEMORY
        memset(object, 0x7f, size);
#endif

        LWLockAcquire(DSA_SCLASS_LOCK(area, size_class), LW_EXCLUSIVE);

        /* Put the object on the span's freelist. */
        Assert(object >= superblock);
        Assert(object < superblock + DSA_SUPERBLOCK_SIZE);
        Assert((object - superblock) % size == 0);
        NextFreeObjectIndex(object) = span->firstfree;
        span->firstfree = (object - superblock) / size;
        ++span->nallocatable;

        /*
         * See if the span needs to moved to a different fullness class, or be
         * freed so its pages can be given back to the segment.
         */
        if (span->nallocatable == 1 && span->fclass == DSA_FULLNESS_CLASSES - 1)
        {
                /*
                 * The block was completely full and is located in the
                 * highest-numbered fullness class, which is never scanned for free
                 * chunks.  We must move it to the next-lower fullness class.
                 */
                unlink_span(area, span);
                add_span_to_fullness_class(area, span, span_pointer,
                                                                   DSA_FULLNESS_CLASSES - 2);

                /*
                 * If this is the only span, and there is no active span, then we
                 * should probably move this span to fullness class 1.  (Otherwise if
                 * you allocate exactly all the objects in the only span, it moves to
                 * class 3, then you free them all, it moves to 2, and then is given
                 * back, leaving no active span).
                 */
        }
        else if (span->nallocatable == span->nmax &&
                         (span->fclass != 1 || span->prevspan != InvalidDsaPointer))
        {
                /*
                 * This entire block is free, and it's not the active block for this
                 * size class.  Return the memory to the free page manager. We don't
                 * do this for the active block to prevent hysteresis: if we
                 * repeatedly allocate and free the only chunk in the active block, it
                 * will be very inefficient if we deallocate and reallocate the block
                 * every time.
                 */
                destroy_superblock(area, span_pointer);
        }

        LWLockRelease(DSA_SCLASS_LOCK(area, size_class));
}

/*
 * Obtain a backend-local address for a dsa_pointer.  'dp' must point to
 * memory allocated by the given area (possibly in another process) that
 * hasn't yet been freed.  This may cause a segment to be mapped into the
 * current process if required, and may cause freed segments to be unmapped.
 */
void *
dsa_get_address(dsa_area *area, dsa_pointer dp)
{
        dsa_segment_index index;
        size_t          offset;

        /* Convert InvalidDsaPointer to NULL. */
        if (!DsaPointerIsValid(dp))
                return NULL;

        /* Process any requests to detach from freed segments. */
        check_for_freed_segments(area);

        /* Break the dsa_pointer into its components. */
        index = DSA_EXTRACT_SEGMENT_NUMBER(dp);
        offset = DSA_EXTRACT_OFFSET(dp);
        Assert(index < DSA_MAX_SEGMENTS);

        /* Check if we need to cause this segment to be mapped in. */
        if (unlikely(area->segment_maps[index].mapped_address == NULL))
        {
                /* Call for effect (we don't need the result). */
                get_segment_by_index(area, index);
        }

        return area->segment_maps[index].mapped_address + offset;
}

/*
 * Pin this area, so that it will continue to exist even if all backends
 * detach from it.  In that case, the area can still be reattached to if a
 * handle has been recorded somewhere.
 */
void
dsa_pin(dsa_area *area)
{
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        if (area->control->pinned)
        {
                LWLockRelease(DSA_AREA_LOCK(area));
                elog(ERROR, "dsa_area already pinned");
        }
        area->control->pinned = true;
        ++area->control->refcnt;
        LWLockRelease(DSA_AREA_LOCK(area));
}

/*
 * Undo the effects of dsa_pin, so that the given area can be freed when no
 * backends are attached to it.  May be called only if dsa_pin has been
 * called.
 */
void
dsa_unpin(dsa_area *area)
{
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        Assert(area->control->refcnt > 1);
        if (!area->control->pinned)
        {
                LWLockRelease(DSA_AREA_LOCK(area));
                elog(ERROR, "dsa_area not pinned");
        }
        area->control->pinned = false;
        --area->control->refcnt;
        LWLockRelease(DSA_AREA_LOCK(area));
}

/*
 * Set the total size limit for this area.  This limit is checked whenever new
 * segments need to be allocated from the operating system.  If the new size
 * limit is already exceeded, this has no immediate effect.
 *
 * Note that the total virtual memory usage may be temporarily larger than
 * this limit when segments have been freed, but not yet detached by all
 * backends that have attached to them.
 */
void
dsa_set_size_limit(dsa_area *area, size_t limit)
{
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        area->control->max_total_segment_size = limit;
        LWLockRelease(DSA_AREA_LOCK(area));
}

/*
 * Aggressively free all spare memory in the hope of returning DSM segments to
 * the operating system.
 */
void
dsa_trim(dsa_area *area)
{
        int                     size_class;

        /*
         * Trim in reverse pool order so we get to the spans-of-spans last, just
         * in case any become entirely free while processing all the other pools.
         */
        for (size_class = DSA_NUM_SIZE_CLASSES - 1; size_class >= 0; --size_class)
        {
                dsa_area_pool *pool = &area->control->pools[size_class];
                dsa_pointer span_pointer;

                if (size_class == DSA_SCLASS_SPAN_LARGE)
                {
                        /* Large object frees give back segments aggressively already. */
                        continue;
                }

                /*
                 * Search fullness class 1 only.  That is where we expect to find an
                 * entirely empty superblock (entirely empty superblocks in other
                 * fullness classes are returned to the free page map by dsa_free).
                 */
                LWLockAcquire(DSA_SCLASS_LOCK(area, size_class), LW_EXCLUSIVE);
                span_pointer = pool->spans[1];
                while (DsaPointerIsValid(span_pointer))
                {
                        dsa_area_span *span = dsa_get_address(area, span_pointer);
                        dsa_pointer next = span->nextspan;

                        if (span->nallocatable == span->nmax)
                                destroy_superblock(area, span_pointer);

                        span_pointer = next;
                }
                LWLockRelease(DSA_SCLASS_LOCK(area, size_class));
        }
}

/*
 * Print out debugging information about the internal state of the shared
 * memory area.
 */
void
dsa_dump(dsa_area *area)
{
        size_t          i,
                                j;

        /*
         * Note: This gives an inconsistent snapshot as it acquires and releases
         * individual locks as it goes...
         */

        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        check_for_freed_segments_locked(area);
        fprintf(stderr, "dsa_area handle %x:\n", area->control->handle);
        fprintf(stderr, "  max_total_segment_size: %zu\n",
                        area->control->max_total_segment_size);
        fprintf(stderr, "  total_segment_size: %zu\n",
                        area->control->total_segment_size);
        fprintf(stderr, "  refcnt: %d\n", area->control->refcnt);
        fprintf(stderr, "  pinned: %c\n", area->control->pinned ? 't' : 'f');
        fprintf(stderr, "  segment bins:\n");
        for (i = 0; i < DSA_NUM_SEGMENT_BINS; ++i)
        {
                if (area->control->segment_bins[i] != DSA_SEGMENT_INDEX_NONE)
                {
                        dsa_segment_index segment_index;

                        if (i == 0)
                                fprintf(stderr,
                                                "    segment bin %zu (no contiguous free pages):\n", i);
                        else
                                fprintf(stderr,
                                                "    segment bin %zu (at least %d contiguous pages free):\n",
                                                i, 1 << (i - 1));
                        segment_index = area->control->segment_bins[i];
                        while (segment_index != DSA_SEGMENT_INDEX_NONE)
                        {
                                dsa_segment_map *segment_map;

                                segment_map =
                                        get_segment_by_index(area, segment_index);

                                fprintf(stderr,
                                                "      segment index %zu, usable_pages = %zu, "
                                                "contiguous_pages = %zu, mapped at %p\n",
                                                segment_index,
                                                segment_map->header->usable_pages,
                                                fpm_largest(segment_map->fpm),
                                                segment_map->mapped_address);
                                segment_index = segment_map->header->next;
                        }
                }
        }
        LWLockRelease(DSA_AREA_LOCK(area));

        fprintf(stderr, "  pools:\n");
        for (i = 0; i < DSA_NUM_SIZE_CLASSES; ++i)
        {
                bool            found = false;

                LWLockAcquire(DSA_SCLASS_LOCK(area, i), LW_EXCLUSIVE);
                for (j = 0; j < DSA_FULLNESS_CLASSES; ++j)
                        if (DsaPointerIsValid(area->control->pools[i].spans[j]))
                                found = true;
                if (found)
                {
                        if (i == DSA_SCLASS_BLOCK_OF_SPANS)
                                fprintf(stderr, "    pool for blocks of span objects:\n");
                        else if (i == DSA_SCLASS_SPAN_LARGE)
                                fprintf(stderr, "    pool for large object spans:\n");
                        else
                                fprintf(stderr,
                                                "    pool for size class %zu (object size %hu bytes):\n",
                                                i, dsa_size_classes[i]);
                        for (j = 0; j < DSA_FULLNESS_CLASSES; ++j)
                        {
                                if (!DsaPointerIsValid(area->control->pools[i].spans[j]))
                                        fprintf(stderr, "      fullness class %zu is empty\n", j);
                                else
                                {
                                        dsa_pointer span_pointer = area->control->pools[i].spans[j];

                                        fprintf(stderr, "      fullness class %zu:\n", j);
                                        while (DsaPointerIsValid(span_pointer))
                                        {
                                                dsa_area_span *span;

                                                span = dsa_get_address(area, span_pointer);
                                                fprintf(stderr,
                                                                "        span descriptor at "
                                                                DSA_POINTER_FORMAT ", superblock at "
                                                                DSA_POINTER_FORMAT
                                                                ", pages = %zu, objects free = %hu/%hu\n",
                                                                span_pointer, span->start, span->npages,
                                                                span->nallocatable, span->nmax);
                                                span_pointer = span->nextspan;
                                        }
                                }
                        }
                }
                LWLockRelease(DSA_SCLASS_LOCK(area, i));
        }
}

/*
 * Return the smallest size that you can successfully provide to
 * dsa_create_in_place.
 */
size_t
dsa_minimum_size(void)
{
        size_t          size;
        int                     pages = 0;

        size = MAXALIGN(sizeof(dsa_area_control)) +
                MAXALIGN(sizeof(FreePageManager));

        /* Figure out how many pages we need, including the page map... */
        while (((size + FPM_PAGE_SIZE - 1) / FPM_PAGE_SIZE) > pages)
        {
                ++pages;
                size += sizeof(dsa_pointer);
        }

        return pages * FPM_PAGE_SIZE;
}

/*
 * Workhorse function for dsa_create and dsa_create_in_place.
 */
static dsa_area *
create_internal(void *place, size_t size,
                                int tranche_id,
                                dsm_handle control_handle,
                                dsm_segment *control_segment)
{
        dsa_area_control *control;
        dsa_area   *area;
        dsa_segment_map *segment_map;
        size_t          usable_pages;
        size_t          total_pages;
        size_t          metadata_bytes;
        int                     i;

        /* Sanity check on the space we have to work in. */
        if (size < dsa_minimum_size())
                elog(ERROR, "dsa_area space must be at least %zu, but %zu provided",
                         dsa_minimum_size(), size);

        /* Now figure out how much space is usable */
        total_pages = size / FPM_PAGE_SIZE;
        metadata_bytes =
                MAXALIGN(sizeof(dsa_area_control)) +
                MAXALIGN(sizeof(FreePageManager)) +
                total_pages * sizeof(dsa_pointer);
        /* Add padding up to next page boundary. */
        if (metadata_bytes % FPM_PAGE_SIZE != 0)
                metadata_bytes += FPM_PAGE_SIZE - (metadata_bytes % FPM_PAGE_SIZE);
        Assert(metadata_bytes <= size);
        usable_pages = (size - metadata_bytes) / FPM_PAGE_SIZE;

        /*
         * Initialize the dsa_area_control object located at the start of the
         * space.
         */
        control = (dsa_area_control *) place;
        memset(place, 0, sizeof(*control));
        control->segment_header.magic =
                DSA_SEGMENT_HEADER_MAGIC ^ control_handle ^ 0;
        control->segment_header.next = DSA_SEGMENT_INDEX_NONE;
        control->segment_header.prev = DSA_SEGMENT_INDEX_NONE;
        control->segment_header.usable_pages = usable_pages;
        control->segment_header.freed = false;
        control->segment_header.size = DSA_INITIAL_SEGMENT_SIZE;
        control->handle = control_handle;
        control->max_total_segment_size = (size_t) -1;
        control->total_segment_size = size;
        control->segment_handles[0] = control_handle;
        for (i = 0; i < DSA_NUM_SEGMENT_BINS; ++i)
                control->segment_bins[i] = DSA_SEGMENT_INDEX_NONE;
        control->refcnt = 1;
        control->lwlock_tranche_id = tranche_id;

        /*
         * Create the dsa_area object that this backend will use to access the
         * area.  Other backends will need to obtain their own dsa_area object by
         * attaching.
         */
        area = palloc(sizeof(dsa_area));
        area->control = control;
        area->resowner = CurrentResourceOwner;
        memset(area->segment_maps, 0, sizeof(dsa_segment_map) * DSA_MAX_SEGMENTS);
        area->high_segment_index = 0;
        area->freed_segment_counter = 0;
        LWLockInitialize(&control->lock, control->lwlock_tranche_id);
        for (i = 0; i < DSA_NUM_SIZE_CLASSES; ++i)
                LWLockInitialize(DSA_SCLASS_LOCK(area, i),
                                                 control->lwlock_tranche_id);

        /* Set up the segment map for this process's mapping. */
        segment_map = &area->segment_maps[0];
        segment_map->segment = control_segment;
        segment_map->mapped_address = place;
        segment_map->header = (dsa_segment_header *) place;
        segment_map->fpm = (FreePageManager *)
                (segment_map->mapped_address +
                 MAXALIGN(sizeof(dsa_area_control)));
        segment_map->pagemap = (dsa_pointer *)
                (segment_map->mapped_address +
                 MAXALIGN(sizeof(dsa_area_control)) +
                 MAXALIGN(sizeof(FreePageManager)));

        /* Set up the free page map. */
        FreePageManagerInitialize(segment_map->fpm, segment_map->mapped_address);
        /* There can be 0 usable pages if size is dsa_minimum_size(). */

        if (usable_pages > 0)
                FreePageManagerPut(segment_map->fpm, metadata_bytes / FPM_PAGE_SIZE,
                                                   usable_pages);

        /* Put this segment into the appropriate bin. */
        control->segment_bins[contiguous_pages_to_segment_bin(usable_pages)] = 0;
        segment_map->header->bin = contiguous_pages_to_segment_bin(usable_pages);

        return area;
}

/*
 * Workhorse function for dsa_attach and dsa_attach_in_place.
 */
static dsa_area *
attach_internal(void *place, dsm_segment *segment, dsa_handle handle)
{
        dsa_area_control *control;
        dsa_area   *area;
        dsa_segment_map *segment_map;

        control = (dsa_area_control *) place;
        Assert(control->handle == handle);
        Assert(control->segment_handles[0] == handle);
        Assert(control->segment_header.magic ==
                   (DSA_SEGMENT_HEADER_MAGIC ^ handle ^ 0));

        /* Build the backend-local area object. */
        area = palloc(sizeof(dsa_area));
        area->control = control;
        area->resowner = CurrentResourceOwner;
        memset(&area->segment_maps[0], 0,
                   sizeof(dsa_segment_map) * DSA_MAX_SEGMENTS);
        area->high_segment_index = 0;

        /* Set up the segment map for this process's mapping. */
        segment_map = &area->segment_maps[0];
        segment_map->segment = segment; /* NULL for in-place */
        segment_map->mapped_address = place;
        segment_map->header = (dsa_segment_header *) segment_map->mapped_address;
        segment_map->fpm = (FreePageManager *)
                (segment_map->mapped_address + MAXALIGN(sizeof(dsa_area_control)));
        segment_map->pagemap = (dsa_pointer *)
                (segment_map->mapped_address + MAXALIGN(sizeof(dsa_area_control)) +
                 MAXALIGN(sizeof(FreePageManager)));

        /* Bump the reference count. */
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        if (control->refcnt == 0)
        {
                /* We can't attach to a DSA area that has already been destroyed. */
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("could not attach to dynamic shared area")));
        }
        ++control->refcnt;
        area->freed_segment_counter = area->control->freed_segment_counter;
        LWLockRelease(DSA_AREA_LOCK(area));

        return area;
}

/*
 * Add a new span to fullness class 1 of the indicated pool.
 */
static void
init_span(dsa_area *area,
                  dsa_pointer span_pointer,
                  dsa_area_pool *pool, dsa_pointer start, size_t npages,
                  uint16 size_class)
{
        dsa_area_span *span = dsa_get_address(area, span_pointer);
        size_t          obsize = dsa_size_classes[size_class];

        /*
         * The per-pool lock must be held because we manipulate the span list for
         * this pool.
         */
        Assert(LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));

        /* Push this span onto the front of the span list for fullness class 1. */
        if (DsaPointerIsValid(pool->spans[1]))
        {
                dsa_area_span *head = (dsa_area_span *)
                        dsa_get_address(area, pool->spans[1]);

                head->prevspan = span_pointer;
        }
        span->pool = DsaAreaPoolToDsaPointer(area, pool);
        span->nextspan = pool->spans[1];
        span->prevspan = InvalidDsaPointer;
        pool->spans[1] = span_pointer;

        span->start = start;
        span->npages = npages;
        span->size_class = size_class;
        span->ninitialized = 0;
        if (size_class == DSA_SCLASS_BLOCK_OF_SPANS)
        {
                /*
                 * A block-of-spans contains its own descriptor, so mark one object as
                 * initialized and reduce the count of allocatable objects by one.
                 * Doing this here has the side effect of also reducing nmax by one,
                 * which is important to make sure we free this object at the correct
                 * time.
                 */
                span->ninitialized = 1;
                span->nallocatable = FPM_PAGE_SIZE / obsize - 1;
        }
        else if (size_class != DSA_SCLASS_SPAN_LARGE)
                span->nallocatable = DSA_SUPERBLOCK_SIZE / obsize;
        span->firstfree = DSA_SPAN_NOTHING_FREE;
        span->nmax = span->nallocatable;
        span->fclass = 1;
}

/*
 * Transfer the first span in one fullness class to the head of another
 * fullness class.
 */
static bool
transfer_first_span(dsa_area *area,
                                        dsa_area_pool *pool, int fromclass, int toclass)
{
        dsa_pointer span_pointer;
        dsa_area_span *span;
        dsa_area_span *nextspan;

        /* Can't do it if source list is empty. */
        span_pointer = pool->spans[fromclass];
        if (!DsaPointerIsValid(span_pointer))
                return false;

        /* Remove span from head of source list. */
        span = dsa_get_address(area, span_pointer);
        pool->spans[fromclass] = span->nextspan;
        if (DsaPointerIsValid(span->nextspan))
        {
                nextspan = (dsa_area_span *)
                        dsa_get_address(area, span->nextspan);
                nextspan->prevspan = InvalidDsaPointer;
        }

        /* Add span to head of target list. */
        span->nextspan = pool->spans[toclass];
        pool->spans[toclass] = span_pointer;
        if (DsaPointerIsValid(span->nextspan))
        {
                nextspan = (dsa_area_span *)
                        dsa_get_address(area, span->nextspan);
                nextspan->prevspan = span_pointer;
        }
        span->fclass = toclass;

        return true;
}

/*
 * Allocate one object of the requested size class from the given area.
 */
static inline dsa_pointer
alloc_object(dsa_area *area, int size_class)
{
        dsa_area_pool *pool = &area->control->pools[size_class];
        dsa_area_span *span;
        dsa_pointer block;
        dsa_pointer result;
        char       *object;
        size_t          size;

        /*
         * Even though ensure_active_superblock can in turn call alloc_object if
         * it needs to allocate a new span, that's always from a different pool,
         * and the order of lock acquisition is always the same, so it's OK that
         * we hold this lock for the duration of this function.
         */
        Assert(!LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));
        LWLockAcquire(DSA_SCLASS_LOCK(area, size_class), LW_EXCLUSIVE);

        /*
         * If there's no active superblock, we must successfully obtain one or
         * fail the request.
         */
        if (!DsaPointerIsValid(pool->spans[1]) &&
                !ensure_active_superblock(area, pool, size_class))
        {
                result = InvalidDsaPointer;
        }
        else
        {
                /*
                 * There should be a block in fullness class 1 at this point, and it
                 * should never be completely full.  Thus we can either pop an object
                 * from the free list or, failing that, initialize a new object.
                 */
                Assert(DsaPointerIsValid(pool->spans[1]));
                span = (dsa_area_span *)
                        dsa_get_address(area, pool->spans[1]);
                Assert(span->nallocatable > 0);
                block = span->start;
                Assert(size_class < DSA_NUM_SIZE_CLASSES);
                size = dsa_size_classes[size_class];
                if (span->firstfree != DSA_SPAN_NOTHING_FREE)
                {
                        result = block + span->firstfree * size;
                        object = dsa_get_address(area, result);
                        span->firstfree = NextFreeObjectIndex(object);
                }
                else
                {
                        result = block + span->ninitialized * size;
                        ++span->ninitialized;
                }
                --span->nallocatable;

                /* If it's now full, move it to the highest-numbered fullness class. */
                if (span->nallocatable == 0)
                        transfer_first_span(area, pool, 1, DSA_FULLNESS_CLASSES - 1);
        }

        Assert(LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));
        LWLockRelease(DSA_SCLASS_LOCK(area, size_class));

        return result;
}

/*
 * Ensure an active (i.e. fullness class 1) superblock, unless all existing
 * superblocks are completely full and no more can be allocated.
 *
 * Fullness classes K of 0..N are loosely intended to represent blocks whose
 * utilization percentage is at least K/N, but we only enforce this rigorously
 * for the highest-numbered fullness class, which always contains exactly
 * those blocks that are completely full.  It's otherwise acceptable for a
 * block to be in a higher-numbered fullness class than the one to which it
 * logically belongs.  In addition, the active block, which is always the
 * first block in fullness class 1, is permitted to have a higher allocation
 * percentage than would normally be allowable for that fullness class; we
 * don't move it until it's completely full, and then it goes to the
 * highest-numbered fullness class.
 *
 * It might seem odd that the active block is the head of fullness class 1
 * rather than fullness class 0, but experience with other allocators has
 * shown that it's usually better to allocate from a block that's moderately
 * full rather than one that's nearly empty.  Insofar as is reasonably
 * possible, we want to avoid performing new allocations in a block that would
 * otherwise become empty soon.
 */
static bool
ensure_active_superblock(dsa_area *area, dsa_area_pool *pool,
                                                 int size_class)
{
        dsa_pointer span_pointer;
        dsa_pointer start_pointer;
        size_t          obsize = dsa_size_classes[size_class];
        size_t          nmax;
        int                     fclass;
        size_t          npages = 1;
        size_t          first_page;
        size_t          i;
        dsa_segment_map *segment_map;

        Assert(LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));

        /*
         * Compute the number of objects that will fit in a block of this size
         * class.  Span-of-spans blocks are just a single page, and the first
         * object isn't available for use because it describes the block-of-spans
         * itself.
         */
        if (size_class == DSA_SCLASS_BLOCK_OF_SPANS)
                nmax = FPM_PAGE_SIZE / obsize - 1;
        else
                nmax = DSA_SUPERBLOCK_SIZE / obsize;

        /*
         * If fullness class 1 is empty, try to find a span to put in it by
         * scanning higher-numbered fullness classes (excluding the last one,
         * whose blocks are certain to all be completely full).
         */
        for (fclass = 2; fclass < DSA_FULLNESS_CLASSES - 1; ++fclass)
        {
                span_pointer = pool->spans[fclass];

                while (DsaPointerIsValid(span_pointer))
                {
                        int                     tfclass;
                        dsa_area_span *span;
                        dsa_area_span *nextspan;
                        dsa_area_span *prevspan;
                        dsa_pointer next_span_pointer;

                        span = (dsa_area_span *)
                                dsa_get_address(area, span_pointer);
                        next_span_pointer = span->nextspan;

                        /* Figure out what fullness class should contain this span. */
                        tfclass = (nmax - span->nallocatable)
                                * (DSA_FULLNESS_CLASSES - 1) / nmax;

                        /* Look up next span. */
                        if (DsaPointerIsValid(span->nextspan))
                                nextspan = (dsa_area_span *)
                                        dsa_get_address(area, span->nextspan);
                        else
                                nextspan = NULL;

                        /*
                         * If utilization has dropped enough that this now belongs in some
                         * other fullness class, move it there.
                         */
                        if (tfclass < fclass)
                        {
                                /* Remove from the current fullness class list. */
                                if (pool->spans[fclass] == span_pointer)
                                {
                                        /* It was the head; remove it. */
                                        Assert(!DsaPointerIsValid(span->prevspan));
                                        pool->spans[fclass] = span->nextspan;
                                        if (nextspan != NULL)
                                                nextspan->prevspan = InvalidDsaPointer;
                                }
                                else
                                {
                                        /* It was not the head. */
                                        Assert(DsaPointerIsValid(span->prevspan));
                                        prevspan = (dsa_area_span *)
                                                dsa_get_address(area, span->prevspan);
                                        prevspan->nextspan = span->nextspan;
                                }
                                if (nextspan != NULL)
                                        nextspan->prevspan = span->prevspan;

                                /* Push onto the head of the new fullness class list. */
                                span->nextspan = pool->spans[tfclass];
                                pool->spans[tfclass] = span_pointer;
                                span->prevspan = InvalidDsaPointer;
                                if (DsaPointerIsValid(span->nextspan))
                                {
                                        nextspan = (dsa_area_span *)
                                                dsa_get_address(area, span->nextspan);
                                        nextspan->prevspan = span_pointer;
                                }
                                span->fclass = tfclass;
                        }

                        /* Advance to next span on list. */
                        span_pointer = next_span_pointer;
                }

                /* Stop now if we found a suitable block. */
                if (DsaPointerIsValid(pool->spans[1]))
                        return true;
        }

        /*
         * If there are no blocks that properly belong in fullness class 1, pick
         * one from some other fullness class and move it there anyway, so that we
         * have an allocation target.  Our last choice is to transfer a block
         * that's almost empty (and might become completely empty soon if left
         * alone), but even that is better than failing, which is what we must do
         * if there are no blocks at all with freespace.
         */
        Assert(!DsaPointerIsValid(pool->spans[1]));
        for (fclass = 2; fclass < DSA_FULLNESS_CLASSES - 1; ++fclass)
                if (transfer_first_span(area, pool, fclass, 1))
                        return true;
        if (!DsaPointerIsValid(pool->spans[1]) &&
                transfer_first_span(area, pool, 0, 1))
                return true;

        /*
         * We failed to find an existing span with free objects, so we need to
         * allocate a new superblock and construct a new span to manage it.
         *
         * First, get a dsa_area_span object to describe the new superblock block
         * ... unless this allocation is for a dsa_area_span object, in which case
         * that's surely not going to work.  We handle that case by storing the
         * span describing a block-of-spans inline.
         */
        if (size_class != DSA_SCLASS_BLOCK_OF_SPANS)
        {
                span_pointer = alloc_object(area, DSA_SCLASS_BLOCK_OF_SPANS);
                if (!DsaPointerIsValid(span_pointer))
                        return false;
                npages = DSA_PAGES_PER_SUPERBLOCK;
        }

        /* Find or create a segment and allocate the superblock. */
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        segment_map = get_best_segment(area, npages);
        if (segment_map == NULL)
        {
                segment_map = make_new_segment(area, npages);
                if (segment_map == NULL)
                {
                        LWLockRelease(DSA_AREA_LOCK(area));
                        return false;
                }
        }

        /*
         * This shouldn't happen: get_best_segment() or make_new_segment()
         * promised that we can successfully allocate npages.
         */
        if (!FreePageManagerGet(segment_map->fpm, npages, &first_page))
                elog(FATAL,
                         "dsa_allocate could not find %zu free pages for superblock",
                         npages);
        LWLockRelease(DSA_AREA_LOCK(area));

        /* Compute the start of the superblock. */
        start_pointer =
                DSA_MAKE_POINTER(get_segment_index(area, segment_map),
                                                 first_page * FPM_PAGE_SIZE);

        /*
         * If this is a block-of-spans, carve the descriptor right out of the
         * allocated space.
         */
        if (size_class == DSA_SCLASS_BLOCK_OF_SPANS)
        {
                /*
                 * We have a pointer into the segment.  We need to build a dsa_pointer
                 * from the segment index and offset into the segment.
                 */
                span_pointer = start_pointer;
        }

        /* Initialize span and pagemap. */
        init_span(area, span_pointer, pool, start_pointer, npages, size_class);
        for (i = 0; i < npages; ++i)
                segment_map->pagemap[first_page + i] = span_pointer;

        return true;
}

/*
 * Return the segment map corresponding to a given segment index, mapping the
 * segment in if necessary.  For internal segment book-keeping, this is called
 * with the area lock held.  It is also called by dsa_free and dsa_get_address
 * without any locking, relying on the fact they have a known live segment
 * index and they always call check_for_freed_segments to ensures that any
 * freed segment occupying the same slot is detached first.
 */
static dsa_segment_map *
get_segment_by_index(dsa_area *area, dsa_segment_index index)
{
        if (unlikely(area->segment_maps[index].mapped_address == NULL))
        {
                dsm_handle      handle;
                dsm_segment *segment;
                dsa_segment_map *segment_map;
                ResourceOwner oldowner;

                /*
                 * If we are reached by dsa_free or dsa_get_address, there must be at
                 * least one object allocated in the referenced segment.  Otherwise,
                 * their caller has a double-free or access-after-free bug, which we
                 * have no hope of detecting.  So we know it's safe to access this
                 * array slot without holding a lock; it won't change underneath us.
                 * Furthermore, we know that we can see the latest contents of the
                 * slot, as explained in check_for_freed_segments, which those
                 * functions call before arriving here.
                 */
                handle = area->control->segment_handles[index];

                /* It's an error to try to access an unused slot. */
                if (handle == DSM_HANDLE_INVALID)
                        elog(ERROR,
                                 "dsa_area could not attach to a segment that has been freed");

                oldowner = CurrentResourceOwner;
                CurrentResourceOwner = area->resowner;
                segment = dsm_attach(handle);
                CurrentResourceOwner = oldowner;
                if (segment == NULL)
                        elog(ERROR, "dsa_area could not attach to segment");
                segment_map = &area->segment_maps[index];
                segment_map->segment = segment;
                segment_map->mapped_address = dsm_segment_address(segment);
                segment_map->header =
                        (dsa_segment_header *) segment_map->mapped_address;
                segment_map->fpm = (FreePageManager *)
                        (segment_map->mapped_address +
                         MAXALIGN(sizeof(dsa_segment_header)));
                segment_map->pagemap = (dsa_pointer *)
                        (segment_map->mapped_address +
                         MAXALIGN(sizeof(dsa_segment_header)) +
                         MAXALIGN(sizeof(FreePageManager)));

                /* Remember the highest index this backend has ever mapped. */
                if (area->high_segment_index < index)
                        area->high_segment_index = index;

                Assert(segment_map->header->magic ==
                           (DSA_SEGMENT_HEADER_MAGIC ^ area->control->handle ^ index));
        }

        /*
         * Callers of dsa_get_address() and dsa_free() don't hold the area lock,
         * but it's a bug in the calling code and undefined behavior if the
         * address is not live (ie if the segment might possibly have been freed,
         * they're trying to use a dangling pointer).
         *
         * For dsa.c code that holds the area lock to manipulate segment_bins
         * lists, it would be a bug if we ever reach a freed segment here.  After
         * it's marked as freed, the only thing any backend should do with it is
         * unmap it, and it should always have done that in
         * check_for_freed_segments_locked() before arriving here to resolve an
         * index to a segment_map.
         *
         * Either way we can assert that we aren't returning a freed segment.
         */
        Assert(!area->segment_maps[index].header->freed);

        return &area->segment_maps[index];
}

/*
 * Return a superblock to the free page manager.  If the underlying segment
 * has become entirely free, then return it to the operating system.
 *
 * The appropriate pool lock must be held.
 */
static void
destroy_superblock(dsa_area *area, dsa_pointer span_pointer)
{
        dsa_area_span *span = dsa_get_address(area, span_pointer);
        int                     size_class = span->size_class;
        dsa_segment_map *segment_map;


        /* Remove it from its fullness class list. */
        unlink_span(area, span);

        /*
         * Note: Here we acquire the area lock while we already hold a per-pool
         * lock.  We never hold the area lock and then take a pool lock, or we
         * could deadlock.
         */
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        check_for_freed_segments_locked(area);
        segment_map =
                get_segment_by_index(area, DSA_EXTRACT_SEGMENT_NUMBER(span->start));
        FreePageManagerPut(segment_map->fpm,
                                           DSA_EXTRACT_OFFSET(span->start) / FPM_PAGE_SIZE,
                                           span->npages);
        /* Check if the segment is now entirely free. */
        if (fpm_largest(segment_map->fpm) == segment_map->header->usable_pages)
        {
                dsa_segment_index index = get_segment_index(area, segment_map);

                /* If it's not the segment with extra control data, free it. */
                if (index != 0)
                {
                        /*
                         * Give it back to the OS, and allow other backends to detect that
                         * they need to detach.
                         */
                        unlink_segment(area, segment_map);
                        segment_map->header->freed = true;
                        Assert(area->control->total_segment_size >=
                                   segment_map->header->size);
                        area->control->total_segment_size -=
                                segment_map->header->size;
                        dsm_unpin_segment(dsm_segment_handle(segment_map->segment));
                        dsm_detach(segment_map->segment);
                        area->control->segment_handles[index] = DSM_HANDLE_INVALID;
                        ++area->control->freed_segment_counter;
                        segment_map->segment = NULL;
                        segment_map->header = NULL;
                        segment_map->mapped_address = NULL;
                }
        }

        /* Move segment to appropriate bin if necessary. */
        if (segment_map->header != NULL)
                rebin_segment(area, segment_map);

        LWLockRelease(DSA_AREA_LOCK(area));

        /*
         * Span-of-spans blocks store the span which describes them within the
         * block itself, so freeing the storage implicitly frees the descriptor
         * also.  If this is a block of any other type, we need to separately free
         * the span object also.  This recursive call to dsa_free will acquire the
         * span pool's lock.  We can't deadlock because the acquisition order is
         * always some other pool and then the span pool.
         */
        if (size_class != DSA_SCLASS_BLOCK_OF_SPANS)
                dsa_free(area, span_pointer);
}

static void
unlink_span(dsa_area *area, dsa_area_span *span)
{
        if (DsaPointerIsValid(span->nextspan))
        {
                dsa_area_span *next = dsa_get_address(area, span->nextspan);

                next->prevspan = span->prevspan;
        }
        if (DsaPointerIsValid(span->prevspan))
        {
                dsa_area_span *prev = dsa_get_address(area, span->prevspan);

                prev->nextspan = span->nextspan;
        }
        else
        {
                dsa_area_pool *pool = dsa_get_address(area, span->pool);

                pool->spans[span->fclass] = span->nextspan;
        }
}

static void
add_span_to_fullness_class(dsa_area *area, dsa_area_span *span,
                                                   dsa_pointer span_pointer,
                                                   int fclass)
{
        dsa_area_pool *pool = dsa_get_address(area, span->pool);

        if (DsaPointerIsValid(pool->spans[fclass]))
        {
                dsa_area_span *head = dsa_get_address(area,
                                                                                          pool->spans[fclass]);

                head->prevspan = span_pointer;
        }
        span->prevspan = InvalidDsaPointer;
        span->nextspan = pool->spans[fclass];
        pool->spans[fclass] = span_pointer;
        span->fclass = fclass;
}

/*
 * Detach from an area that was either created or attached to by this process.
 */
void
dsa_detach(dsa_area *area)
{
        int                     i;

        /* Detach from all segments. */
        for (i = 0; i <= area->high_segment_index; ++i)
                if (area->segment_maps[i].segment != NULL)
                        dsm_detach(area->segment_maps[i].segment);

        /*
         * Note that 'detaching' (= detaching from DSM segments) doesn't include
         * 'releasing' (= adjusting the reference count).  It would be nice to
         * combine these operations, but client code might never get around to
         * calling dsa_detach because of an error path, and a detach hook on any
         * particular segment is too late to detach other segments in the area
         * without risking a 'leak' warning in the non-error path.
         */

        /* Free the backend-local area object. */
        pfree(area);
}

/*
 * Unlink a segment from the bin that contains it.
 */
static void
unlink_segment(dsa_area *area, dsa_segment_map *segment_map)
{
        if (segment_map->header->prev != DSA_SEGMENT_INDEX_NONE)
        {
                dsa_segment_map *prev;

                prev = get_segment_by_index(area, segment_map->header->prev);
                prev->header->next = segment_map->header->next;
        }
        else
        {
                Assert(area->control->segment_bins[segment_map->header->bin] ==
                           get_segment_index(area, segment_map));
                area->control->segment_bins[segment_map->header->bin] =
                        segment_map->header->next;
        }
        if (segment_map->header->next != DSA_SEGMENT_INDEX_NONE)
        {
                dsa_segment_map *next;

                next = get_segment_by_index(area, segment_map->header->next);
                next->header->prev = segment_map->header->prev;
        }
}

/*
 * Find a segment that could satisfy a request for 'npages' of contiguous
 * memory, or return NULL if none can be found.  This may involve attaching to
 * segments that weren't previously attached so that we can query their free
 * pages map.
 */
static dsa_segment_map *
get_best_segment(dsa_area *area, size_t npages)
{
        size_t          bin;

        Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));
        check_for_freed_segments_locked(area);

        /*
         * Start searching from the first bin that *might* have enough contiguous
         * pages.
         */
        for (bin = contiguous_pages_to_segment_bin(npages);
                 bin < DSA_NUM_SEGMENT_BINS;
                 ++bin)
        {
                /*
                 * The minimum contiguous size that any segment in this bin should
                 * have.  We'll re-bin if we see segments with fewer.
                 */
                size_t          threshold = (size_t) 1 << (bin - 1);
                dsa_segment_index segment_index;

                /* Search this bin for a segment with enough contiguous space. */
                segment_index = area->control->segment_bins[bin];
                while (segment_index != DSA_SEGMENT_INDEX_NONE)
                {
                        dsa_segment_map *segment_map;
                        dsa_segment_index next_segment_index;
                        size_t          contiguous_pages;

                        segment_map = get_segment_by_index(area, segment_index);
                        next_segment_index = segment_map->header->next;
                        contiguous_pages = fpm_largest(segment_map->fpm);

                        /* Not enough for the request, still enough for this bin. */
                        if (contiguous_pages >= threshold && contiguous_pages < npages)
                        {
                                segment_index = next_segment_index;
                                continue;
                        }

                        /* Re-bin it if it's no longer in the appropriate bin. */
                        if (contiguous_pages < threshold)
                        {
                                rebin_segment(area, segment_map);

                                /*
                                 * But fall through to see if it's enough to satisfy this
                                 * request anyway....
                                 */
                        }

                        /* Check if we are done. */
                        if (contiguous_pages >= npages)
                                return segment_map;

                        /* Continue searching the same bin. */
                        segment_index = next_segment_index;
                }
        }

        /* Not found. */
        return NULL;
}

/*
 * Create a new segment that can handle at least requested_pages.  Returns
 * NULL if the requested total size limit or maximum allowed number of
 * segments would be exceeded.
 */
static dsa_segment_map *
make_new_segment(dsa_area *area, size_t requested_pages)
{
        dsa_segment_index new_index;
        size_t          metadata_bytes;
        size_t          total_size;
        size_t          total_pages;
        size_t          usable_pages;
        dsa_segment_map *segment_map;
        dsm_segment *segment;
        ResourceOwner oldowner;

        Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));

        /* Find a segment slot that is not in use (linearly for now). */
        for (new_index = 1; new_index < DSA_MAX_SEGMENTS; ++new_index)
        {
                if (area->control->segment_handles[new_index] == DSM_HANDLE_INVALID)
                        break;
        }
        if (new_index == DSA_MAX_SEGMENTS)
                return NULL;

        /*
         * If the total size limit is already exceeded, then we exit early and
         * avoid arithmetic wraparound in the unsigned expressions below.
         */
        if (area->control->total_segment_size >=
                area->control->max_total_segment_size)
                return NULL;

        /*
         * The size should be at least as big as requested, and at least big
         * enough to follow a geometric series that approximately doubles the
         * total storage each time we create a new segment.  We use geometric
         * growth because the underlying DSM system isn't designed for large
         * numbers of segments (otherwise we might even consider just using one
         * DSM segment for each large allocation and for each superblock, and then
         * we wouldn't need to use FreePageManager).
         *
         * We decide on a total segment size first, so that we produce tidy
         * power-of-two sized segments.  This is a good property to have if we
         * move to huge pages in the future.  Then we work back to the number of
         * pages we can fit.
         */
        total_size = DSA_INITIAL_SEGMENT_SIZE *
                ((size_t) 1 << (new_index / DSA_NUM_SEGMENTS_AT_EACH_SIZE));
        total_size = Min(total_size, DSA_MAX_SEGMENT_SIZE);
        total_size = Min(total_size,
                                         area->control->max_total_segment_size -
                                         area->control->total_segment_size);

        total_pages = total_size / FPM_PAGE_SIZE;
        metadata_bytes =
                MAXALIGN(sizeof(dsa_segment_header)) +
                MAXALIGN(sizeof(FreePageManager)) +
                sizeof(dsa_pointer) * total_pages;

        /* Add padding up to next page boundary. */
        if (metadata_bytes % FPM_PAGE_SIZE != 0)
                metadata_bytes += FPM_PAGE_SIZE - (metadata_bytes % FPM_PAGE_SIZE);
        if (total_size <= metadata_bytes)
                return NULL;
        usable_pages = (total_size - metadata_bytes) / FPM_PAGE_SIZE;
        Assert(metadata_bytes + usable_pages * FPM_PAGE_SIZE <= total_size);

        /* See if that is enough... */
        if (requested_pages > usable_pages)
        {
                /*
                 * We'll make an odd-sized segment, working forward from the requested
                 * number of pages.
                 */
                usable_pages = requested_pages;
                metadata_bytes =
                        MAXALIGN(sizeof(dsa_segment_header)) +
                        MAXALIGN(sizeof(FreePageManager)) +
                        usable_pages * sizeof(dsa_pointer);

                /* Add padding up to next page boundary. */
                if (metadata_bytes % FPM_PAGE_SIZE != 0)
                        metadata_bytes += FPM_PAGE_SIZE - (metadata_bytes % FPM_PAGE_SIZE);
                total_size = metadata_bytes + usable_pages * FPM_PAGE_SIZE;

                /* Is that too large for dsa_pointer's addressing scheme? */
                if (total_size > DSA_MAX_SEGMENT_SIZE)
                        return NULL;

                /* Would that exceed the limit? */
                if (total_size > area->control->max_total_segment_size -
                        area->control->total_segment_size)
                        return NULL;
        }

        /* Create the segment. */
        oldowner = CurrentResourceOwner;
        CurrentResourceOwner = area->resowner;
        segment = dsm_create(total_size, 0);
        CurrentResourceOwner = oldowner;
        if (segment == NULL)
                return NULL;
        dsm_pin_segment(segment);

        /* Store the handle in shared memory to be found by index. */
        area->control->segment_handles[new_index] =
                dsm_segment_handle(segment);
        /* Track the highest segment index in the history of the area. */
        if (area->control->high_segment_index < new_index)
                area->control->high_segment_index = new_index;
        /* Track the highest segment index this backend has ever mapped. */
        if (area->high_segment_index < new_index)
                area->high_segment_index = new_index;
        /* Track total size of all segments. */
        area->control->total_segment_size += total_size;
        Assert(area->control->total_segment_size <=
                   area->control->max_total_segment_size);

        /* Build a segment map for this segment in this backend. */
        segment_map = &area->segment_maps[new_index];
        segment_map->segment = segment;
        segment_map->mapped_address = dsm_segment_address(segment);
        segment_map->header = (dsa_segment_header *) segment_map->mapped_address;
        segment_map->fpm = (FreePageManager *)
                (segment_map->mapped_address +
                 MAXALIGN(sizeof(dsa_segment_header)));
        segment_map->pagemap = (dsa_pointer *)
                (segment_map->mapped_address +
                 MAXALIGN(sizeof(dsa_segment_header)) +
                 MAXALIGN(sizeof(FreePageManager)));

        /* Set up the free page map. */
        FreePageManagerInitialize(segment_map->fpm, segment_map->mapped_address);
        FreePageManagerPut(segment_map->fpm, metadata_bytes / FPM_PAGE_SIZE,
                                           usable_pages);

        /* Set up the segment header and put it in the appropriate bin. */
        segment_map->header->magic =
                DSA_SEGMENT_HEADER_MAGIC ^ area->control->handle ^ new_index;
        segment_map->header->usable_pages = usable_pages;
        segment_map->header->size = total_size;
        segment_map->header->bin = contiguous_pages_to_segment_bin(usable_pages);
        segment_map->header->prev = DSA_SEGMENT_INDEX_NONE;
        segment_map->header->next =
                area->control->segment_bins[segment_map->header->bin];
        segment_map->header->freed = false;
        area->control->segment_bins[segment_map->header->bin] = new_index;
        if (segment_map->header->next != DSA_SEGMENT_INDEX_NONE)
        {
                dsa_segment_map *next =
                        get_segment_by_index(area, segment_map->header->next);

                Assert(next->header->bin == segment_map->header->bin);
                next->header->prev = new_index;
        }

        return segment_map;
}

/*
 * Check if any segments have been freed by destroy_superblock, so we can
 * detach from them in this backend.  This function is called by
 * dsa_get_address and dsa_free to make sure that a dsa_pointer they have
 * received can be resolved to the correct segment.
 *
 * The danger we want to defend against is that there could be an old segment
 * mapped into a given slot in this backend, and the dsa_pointer they have
 * might refer to some new segment in the same slot.  So those functions must
 * be sure to process all instructions to detach from a freed segment that had
 * been generated by the time this process received the dsa_pointer, before
 * they call get_segment_by_index.
 */
static void
check_for_freed_segments(dsa_area *area)
{
        size_t          freed_segment_counter;

        /*
         * Any other process that has freed a segment has incremented
         * freed_segment_counter while holding an LWLock, and that must precede
         * any backend creating a new segment in the same slot while holding an
         * LWLock, and that must precede the creation of any dsa_pointer pointing
         * into the new segment which might reach us here, and the caller must
         * have sent the dsa_pointer to this process using appropriate memory
         * synchronization (some kind of locking or atomic primitive or system
         * call).  So all we need to do on the reading side is ask for the load of
         * freed_segment_counter to follow the caller's load of the dsa_pointer it
         * has, and we can be sure to detect any segments that had been freed as
         * of the time that the dsa_pointer reached this process.
         */
        pg_read_barrier();
        freed_segment_counter = area->control->freed_segment_counter;
        if (unlikely(area->freed_segment_counter != freed_segment_counter))
        {
                /* Check all currently mapped segments to find what's been freed. */
                LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
                check_for_freed_segments_locked(area);
                LWLockRelease(DSA_AREA_LOCK(area));
        }
}

/*
 * Workhorse for check_for_freed_segments(), and also used directly in path
 * where the area lock is already held.  This should be called after acquiring
 * the lock but before looking up any segment by index number, to make sure we
 * unmap any stale segments that might have previously had the same index as a
 * current segment.
 */
static void
check_for_freed_segments_locked(dsa_area *area)
{
        size_t          freed_segment_counter;
        int                     i;

        Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));
        freed_segment_counter = area->control->freed_segment_counter;
        if (unlikely(area->freed_segment_counter != freed_segment_counter))
        {
                for (i = 0; i <= area->high_segment_index; ++i)
                {
                        if (area->segment_maps[i].header != NULL &&
                                area->segment_maps[i].header->freed)
                        {
                                dsm_detach(area->segment_maps[i].segment);
                                area->segment_maps[i].segment = NULL;
                                area->segment_maps[i].header = NULL;
                                area->segment_maps[i].mapped_address = NULL;
                        }
                }
                area->freed_segment_counter = freed_segment_counter;
        }
}

/*
 * Re-bin segment if it's no longer in the appropriate bin.
 */
static void
rebin_segment(dsa_area *area, dsa_segment_map *segment_map)
{
        size_t          new_bin;
        dsa_segment_index segment_index;

        new_bin = contiguous_pages_to_segment_bin(fpm_largest(segment_map->fpm));
        if (segment_map->header->bin == new_bin)
                return;

        /* Remove it from its current bin. */
        unlink_segment(area, segment_map);

        /* Push it onto the front of its new bin. */
        segment_index = get_segment_index(area, segment_map);
        segment_map->header->prev = DSA_SEGMENT_INDEX_NONE;
        segment_map->header->next = area->control->segment_bins[new_bin];
        segment_map->header->bin = new_bin;
        area->control->segment_bins[new_bin] = segment_index;
        if (segment_map->header->next != DSA_SEGMENT_INDEX_NONE)
        {
                dsa_segment_map *next;

                next = get_segment_by_index(area, segment_map->header->next);
                Assert(next->header->bin == new_bin);
                next->header->prev = segment_index;
        }
}