buf_init.c

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/*-------------------------------------------------------------------------
 *
 * buf_init.c
 *    buffer manager initialization routines
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/buffer/buf_init.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "storage/aio.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/proclist.h"
#include "storage/shmem.h"
#include "storage/subsystems.h"
 
BufferDescPadded *BufferDescriptors;
char       *BufferBlocks;
ConditionVariableMinimallyPadded *BufferIOCVArray;
WritebackContext BackendWritebackContext;
CkptSortItem *CkptBufferIds;
 
static void BufferManagerShmemRequest(void *arg);
static void BufferManagerShmemInit(void *arg);
static void BufferManagerShmemAttach(void *arg);
 
const ShmemCallbacks BufferManagerShmemCallbacks = {
    .request_fn = BufferManagerShmemRequest,
    .init_fn = BufferManagerShmemInit,
    .attach_fn = BufferManagerShmemAttach,
};
 
/*
 * Data Structures:
 *      buffers live in a freelist and a lookup data structure.
 *
 *
 * Buffer Lookup:
 *      Two important notes.  First, the buffer has to be
 *      available for lookup BEFORE an IO begins.  Otherwise
 *      a second process trying to read the buffer will
 *      allocate its own copy and the buffer pool will
 *      become inconsistent.
 *
 * Buffer Replacement:
 *      see freelist.c.  A buffer cannot be replaced while in
 *      use either by data manager or during IO.
 *
 *
 * Synchronization/Locking:
 *
 * IO_IN_PROGRESS -- this is a flag in the buffer descriptor.
 *      It must be set when an IO is initiated and cleared at
 *      the end of the IO.  It is there to make sure that one
 *      process doesn't start to use a buffer while another is
 *      faulting it in.  see WaitIO and related routines.
 *
 * refcount --  Counts the number of processes holding pins on a buffer.
 *      A buffer is pinned during IO and immediately after a BufferAlloc().
 *      Pins must be released before end of transaction.  For efficiency the
 *      shared refcount isn't increased if an individual backend pins a buffer
 *      multiple times. Check the PrivateRefCount infrastructure in bufmgr.c.
 */
 
 
/*
 * Register shared memory area for the buffer pool.
 */
static void
BufferManagerShmemRequest(void *arg)
{
    ShmemRequestStruct(.name = "Buffer Descriptors",
                       .size = NBuffers * sizeof(BufferDescPadded),
    /* Align descriptors to a cacheline boundary. */
                       .alignment = PG_CACHE_LINE_SIZE,
                       .ptr = (void **) &BufferDescriptors,
        );
 
    ShmemRequestStruct(.name = "Buffer Blocks",
                       .size = NBuffers * (Size) BLCKSZ,
    /* Align buffer pool on IO page size boundary. */
                       .alignment = PG_IO_ALIGN_SIZE,
                       .ptr = (void **) &BufferBlocks,
        );
 
    ShmemRequestStruct(.name = "Buffer IO Condition Variables",
                       .size = NBuffers * sizeof(ConditionVariableMinimallyPadded),
    /* Align descriptors to a cacheline boundary. */
                       .alignment = PG_CACHE_LINE_SIZE,
                       .ptr = (void **) &BufferIOCVArray,
        );
 
    /*
     * The array used to sort to-be-checkpointed buffer ids is located in
     * shared memory, to avoid having to allocate significant amounts of
     * memory at runtime. As that'd be in the middle of a checkpoint, or when
     * the checkpointer is restarted, memory allocation failures would be
     * painful.
     */
    ShmemRequestStruct(.name = "Checkpoint BufferIds",
                       .size = NBuffers * sizeof(CkptSortItem),
                       .ptr = (void **) &CkptBufferIds,
        );
}
 
/*
 * Initialize shared buffer pool
 *
 * This is called once during shared-memory initialization (either in the
 * postmaster, or in a standalone backend).
 */
static void
BufferManagerShmemInit(void *arg)
{
    /*
     * Initialize all the buffer headers.
     */
    for (int i = 0; i < NBuffers; i++)
    {
        BufferDesc *buf = GetBufferDescriptor(i);
 
        ClearBufferTag(&buf->tag);
 
        pg_atomic_init_u64(&buf->state, 0);
        buf->wait_backend_pgprocno = INVALID_PROC_NUMBER;
 
        buf->buf_id = i;
 
        pgaio_wref_clear(&buf->io_wref);
 
        proclist_init(&buf->lock_waiters);
        ConditionVariableInit(BufferDescriptorGetIOCV(buf));
    }
 
    /* Initialize per-backend file flush context */
    WritebackContextInit(&BackendWritebackContext,
                         &backend_flush_after);
}
 
static void
BufferManagerShmemAttach(void *arg)
{
    /* Initialize per-backend file flush context */
    WritebackContextInit(&BackendWritebackContext,
                         &backend_flush_after);
}