greenplumn dsa 源码
greenplumn dsa 代码
文件路径：/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) 1996-2019, 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 "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"
/*
* 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 4
/*
* 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.
*/
#define contiguous_pages_to_segment_bin(n) Min(fls(n), 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;
/* Has the mapping been pinned? */
bool mapping_pinned;
/*
* 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);
/*
* 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 != DSM_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, DSM_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;
Assert(!area->mapping_pinned);
area->mapping_pinned = true;
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);
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;
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;
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;
memset(&control->segment_handles[0], 0,
sizeof(dsm_handle) * DSA_MAX_SEGMENTS);
control->segment_handles[0] = control_handle;
for (i = 0; i < DSA_NUM_SEGMENT_BINS; ++i)
control->segment_bins[i] = DSA_SEGMENT_INDEX_NONE;
control->high_segment_index = 0;
control->refcnt = 1;
control->freed_segment_counter = 0;
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->mapping_pinned = false;
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->mapping_pinned = false;
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;
/*
* 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");
segment = dsm_attach(handle);
if (segment == NULL)
elog(ERROR, "dsa_area could not attach to segment");
if (area->mapping_pinned)
dsm_pin_mapping(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;
}
}
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)
{
size_t new_bin;
new_bin = contiguous_pages_to_segment_bin(contiguous_pages);
/* Remove it from its current bin. */
unlink_segment(area, segment_map);
/* Push it onto the front of its new bin. */
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;
}
/*
* 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;
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. */
segment = dsm_create(total_size, 0);
if (segment == NULL)
return NULL;
dsm_pin_segment(segment);
if (area->mapping_pinned)
dsm_pin_mapping(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
* free_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;
}
}
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