slru.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * slru.c
 *      Simple LRU buffering for wrap-around-able permanent metadata
 *
 * This module is used to maintain various pieces of transaction status
 * indexed by TransactionId (such as commit status, parent transaction ID,
 * commit timestamp), as well as storage for multixacts, serializable
 * isolation locks and NOTIFY traffic.  Extensions can define their own
 * SLRUs, too.
 *
 * Under ordinary circumstances we expect that write traffic will occur
 * mostly to the latest page (and to the just-prior page, soon after a
 * page transition).  Read traffic will probably touch a larger span of
 * pages, but a relatively small number of buffers should be sufficient.
 *
 * We use a simple least-recently-used scheme to manage a pool of shared
 * page buffers, split in banks by the lowest bits of the page number, and
 * the management algorithm only processes the bank to which the desired
 * page belongs, so a linear search is sufficient; there's no need for a
 * hashtable or anything fancy.  The algorithm is straight LRU except that
 * we will never swap out the latest page (since we know it's going to be
 * hit again eventually).
 *
 * We use per-bank control LWLocks to protect the shared data structures,
 * plus per-buffer LWLocks that synchronize I/O for each buffer.  The
 * bank's control lock must be held to examine or modify any of the bank's
 * shared state.  A process that is reading in or writing out a page
 * buffer does not hold the control lock, only the per-buffer lock for the
 * buffer it is working on.  One exception is latest_page_number, which is
 * read and written using atomic ops.
 *
 * "Holding the bank control lock" means exclusive lock in all cases
 * except for SimpleLruReadPage_ReadOnly(); see comments for
 * SlruRecentlyUsed() for the implications of that.
 *
 * When initiating I/O on a buffer, we acquire the per-buffer lock exclusively
 * before releasing the control lock.  The per-buffer lock is released after
 * completing the I/O, re-acquiring the control lock, and updating the shared
 * state.  (Deadlock is not possible here, because we never try to initiate
 * I/O when someone else is already doing I/O on the same buffer.)
 * To wait for I/O to complete, release the control lock, acquire the
 * per-buffer lock in shared mode, immediately release the per-buffer lock,
 * reacquire the control lock, and then recheck state (since arbitrary things
 * could have happened while we didn't have the lock).
 *
 * As with the regular buffer manager, it is possible for another process
 * to re-dirty a page that is currently being written out.  This is handled
 * by re-setting the page's page_dirty flag.
 *
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/slru.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
 
#include "access/slru.h"
#include "access/transam.h"
#include "access/xlog.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/fd.h"
#include "storage/shmem.h"
#include "utils/guc.h"
 
/*
 * Converts segment number to the filename of the segment.
 *
 * "path" should point to a buffer at least MAXPGPATH characters long.
 *
 * If ctl->long_segment_names is true, segno can be in the range [0, 2^60-1].
 * The resulting file name is made of 15 characters, e.g. dir/123456789ABCDEF.
 *
 * If ctl->long_segment_names is false, segno can be in the range [0, 2^24-1].
 * The resulting file name is made of 4 to 6 characters, as of:
 *
 *  dir/1234   for [0, 2^16-1]
 *  dir/12345  for [2^16, 2^20-1]
 *  dir/123456 for [2^20, 2^24-1]
 */
static inline int
SlruFileName(SlruCtl ctl, char *path, int64 segno)
{
    if (ctl->long_segment_names)
    {
        /*
         * We could use 16 characters here but the disadvantage would be that
         * the SLRU segments will be hard to distinguish from WAL segments.
         *
         * For this reason we use 15 characters. It is enough but also means
         * that in the future we can't decrease SLRU_PAGES_PER_SEGMENT easily.
         */
        Assert(segno >= 0 && segno <= INT64CONST(0xFFFFFFFFFFFFFFF));
        return snprintf(path, MAXPGPATH, "%s/%015" PRIX64, ctl->Dir, segno);
    }
    else
    {
        /*
         * Despite the fact that %04X format string is used up to 24 bit
         * integers are allowed. See SlruCorrectSegmentFilenameLength()
         */
        Assert(segno >= 0 && segno <= INT64CONST(0xFFFFFF));
        return snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir,
                        (unsigned int) segno);
    }
}
 
/*
 * During SimpleLruWriteAll(), we will usually not need to write more than one
 * or two physical files, but we may need to write several pages per file.  We
 * can consolidate the I/O requests by leaving files open until control returns
 * to SimpleLruWriteAll().  This data structure remembers which files are open.
 */
#define MAX_WRITEALL_BUFFERS    16
 
typedef struct SlruWriteAllData
{
    int         num_files;      /* # files actually open */
    int         fd[MAX_WRITEALL_BUFFERS];   /* their FD's */
    int64       segno[MAX_WRITEALL_BUFFERS];    /* their log seg#s */
} SlruWriteAllData;
 
typedef struct SlruWriteAllData *SlruWriteAll;
 
 
/*
 * Bank size for the slot array.  Pages are assigned a bank according to their
 * page number, with each bank being this size.  We want a power of 2 so that
 * we can determine the bank number for a page with just bit shifting; we also
 * want to keep the bank size small so that LRU victim search is fast.  16
 * buffers per bank seems a good number.
 */
#define SLRU_BANK_BITSHIFT      4
#define SLRU_BANK_SIZE          (1 << SLRU_BANK_BITSHIFT)
 
/*
 * Macro to get the bank number to which the slot belongs.
 */
#define SlotGetBankNumber(slotno)   ((slotno) >> SLRU_BANK_BITSHIFT)
 
 
/*
 * Populate a file tag describing a segment file.  We only use the segment
 * number, since we can derive everything else we need by having separate
 * sync handler functions for clog, multixact etc.
 */
#define INIT_SLRUFILETAG(a,xx_handler,xx_segno) \
( \
    memset(&(a), 0, sizeof(FileTag)), \
    (a).handler = (xx_handler), \
    (a).segno = (xx_segno) \
)
 
/* Saved info for SlruReportIOError */
typedef enum
{
    SLRU_OPEN_FAILED,
    SLRU_SEEK_FAILED,
    SLRU_READ_FAILED,
    SLRU_WRITE_FAILED,
    SLRU_FSYNC_FAILED,
    SLRU_CLOSE_FAILED,
} SlruErrorCause;
 
static SlruErrorCause slru_errcause;
static int  slru_errno;
 
 
static void SimpleLruZeroLSNs(SlruCtl ctl, int slotno);
static void SimpleLruWaitIO(SlruCtl ctl, int slotno);
static void SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata);
static bool SlruPhysicalReadPage(SlruCtl ctl, int64 pageno, int slotno);
static bool SlruPhysicalWritePage(SlruCtl ctl, int64 pageno, int slotno,
                                  SlruWriteAll fdata);
static void SlruReportIOError(SlruCtl ctl, int64 pageno, TransactionId xid);
static int  SlruSelectLRUPage(SlruCtl ctl, int64 pageno);
 
static bool SlruScanDirCbDeleteCutoff(SlruCtl ctl, char *filename,
                                      int64 segpage, void *data);
static void SlruInternalDeleteSegment(SlruCtl ctl, int64 segno);
static inline void SlruRecentlyUsed(SlruShared shared, int slotno);
 
 
/*
 * Initialization of shared memory
 */
 
Size
SimpleLruShmemSize(int nslots, int nlsns)
{
    int         nbanks = nslots / SLRU_BANK_SIZE;
    Size        sz;
 
    Assert(nslots <= SLRU_MAX_ALLOWED_BUFFERS);
    Assert(nslots % SLRU_BANK_SIZE == 0);
 
    /* we assume nslots isn't so large as to risk overflow */
    sz = MAXALIGN(sizeof(SlruSharedData));
    sz += MAXALIGN(nslots * sizeof(char *));    /* page_buffer[] */
    sz += MAXALIGN(nslots * sizeof(SlruPageStatus));    /* page_status[] */
    sz += MAXALIGN(nslots * sizeof(bool));  /* page_dirty[] */
    sz += MAXALIGN(nslots * sizeof(int64)); /* page_number[] */
    sz += MAXALIGN(nslots * sizeof(int));   /* page_lru_count[] */
    sz += MAXALIGN(nslots * sizeof(LWLockPadded));  /* buffer_locks[] */
    sz += MAXALIGN(nbanks * sizeof(LWLockPadded));  /* bank_locks[] */
    sz += MAXALIGN(nbanks * sizeof(int));   /* bank_cur_lru_count[] */
 
    if (nlsns > 0)
        sz += MAXALIGN(nslots * nlsns * sizeof(XLogRecPtr));    /* group_lsn[] */
 
    return BUFFERALIGN(sz) + BLCKSZ * nslots;
}
 
/*
 * Determine a number of SLRU buffers to use.
 *
 * We simply divide shared_buffers by the divisor given and cap
 * that at the maximum given; but always at least SLRU_BANK_SIZE.
 * Round down to the nearest multiple of SLRU_BANK_SIZE.
 */
int
SimpleLruAutotuneBuffers(int divisor, int max)
{
    return Min(max - (max % SLRU_BANK_SIZE),
               Max(SLRU_BANK_SIZE,
                   NBuffers / divisor - (NBuffers / divisor) % SLRU_BANK_SIZE));
}
 
/*
 * Initialize, or attach to, a simple LRU cache in shared memory.
 *
 * ctl: address of local (unshared) control structure.
 * name: name of SLRU.  (This is user-visible, pick with care!)
 * nslots: number of page slots to use.
 * nlsns: number of LSN groups per page (set to zero if not relevant).
 * subdir: PGDATA-relative subdirectory that will contain the files.
 * buffer_tranche_id: tranche ID to use for the SLRU's per-buffer LWLocks.
 * bank_tranche_id: tranche ID to use for the bank LWLocks.
 * sync_handler: which set of functions to use to handle sync requests
 * long_segment_names: use short or long segment names
 */
void
SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
              const char *subdir, int buffer_tranche_id, int bank_tranche_id,
              SyncRequestHandler sync_handler, bool long_segment_names)
{
    SlruShared  shared;
    bool        found;
    int         nbanks = nslots / SLRU_BANK_SIZE;
 
    Assert(nslots <= SLRU_MAX_ALLOWED_BUFFERS);
 
    shared = (SlruShared) ShmemInitStruct(name,
                                          SimpleLruShmemSize(nslots, nlsns),
                                          &found);
 
    if (!IsUnderPostmaster)
    {
        /* Initialize locks and shared memory area */
        char       *ptr;
        Size        offset;
 
        Assert(!found);
 
        memset(shared, 0, sizeof(SlruSharedData));
 
        shared->num_slots = nslots;
        shared->lsn_groups_per_page = nlsns;
 
        pg_atomic_init_u64(&shared->latest_page_number, 0);
 
        shared->slru_stats_idx = pgstat_get_slru_index(name);
 
        ptr = (char *) shared;
        offset = MAXALIGN(sizeof(SlruSharedData));
        shared->page_buffer = (char **) (ptr + offset);
        offset += MAXALIGN(nslots * sizeof(char *));
        shared->page_status = (SlruPageStatus *) (ptr + offset);
        offset += MAXALIGN(nslots * sizeof(SlruPageStatus));
        shared->page_dirty = (bool *) (ptr + offset);
        offset += MAXALIGN(nslots * sizeof(bool));
        shared->page_number = (int64 *) (ptr + offset);
        offset += MAXALIGN(nslots * sizeof(int64));
        shared->page_lru_count = (int *) (ptr + offset);
        offset += MAXALIGN(nslots * sizeof(int));
 
        /* Initialize LWLocks */
        shared->buffer_locks = (LWLockPadded *) (ptr + offset);
        offset += MAXALIGN(nslots * sizeof(LWLockPadded));
        shared->bank_locks = (LWLockPadded *) (ptr + offset);
        offset += MAXALIGN(nbanks * sizeof(LWLockPadded));
        shared->bank_cur_lru_count = (int *) (ptr + offset);
        offset += MAXALIGN(nbanks * sizeof(int));
 
        if (nlsns > 0)
        {
            shared->group_lsn = (XLogRecPtr *) (ptr + offset);
            offset += MAXALIGN(nslots * nlsns * sizeof(XLogRecPtr));
        }
 
        ptr += BUFFERALIGN(offset);
        for (int slotno = 0; slotno < nslots; slotno++)
        {
            LWLockInitialize(&shared->buffer_locks[slotno].lock,
                             buffer_tranche_id);
 
            shared->page_buffer[slotno] = ptr;
            shared->page_status[slotno] = SLRU_PAGE_EMPTY;
            shared->page_dirty[slotno] = false;
            shared->page_lru_count[slotno] = 0;
            ptr += BLCKSZ;
        }
 
        /* Initialize the slot banks. */
        for (int bankno = 0; bankno < nbanks; bankno++)
        {
            LWLockInitialize(&shared->bank_locks[bankno].lock, bank_tranche_id);
            shared->bank_cur_lru_count[bankno] = 0;
        }
 
        /* Should fit to estimated shmem size */
        Assert(ptr - (char *) shared <= SimpleLruShmemSize(nslots, nlsns));
    }
    else
    {
        Assert(found);
        Assert(shared->num_slots == nslots);
    }
 
    /*
     * Initialize the unshared control struct, including directory path. We
     * assume caller set PagePrecedes.
     */
    ctl->shared = shared;
    ctl->sync_handler = sync_handler;
    ctl->long_segment_names = long_segment_names;
    ctl->nbanks = nbanks;
    strlcpy(ctl->Dir, subdir, sizeof(ctl->Dir));
}
 
/*
 * Helper function for GUC check_hook to check whether slru buffers are in
 * multiples of SLRU_BANK_SIZE.
 */
bool
check_slru_buffers(const char *name, int *newval)
{
    /* Valid values are multiples of SLRU_BANK_SIZE */
    if (*newval % SLRU_BANK_SIZE == 0)
        return true;
 
    GUC_check_errdetail("\"%s\" must be a multiple of %d.", name,
                        SLRU_BANK_SIZE);
    return false;
}
 
/*
 * Initialize (or reinitialize) a page to zeroes.
 *
 * The page is not actually written, just set up in shared memory.
 * The slot number of the new page is returned.
 *
 * Bank lock must be held at entry, and will be held at exit.
 */
int
SimpleLruZeroPage(SlruCtl ctl, int64 pageno)
{
    SlruShared  shared = ctl->shared;
    int         slotno;
 
    Assert(LWLockHeldByMeInMode(SimpleLruGetBankLock(ctl, pageno), LW_EXCLUSIVE));
 
    /* Find a suitable buffer slot for the page */
    slotno = SlruSelectLRUPage(ctl, pageno);
    Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
           (shared->page_status[slotno] == SLRU_PAGE_VALID &&
            !shared->page_dirty[slotno]) ||
           shared->page_number[slotno] == pageno);
 
    /* Mark the slot as containing this page */
    shared->page_number[slotno] = pageno;
    shared->page_status[slotno] = SLRU_PAGE_VALID;
    shared->page_dirty[slotno] = true;
    SlruRecentlyUsed(shared, slotno);
 
    /* Set the buffer to zeroes */
    MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
 
    /* Set the LSNs for this new page to zero */
    SimpleLruZeroLSNs(ctl, slotno);
 
    /*
     * Assume this page is now the latest active page.
     *
     * Note that because both this routine and SlruSelectLRUPage run with a
     * SLRU bank lock held, it is not possible for this to be zeroing a page
     * that SlruSelectLRUPage is going to evict simultaneously.  Therefore,
     * there's no memory barrier here.
     */
    pg_atomic_write_u64(&shared->latest_page_number, pageno);
 
    /* update the stats counter of zeroed pages */
    pgstat_count_slru_blocks_zeroed(shared->slru_stats_idx);
 
    return slotno;
}
 
/*
 * Zero all the LSNs we store for this slru page.
 *
 * This should be called each time we create a new page, and each time we read
 * in a page from disk into an existing buffer.  (Such an old page cannot
 * have any interesting LSNs, since we'd have flushed them before writing
 * the page in the first place.)
 *
 * This assumes that InvalidXLogRecPtr is bitwise-all-0.
 */
static void
SimpleLruZeroLSNs(SlruCtl ctl, int slotno)
{
    SlruShared  shared = ctl->shared;
 
    if (shared->lsn_groups_per_page > 0)
        MemSet(&shared->group_lsn[slotno * shared->lsn_groups_per_page], 0,
               shared->lsn_groups_per_page * sizeof(XLogRecPtr));
}
 
/*
 * This is a convenience wrapper for the common case of zeroing a page and
 * immediately flushing it to disk.
 *
 * SLRU bank lock is acquired and released here.
 */
void
SimpleLruZeroAndWritePage(SlruCtl ctl, int64 pageno)
{
    int         slotno;
    LWLock     *lock;
 
    lock = SimpleLruGetBankLock(ctl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);
 
    /* Create and zero the page */
    slotno = SimpleLruZeroPage(ctl, pageno);
 
    /* Make sure it's written out */
    SimpleLruWritePage(ctl, slotno);
    Assert(!ctl->shared->page_dirty[slotno]);
 
    LWLockRelease(lock);
}
 
/*
 * Wait for any active I/O on a page slot to finish.  (This does not
 * guarantee that new I/O hasn't been started before we return, though.
 * In fact the slot might not even contain the same page anymore.)
 *
 * Bank lock must be held at entry, and will be held at exit.
 */
static void
SimpleLruWaitIO(SlruCtl ctl, int slotno)
{
    SlruShared  shared = ctl->shared;
    int         bankno = SlotGetBankNumber(slotno);
 
    Assert(shared->page_status[slotno] != SLRU_PAGE_EMPTY);
 
    /* See notes at top of file */
    LWLockRelease(&shared->bank_locks[bankno].lock);
    LWLockAcquire(&shared->buffer_locks[slotno].lock, LW_SHARED);
    LWLockRelease(&shared->buffer_locks[slotno].lock);
    LWLockAcquire(&shared->bank_locks[bankno].lock, LW_EXCLUSIVE);
 
    /*
     * If the slot is still in an io-in-progress state, then either someone
     * already started a new I/O on the slot, or a previous I/O failed and
     * neglected to reset the page state.  That shouldn't happen, really, but
     * it seems worth a few extra cycles to check and recover from it. We can
     * cheaply test for failure by seeing if the buffer lock is still held (we
     * assume that transaction abort would release the lock).
     */
    if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS ||
        shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS)
    {
        if (LWLockConditionalAcquire(&shared->buffer_locks[slotno].lock, LW_SHARED))
        {
            /* indeed, the I/O must have failed */
            if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS)
                shared->page_status[slotno] = SLRU_PAGE_EMPTY;
            else                /* write_in_progress */
            {
                shared->page_status[slotno] = SLRU_PAGE_VALID;
                shared->page_dirty[slotno] = true;
            }
            LWLockRelease(&shared->buffer_locks[slotno].lock);
        }
    }
}
 
/*
 * Find a page in a shared buffer, reading it in if necessary.
 * The page number must correspond to an already-initialized page.
 *
 * If write_ok is true then it is OK to return a page that is in
 * WRITE_IN_PROGRESS state; it is the caller's responsibility to be sure
 * that modification of the page is safe.  If write_ok is false then we
 * will not return the page until it is not undergoing active I/O.
 *
 * The passed-in xid is used only for error reporting, and may be
 * InvalidTransactionId if no specific xid is associated with the action.
 *
 * Return value is the shared-buffer slot number now holding the page.
 * The buffer's LRU access info is updated.
 *
 * The correct bank lock must be held at entry, and will be held at exit.
 */
int
SimpleLruReadPage(SlruCtl ctl, int64 pageno, bool write_ok,
                  TransactionId xid)
{
    SlruShared  shared = ctl->shared;
    LWLock     *banklock = SimpleLruGetBankLock(ctl, pageno);
 
    Assert(LWLockHeldByMeInMode(banklock, LW_EXCLUSIVE));
 
    /* Outer loop handles restart if we must wait for someone else's I/O */
    for (;;)
    {
        int         slotno;
        bool        ok;
 
        /* See if page already is in memory; if not, pick victim slot */
        slotno = SlruSelectLRUPage(ctl, pageno);
 
        /* Did we find the page in memory? */
        if (shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
            shared->page_number[slotno] == pageno)
        {
            /*
             * If page is still being read in, we must wait for I/O.  Likewise
             * if the page is being written and the caller said that's not OK.
             */
            if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS ||
                (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS &&
                 !write_ok))
            {
                SimpleLruWaitIO(ctl, slotno);
                /* Now we must recheck state from the top */
                continue;
            }
            /* Otherwise, it's ready to use */
            SlruRecentlyUsed(shared, slotno);
 
            /* update the stats counter of pages found in the SLRU */
            pgstat_count_slru_blocks_hit(shared->slru_stats_idx);
 
            return slotno;
        }
 
        /* We found no match; assert we selected a freeable slot */
        Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
               (shared->page_status[slotno] == SLRU_PAGE_VALID &&
                !shared->page_dirty[slotno]));
 
        /* Mark the slot read-busy */
        shared->page_number[slotno] = pageno;
        shared->page_status[slotno] = SLRU_PAGE_READ_IN_PROGRESS;
        shared->page_dirty[slotno] = false;
 
        /* Acquire per-buffer lock (cannot deadlock, see notes at top) */
        LWLockAcquire(&shared->buffer_locks[slotno].lock, LW_EXCLUSIVE);
 
        /* Release bank lock while doing I/O */
        LWLockRelease(banklock);
 
        /* Do the read */
        ok = SlruPhysicalReadPage(ctl, pageno, slotno);
 
        /* Set the LSNs for this newly read-in page to zero */
        SimpleLruZeroLSNs(ctl, slotno);
 
        /* Re-acquire bank control lock and update page state */
        LWLockAcquire(banklock, LW_EXCLUSIVE);
 
        Assert(shared->page_number[slotno] == pageno &&
               shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS &&
               !shared->page_dirty[slotno]);
 
        shared->page_status[slotno] = ok ? SLRU_PAGE_VALID : SLRU_PAGE_EMPTY;
 
        LWLockRelease(&shared->buffer_locks[slotno].lock);
 
        /* Now it's okay to ereport if we failed */
        if (!ok)
            SlruReportIOError(ctl, pageno, xid);
 
        SlruRecentlyUsed(shared, slotno);
 
        /* update the stats counter of pages not found in SLRU */
        pgstat_count_slru_blocks_read(shared->slru_stats_idx);
 
        return slotno;
    }
}
 
/*
 * Find a page in a shared buffer, reading it in if necessary.
 * The page number must correspond to an already-initialized page.
 * The caller must intend only read-only access to the page.
 *
 * The passed-in xid is used only for error reporting, and may be
 * InvalidTransactionId if no specific xid is associated with the action.
 *
 * Return value is the shared-buffer slot number now holding the page.
 * The buffer's LRU access info is updated.
 *
 * Bank control lock must NOT be held at entry, but will be held at exit.
 * It is unspecified whether the lock will be shared or exclusive.
 */
int
SimpleLruReadPage_ReadOnly(SlruCtl ctl, int64 pageno, TransactionId xid)
{
    SlruShared  shared = ctl->shared;
    LWLock     *banklock = SimpleLruGetBankLock(ctl, pageno);
    int         bankno = pageno % ctl->nbanks;
    int         bankstart = bankno * SLRU_BANK_SIZE;
    int         bankend = bankstart + SLRU_BANK_SIZE;
 
    /* Try to find the page while holding only shared lock */
    LWLockAcquire(banklock, LW_SHARED);
 
    /* See if page is already in a buffer */
    for (int slotno = bankstart; slotno < bankend; slotno++)
    {
        if (shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
            shared->page_number[slotno] == pageno &&
            shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
        {
            /* See comments for SlruRecentlyUsed() */
            SlruRecentlyUsed(shared, slotno);
 
            /* update the stats counter of pages found in the SLRU */
            pgstat_count_slru_blocks_hit(shared->slru_stats_idx);
 
            return slotno;
        }
    }
 
    /* No luck, so switch to normal exclusive lock and do regular read */
    LWLockRelease(banklock);
    LWLockAcquire(banklock, LW_EXCLUSIVE);
 
    return SimpleLruReadPage(ctl, pageno, true, xid);
}
 
/*
 * Write a page from a shared buffer, if necessary.
 * Does nothing if the specified slot is not dirty.
 *
 * NOTE: only one write attempt is made here.  Hence, it is possible that
 * the page is still dirty at exit (if someone else re-dirtied it during
 * the write).  However, we *do* attempt a fresh write even if the page
 * is already being written; this is for checkpoints.
 *
 * Bank lock must be held at entry, and will be held at exit.
 */
static void
SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata)
{
    SlruShared  shared = ctl->shared;
    int64       pageno = shared->page_number[slotno];
    int         bankno = SlotGetBankNumber(slotno);
    bool        ok;
 
    Assert(shared->page_status[slotno] != SLRU_PAGE_EMPTY);
    Assert(LWLockHeldByMeInMode(SimpleLruGetBankLock(ctl, pageno), LW_EXCLUSIVE));
 
    /* If a write is in progress, wait for it to finish */
    while (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS &&
           shared->page_number[slotno] == pageno)
    {
        SimpleLruWaitIO(ctl, slotno);
    }
 
    /*
     * Do nothing if page is not dirty, or if buffer no longer contains the
     * same page we were called for.
     */
    if (!shared->page_dirty[slotno] ||
        shared->page_status[slotno] != SLRU_PAGE_VALID ||
        shared->page_number[slotno] != pageno)
        return;
 
    /*
     * Mark the slot write-busy, and clear the dirtybit.  After this point, a
     * transaction status update on this page will mark it dirty again.
     */
    shared->page_status[slotno] = SLRU_PAGE_WRITE_IN_PROGRESS;
    shared->page_dirty[slotno] = false;
 
    /* Acquire per-buffer lock (cannot deadlock, see notes at top) */
    LWLockAcquire(&shared->buffer_locks[slotno].lock, LW_EXCLUSIVE);
 
    /* Release bank lock while doing I/O */
    LWLockRelease(&shared->bank_locks[bankno].lock);
 
    /* Do the write */
    ok = SlruPhysicalWritePage(ctl, pageno, slotno, fdata);
 
    /* If we failed, and we're in a flush, better close the files */
    if (!ok && fdata)
    {
        for (int i = 0; i < fdata->num_files; i++)
            CloseTransientFile(fdata->fd[i]);
    }
 
    /* Re-acquire bank lock and update page state */
    LWLockAcquire(&shared->bank_locks[bankno].lock, LW_EXCLUSIVE);
 
    Assert(shared->page_number[slotno] == pageno &&
           shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS);
 
    /* If we failed to write, mark the page dirty again */
    if (!ok)
        shared->page_dirty[slotno] = true;
 
    shared->page_status[slotno] = SLRU_PAGE_VALID;
 
    LWLockRelease(&shared->buffer_locks[slotno].lock);
 
    /* Now it's okay to ereport if we failed */
    if (!ok)
        SlruReportIOError(ctl, pageno, InvalidTransactionId);
 
    /* If part of a checkpoint, count this as a SLRU buffer written. */
    if (fdata)
    {
        CheckpointStats.ckpt_slru_written++;
        PendingCheckpointerStats.slru_written++;
    }
}
 
/*
 * Wrapper of SlruInternalWritePage, for external callers.
 * fdata is always passed a NULL here.
 */
void
SimpleLruWritePage(SlruCtl ctl, int slotno)
{
    Assert(ctl->shared->page_status[slotno] != SLRU_PAGE_EMPTY);
 
    SlruInternalWritePage(ctl, slotno, NULL);
}
 
/*
 * Return whether the given page exists on disk.
 *
 * A false return means that either the file does not exist, or that it's not
 * large enough to contain the given page.
 */
bool
SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int64 pageno)
{
    int64       segno = pageno / SLRU_PAGES_PER_SEGMENT;
    int         rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
    int         offset = rpageno * BLCKSZ;
    char        path[MAXPGPATH];
    int         fd;
    bool        result;
    off_t       endpos;
 
    /* update the stats counter of checked pages */
    pgstat_count_slru_blocks_exists(ctl->shared->slru_stats_idx);
 
    SlruFileName(ctl, path, segno);
 
    fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
    if (fd < 0)
    {
        /* expected: file doesn't exist */
        if (errno == ENOENT)
            return false;
 
        /* report error normally */
        slru_errcause = SLRU_OPEN_FAILED;
        slru_errno = errno;
        SlruReportIOError(ctl, pageno, 0);
    }
 
    if ((endpos = lseek(fd, 0, SEEK_END)) < 0)
    {
        slru_errcause = SLRU_SEEK_FAILED;
        slru_errno = errno;
        SlruReportIOError(ctl, pageno, 0);
    }
 
    result = endpos >= (off_t) (offset + BLCKSZ);
 
    if (CloseTransientFile(fd) != 0)
    {
        slru_errcause = SLRU_CLOSE_FAILED;
        slru_errno = errno;
        return false;
    }
 
    return result;
}
 
/*
 * Physical read of a (previously existing) page into a buffer slot
 *
 * On failure, we cannot just ereport(ERROR) since caller has put state in
 * shared memory that must be undone.  So, we return false and save enough
 * info in static variables to let SlruReportIOError make the report.
 *
 * For now, assume it's not worth keeping a file pointer open across
 * read/write operations.  We could cache one virtual file pointer ...
 */
static bool
SlruPhysicalReadPage(SlruCtl ctl, int64 pageno, int slotno)
{
    SlruShared  shared = ctl->shared;
    int64       segno = pageno / SLRU_PAGES_PER_SEGMENT;
    int         rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
    off_t       offset = rpageno * BLCKSZ;
    char        path[MAXPGPATH];
    int         fd;
 
    SlruFileName(ctl, path, segno);
 
    /*
     * In a crash-and-restart situation, it's possible for us to receive
     * commands to set the commit status of transactions whose bits are in
     * already-truncated segments of the commit log (see notes in
     * SlruPhysicalWritePage).  Hence, if we are InRecovery, allow the case
     * where the file doesn't exist, and return zeroes instead.
     */
    fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
    if (fd < 0)
    {
        if (errno != ENOENT || !InRecovery)
        {
            slru_errcause = SLRU_OPEN_FAILED;
            slru_errno = errno;
            return false;
        }
 
        ereport(LOG,
                (errmsg("file \"%s\" doesn't exist, reading as zeroes",
                        path)));
        MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
        return true;
    }
 
    errno = 0;
    pgstat_report_wait_start(WAIT_EVENT_SLRU_READ);
    if (pg_pread(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
    {
        pgstat_report_wait_end();
        slru_errcause = SLRU_READ_FAILED;
        slru_errno = errno;
        CloseTransientFile(fd);
        return false;
    }
    pgstat_report_wait_end();
 
    if (CloseTransientFile(fd) != 0)
    {
        slru_errcause = SLRU_CLOSE_FAILED;
        slru_errno = errno;
        return false;
    }
 
    return true;
}
 
/*
 * Physical write of a page from a buffer slot
 *
 * On failure, we cannot just ereport(ERROR) since caller has put state in
 * shared memory that must be undone.  So, we return false and save enough
 * info in static variables to let SlruReportIOError make the report.
 *
 * For now, assume it's not worth keeping a file pointer open across
 * independent read/write operations.  We do batch operations during
 * SimpleLruWriteAll, though.
 *
 * fdata is NULL for a standalone write, pointer to open-file info during
 * SimpleLruWriteAll.
 */
static bool
SlruPhysicalWritePage(SlruCtl ctl, int64 pageno, int slotno, SlruWriteAll fdata)
{
    SlruShared  shared = ctl->shared;
    int64       segno = pageno / SLRU_PAGES_PER_SEGMENT;
    int         rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
    off_t       offset = rpageno * BLCKSZ;
    char        path[MAXPGPATH];
    int         fd = -1;
 
    /* update the stats counter of written pages */
    pgstat_count_slru_blocks_written(shared->slru_stats_idx);
 
    /*
     * Honor the write-WAL-before-data rule, if appropriate, so that we do not
     * write out data before associated WAL records.  This is the same action
     * performed during FlushBuffer() in the main buffer manager.
     */
    if (shared->group_lsn != NULL)
    {
        /*
         * We must determine the largest async-commit LSN for the page. This
         * is a bit tedious, but since this entire function is a slow path
         * anyway, it seems better to do this here than to maintain a per-page
         * LSN variable (which'd need an extra comparison in the
         * transaction-commit path).
         */
        XLogRecPtr  max_lsn;
        int         lsnindex;
 
        lsnindex = slotno * shared->lsn_groups_per_page;
        max_lsn = shared->group_lsn[lsnindex++];
        for (int lsnoff = 1; lsnoff < shared->lsn_groups_per_page; lsnoff++)
        {
            XLogRecPtr  this_lsn = shared->group_lsn[lsnindex++];
 
            if (max_lsn < this_lsn)
                max_lsn = this_lsn;
        }
 
        if (!XLogRecPtrIsInvalid(max_lsn))
        {
            /*
             * As noted above, elog(ERROR) is not acceptable here, so if
             * XLogFlush were to fail, we must PANIC.  This isn't much of a
             * restriction because XLogFlush is just about all critical
             * section anyway, but let's make sure.
             */
            START_CRIT_SECTION();
            XLogFlush(max_lsn);
            END_CRIT_SECTION();
        }
    }
 
    /*
     * During a SimpleLruWriteAll, we may already have the desired file open.
     */
    if (fdata)
    {
        for (int i = 0; i < fdata->num_files; i++)
        {
            if (fdata->segno[i] == segno)
            {
                fd = fdata->fd[i];
                break;
            }
        }
    }
 
    if (fd < 0)
    {
        /*
         * If the file doesn't already exist, we should create it.  It is
         * possible for this to need to happen when writing a page that's not
         * first in its segment; we assume the OS can cope with that. (Note:
         * it might seem that it'd be okay to create files only when
         * SimpleLruZeroPage is called for the first page of a segment.
         * However, if after a crash and restart the REDO logic elects to
         * replay the log from a checkpoint before the latest one, then it's
         * possible that we will get commands to set transaction status of
         * transactions that have already been truncated from the commit log.
         * Easiest way to deal with that is to accept references to
         * nonexistent files here and in SlruPhysicalReadPage.)
         *
         * Note: it is possible for more than one backend to be executing this
         * code simultaneously for different pages of the same file. Hence,
         * don't use O_EXCL or O_TRUNC or anything like that.
         */
        SlruFileName(ctl, path, segno);
        fd = OpenTransientFile(path, O_RDWR | O_CREAT | PG_BINARY);
        if (fd < 0)
        {
            slru_errcause = SLRU_OPEN_FAILED;
            slru_errno = errno;
            return false;
        }
 
        if (fdata)
        {
            if (fdata->num_files < MAX_WRITEALL_BUFFERS)
            {
                fdata->fd[fdata->num_files] = fd;
                fdata->segno[fdata->num_files] = segno;
                fdata->num_files++;
            }
            else
            {
                /*
                 * In the unlikely event that we exceed MAX_WRITEALL_BUFFERS,
                 * fall back to treating it as a standalone write.
                 */
                fdata = NULL;
            }
        }
    }
 
    errno = 0;
    pgstat_report_wait_start(WAIT_EVENT_SLRU_WRITE);
    if (pg_pwrite(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
    {
        pgstat_report_wait_end();
        /* if write didn't set errno, assume problem is no disk space */
        if (errno == 0)
            errno = ENOSPC;
        slru_errcause = SLRU_WRITE_FAILED;
        slru_errno = errno;
        if (!fdata)
            CloseTransientFile(fd);
        return false;
    }
    pgstat_report_wait_end();
 
    /* Queue up a sync request for the checkpointer. */
    if (ctl->sync_handler != SYNC_HANDLER_NONE)
    {
        FileTag     tag;
 
        INIT_SLRUFILETAG(tag, ctl->sync_handler, segno);
        if (!RegisterSyncRequest(&tag, SYNC_REQUEST, false))
        {
            /* No space to enqueue sync request.  Do it synchronously. */
            pgstat_report_wait_start(WAIT_EVENT_SLRU_SYNC);
            if (pg_fsync(fd) != 0)
            {
                pgstat_report_wait_end();
                slru_errcause = SLRU_FSYNC_FAILED;
                slru_errno = errno;
                CloseTransientFile(fd);
                return false;
            }
            pgstat_report_wait_end();
        }
    }
 
    /* Close file, unless part of flush request. */
    if (!fdata)
    {
        if (CloseTransientFile(fd) != 0)
        {
            slru_errcause = SLRU_CLOSE_FAILED;
            slru_errno = errno;
            return false;
        }
    }
 
    return true;
}
 
/*
 * Issue the error message after failure of SlruPhysicalReadPage or
 * SlruPhysicalWritePage.  Call this after cleaning up shared-memory state.
 */
static void
SlruReportIOError(SlruCtl ctl, int64 pageno, TransactionId xid)
{
    int64       segno = pageno / SLRU_PAGES_PER_SEGMENT;
    int         rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
    int         offset = rpageno * BLCKSZ;
    char        path[MAXPGPATH];
 
    SlruFileName(ctl, path, segno);
    errno = slru_errno;
    switch (slru_errcause)
    {
        case SLRU_OPEN_FAILED:
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not access status of transaction %u", xid),
                     errdetail("Could not open file \"%s\": %m.", path)));
            break;
        case SLRU_SEEK_FAILED:
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not access status of transaction %u", xid),
                     errdetail("Could not seek in file \"%s\" to offset %d: %m.",
                               path, offset)));
            break;
        case SLRU_READ_FAILED:
            if (errno)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not access status of transaction %u", xid),
                         errdetail("Could not read from file \"%s\" at offset %d: %m.",
                                   path, offset)));
            else
                ereport(ERROR,
                        (errmsg("could not access status of transaction %u", xid),
                         errdetail("Could not read from file \"%s\" at offset %d: read too few bytes.", path, offset)));
            break;
        case SLRU_WRITE_FAILED:
            if (errno)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not access status of transaction %u", xid),
                         errdetail("Could not write to file \"%s\" at offset %d: %m.",
                                   path, offset)));
            else
                ereport(ERROR,
                        (errmsg("could not access status of transaction %u", xid),
                         errdetail("Could not write to file \"%s\" at offset %d: wrote too few bytes.",
                                   path, offset)));
            break;
        case SLRU_FSYNC_FAILED:
            ereport(data_sync_elevel(ERROR),
                    (errcode_for_file_access(),
                     errmsg("could not access status of transaction %u", xid),
                     errdetail("Could not fsync file \"%s\": %m.",
                               path)));
            break;
        case SLRU_CLOSE_FAILED:
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not access status of transaction %u", xid),
                     errdetail("Could not close file \"%s\": %m.",
                               path)));
            break;
        default:
            /* can't get here, we trust */
            elog(ERROR, "unrecognized SimpleLru error cause: %d",
                 (int) slru_errcause);
            break;
    }
}
 
/*
 * Mark a buffer slot "most recently used".
 */
static inline void
SlruRecentlyUsed(SlruShared shared, int slotno)
{
    int         bankno = SlotGetBankNumber(slotno);
    int         new_lru_count = shared->bank_cur_lru_count[bankno];
 
    Assert(shared->page_status[slotno] != SLRU_PAGE_EMPTY);
 
    /*
     * The reason for the if-test is that there are often many consecutive
     * accesses to the same page (particularly the latest page).  By
     * suppressing useless increments of bank_cur_lru_count, we reduce the
     * probability that old pages' counts will "wrap around" and make them
     * appear recently used.
     *
     * We allow this code to be executed concurrently by multiple processes
     * within SimpleLruReadPage_ReadOnly().  As long as int reads and writes
     * are atomic, this should not cause any completely-bogus values to enter
     * the computation.  However, it is possible for either bank_cur_lru_count
     * or individual page_lru_count entries to be "reset" to lower values than
     * they should have, in case a process is delayed while it executes this
     * function.  With care in SlruSelectLRUPage(), this does little harm, and
     * in any case the absolute worst possible consequence is a nonoptimal
     * choice of page to evict.  The gain from allowing concurrent reads of
     * SLRU pages seems worth it.
     */
    if (new_lru_count != shared->page_lru_count[slotno])
    {
        shared->bank_cur_lru_count[bankno] = ++new_lru_count;
        shared->page_lru_count[slotno] = new_lru_count;
    }
}
 
/*
 * Select the slot to re-use when we need a free slot for the given page.
 *
 * The target page number is passed not only because we need to know the
 * correct bank to use, but also because we need to consider the possibility
 * that some other process reads in the target page while we are doing I/O to
 * free a slot.  Hence, check or recheck to see if any slot already holds the
 * target page, and return that slot if so.  Thus, the returned slot is
 * *either* a slot already holding the pageno (could be any state except
 * EMPTY), *or* a freeable slot (state EMPTY or CLEAN).
 *
 * The correct bank lock must be held at entry, and will be held at exit.
 */
static int
SlruSelectLRUPage(SlruCtl ctl, int64 pageno)
{
    SlruShared  shared = ctl->shared;
 
    /* Outer loop handles restart after I/O */
    for (;;)
    {
        int         cur_count;
        int         bestvalidslot = 0;  /* keep compiler quiet */
        int         best_valid_delta = -1;
        int64       best_valid_page_number = 0; /* keep compiler quiet */
        int         bestinvalidslot = 0;    /* keep compiler quiet */
        int         best_invalid_delta = -1;
        int64       best_invalid_page_number = 0;   /* keep compiler quiet */
        int         bankno = pageno % ctl->nbanks;
        int         bankstart = bankno * SLRU_BANK_SIZE;
        int         bankend = bankstart + SLRU_BANK_SIZE;
 
        Assert(LWLockHeldByMe(SimpleLruGetBankLock(ctl, pageno)));
 
        /* See if page already has a buffer assigned */
        for (int slotno = bankstart; slotno < bankend; slotno++)
        {
            if (shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
                shared->page_number[slotno] == pageno)
                return slotno;
        }
 
        /*
         * If we find any EMPTY slot, just select that one. Else choose a
         * victim page to replace.  We normally take the least recently used
         * valid page, but we will never take the slot containing
         * latest_page_number, even if it appears least recently used.  We
         * will select a slot that is already I/O busy only if there is no
         * other choice: a read-busy slot will not be least recently used once
         * the read finishes, and waiting for an I/O on a write-busy slot is
         * inferior to just picking some other slot.  Testing shows the slot
         * we pick instead will often be clean, allowing us to begin a read at
         * once.
         *
         * Normally the page_lru_count values will all be different and so
         * there will be a well-defined LRU page.  But since we allow
         * concurrent execution of SlruRecentlyUsed() within
         * SimpleLruReadPage_ReadOnly(), it is possible that multiple pages
         * acquire the same lru_count values.  In that case we break ties by
         * choosing the furthest-back page.
         *
         * Notice that this next line forcibly advances cur_lru_count to a
         * value that is certainly beyond any value that will be in the
         * page_lru_count array after the loop finishes.  This ensures that
         * the next execution of SlruRecentlyUsed will mark the page newly
         * used, even if it's for a page that has the current counter value.
         * That gets us back on the path to having good data when there are
         * multiple pages with the same lru_count.
         */
        cur_count = (shared->bank_cur_lru_count[bankno])++;
        for (int slotno = bankstart; slotno < bankend; slotno++)
        {
            int         this_delta;
            int64       this_page_number;
 
            if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
                return slotno;
 
            this_delta = cur_count - shared->page_lru_count[slotno];
            if (this_delta < 0)
            {
                /*
                 * Clean up in case shared updates have caused cur_count
                 * increments to get "lost".  We back off the page counts,
                 * rather than trying to increase cur_count, to avoid any
                 * question of infinite loops or failure in the presence of
                 * wrapped-around counts.
                 */
                shared->page_lru_count[slotno] = cur_count;
                this_delta = 0;
            }
 
            /*
             * If this page is the one most recently zeroed, don't consider it
             * an eviction candidate. See comments in SimpleLruZeroPage for an
             * explanation about the lack of a memory barrier here.
             */
            this_page_number = shared->page_number[slotno];
            if (this_page_number ==
                pg_atomic_read_u64(&shared->latest_page_number))
                continue;
 
            if (shared->page_status[slotno] == SLRU_PAGE_VALID)
            {
                if (this_delta > best_valid_delta ||
                    (this_delta == best_valid_delta &&
                     ctl->PagePrecedes(this_page_number,
                                       best_valid_page_number)))
                {
                    bestvalidslot = slotno;
                    best_valid_delta = this_delta;
                    best_valid_page_number = this_page_number;
                }
            }
            else
            {
                if (this_delta > best_invalid_delta ||
                    (this_delta == best_invalid_delta &&
                     ctl->PagePrecedes(this_page_number,
                                       best_invalid_page_number)))
                {
                    bestinvalidslot = slotno;
                    best_invalid_delta = this_delta;
                    best_invalid_page_number = this_page_number;
                }
            }
        }
 
        /*
         * If all pages (except possibly the latest one) are I/O busy, we'll
         * have to wait for an I/O to complete and then retry.  In that
         * unhappy case, we choose to wait for the I/O on the least recently
         * used slot, on the assumption that it was likely initiated first of
         * all the I/Os in progress and may therefore finish first.
         */
        if (best_valid_delta < 0)
        {
            SimpleLruWaitIO(ctl, bestinvalidslot);
            continue;
        }
 
        /*
         * If the selected page is clean, we're set.
         */
        if (!shared->page_dirty[bestvalidslot])
            return bestvalidslot;
 
        /*
         * Write the page.
         */
        SlruInternalWritePage(ctl, bestvalidslot, NULL);
 
        /*
         * Now loop back and try again.  This is the easiest way of dealing
         * with corner cases such as the victim page being re-dirtied while we
         * wrote it.
         */
    }
}
 
/*
 * Write dirty pages to disk during checkpoint or database shutdown.  Flushing
 * is deferred until the next call to ProcessSyncRequests(), though we do fsync
 * the containing directory here to make sure that newly created directory
 * entries are on disk.
 */
void
SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied)
{
    SlruShared  shared = ctl->shared;
    SlruWriteAllData fdata;
    int64       pageno = 0;
    int         prevbank = SlotGetBankNumber(0);
    bool        ok;
 
    /* update the stats counter of flushes */
    pgstat_count_slru_flush(shared->slru_stats_idx);
 
    /*
     * Find and write dirty pages
     */
    fdata.num_files = 0;
 
    LWLockAcquire(&shared->bank_locks[prevbank].lock, LW_EXCLUSIVE);
 
    for (int slotno = 0; slotno < shared->num_slots; slotno++)
    {
        int         curbank = SlotGetBankNumber(slotno);
 
        /*
         * If the current bank lock is not same as the previous bank lock then
         * release the previous lock and acquire the new lock.
         */
        if (curbank != prevbank)
        {
            LWLockRelease(&shared->bank_locks[prevbank].lock);
            LWLockAcquire(&shared->bank_locks[curbank].lock, LW_EXCLUSIVE);
            prevbank = curbank;
        }
 
        /* Do nothing if slot is unused */
        if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
            continue;
 
        SlruInternalWritePage(ctl, slotno, &fdata);
 
        /*
         * In some places (e.g. checkpoints), we cannot assert that the slot
         * is clean now, since another process might have re-dirtied it
         * already.  That's okay.
         */
        Assert(allow_redirtied ||
               shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
               (shared->page_status[slotno] == SLRU_PAGE_VALID &&
                !shared->page_dirty[slotno]));
    }
 
    LWLockRelease(&shared->bank_locks[prevbank].lock);
 
    /*
     * Now close any files that were open
     */
    ok = true;
    for (int i = 0; i < fdata.num_files; i++)
    {
        if (CloseTransientFile(fdata.fd[i]) != 0)
        {
            slru_errcause = SLRU_CLOSE_FAILED;
            slru_errno = errno;
            pageno = fdata.segno[i] * SLRU_PAGES_PER_SEGMENT;
            ok = false;
        }
    }
    if (!ok)
        SlruReportIOError(ctl, pageno, InvalidTransactionId);
 
    /* Ensure that directory entries for new files are on disk. */
    if (ctl->sync_handler != SYNC_HANDLER_NONE)
        fsync_fname(ctl->Dir, true);
}
 
/*
 * Remove all segments before the one holding the passed page number
 *
 * All SLRUs prevent concurrent calls to this function, either with an LWLock
 * or by calling it only as part of a checkpoint.  Mutual exclusion must begin
 * before computing cutoffPage.  Mutual exclusion must end after any limit
 * update that would permit other backends to write fresh data into the
 * segment immediately preceding the one containing cutoffPage.  Otherwise,
 * when the SLRU is quite full, SimpleLruTruncate() might delete that segment
 * after it has accrued freshly-written data.
 */
void
SimpleLruTruncate(SlruCtl ctl, int64 cutoffPage)
{
    SlruShared  shared = ctl->shared;
    int         prevbank;
 
    /* update the stats counter of truncates */
    pgstat_count_slru_truncate(shared->slru_stats_idx);
 
    /*
     * Scan shared memory and remove any pages preceding the cutoff page, to
     * ensure we won't rewrite them later.  (Since this is normally called in
     * or just after a checkpoint, any dirty pages should have been flushed
     * already ... we're just being extra careful here.)
     */
restart:
 
    /*
     * An important safety check: the current endpoint page must not be
     * eligible for removal.  This check is just a backstop against wraparound
     * bugs elsewhere in SLRU handling, so we don't care if we read a slightly
     * outdated value; therefore we don't add a memory barrier.
     */
    if (ctl->PagePrecedes(pg_atomic_read_u64(&shared->latest_page_number),
                          cutoffPage))
    {
        ereport(LOG,
                (errmsg("could not truncate directory \"%s\": apparent wraparound",
                        ctl->Dir)));
        return;
    }
 
    prevbank = SlotGetBankNumber(0);
    LWLockAcquire(&shared->bank_locks[prevbank].lock, LW_EXCLUSIVE);
    for (int slotno = 0; slotno < shared->num_slots; slotno++)
    {
        int         curbank = SlotGetBankNumber(slotno);
 
        /*
         * If the current bank lock is not same as the previous bank lock then
         * release the previous lock and acquire the new lock.
         */
        if (curbank != prevbank)
        {
            LWLockRelease(&shared->bank_locks[prevbank].lock);
            LWLockAcquire(&shared->bank_locks[curbank].lock, LW_EXCLUSIVE);
            prevbank = curbank;
        }
 
        if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
            continue;
        if (!ctl->PagePrecedes(shared->page_number[slotno], cutoffPage))
            continue;
 
        /*
         * If page is clean, just change state to EMPTY (expected case).
         */
        if (shared->page_status[slotno] == SLRU_PAGE_VALID &&
            !shared->page_dirty[slotno])
        {
            shared->page_status[slotno] = SLRU_PAGE_EMPTY;
            continue;
        }
 
        /*
         * Hmm, we have (or may have) I/O operations acting on the page, so
         * we've got to wait for them to finish and then start again. This is
         * the same logic as in SlruSelectLRUPage.  (XXX if page is dirty,
         * wouldn't it be OK to just discard it without writing it?
         * SlruMayDeleteSegment() uses a stricter qualification, so we might
         * not delete this page in the end; even if we don't delete it, we
         * won't have cause to read its data again.  For now, keep the logic
         * the same as it was.)
         */
        if (shared->page_status[slotno] == SLRU_PAGE_VALID)
            SlruInternalWritePage(ctl, slotno, NULL);
        else
            SimpleLruWaitIO(ctl, slotno);
 
        LWLockRelease(&shared->bank_locks[prevbank].lock);
        goto restart;
    }
 
    LWLockRelease(&shared->bank_locks[prevbank].lock);
 
    /* Now we can remove the old segment(s) */
    (void) SlruScanDirectory(ctl, SlruScanDirCbDeleteCutoff, &cutoffPage);
}
 
/*
 * Delete an individual SLRU segment.
 *
 * NB: This does not touch the SLRU buffers themselves, callers have to ensure
 * they either can't yet contain anything, or have already been cleaned out.
 */
static void
SlruInternalDeleteSegment(SlruCtl ctl, int64 segno)
{
    char        path[MAXPGPATH];
 
    /* Forget any fsync requests queued for this segment. */
    if (ctl->sync_handler != SYNC_HANDLER_NONE)
    {
        FileTag     tag;
 
        INIT_SLRUFILETAG(tag, ctl->sync_handler, segno);
        RegisterSyncRequest(&tag, SYNC_FORGET_REQUEST, true);
    }
 
    /* Unlink the file. */
    SlruFileName(ctl, path, segno);
    ereport(DEBUG2, (errmsg_internal("removing file \"%s\"", path)));
    unlink(path);
}
 
/*
 * Delete an individual SLRU segment, identified by the segment number.
 */
void
SlruDeleteSegment(SlruCtl ctl, int64 segno)
{
    SlruShared  shared = ctl->shared;
    int         prevbank = SlotGetBankNumber(0);
    bool        did_write;
 
    /* Clean out any possibly existing references to the segment. */
    LWLockAcquire(&shared->bank_locks[prevbank].lock, LW_EXCLUSIVE);
restart:
    did_write = false;
    for (int slotno = 0; slotno < shared->num_slots; slotno++)
    {
        int64       pagesegno;
        int         curbank = SlotGetBankNumber(slotno);
 
        /*
         * If the current bank lock is not same as the previous bank lock then
         * release the previous lock and acquire the new lock.
         */
        if (curbank != prevbank)
        {
            LWLockRelease(&shared->bank_locks[prevbank].lock);
            LWLockAcquire(&shared->bank_locks[curbank].lock, LW_EXCLUSIVE);
            prevbank = curbank;
        }
 
        if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
            continue;
 
        pagesegno = shared->page_number[slotno] / SLRU_PAGES_PER_SEGMENT;
        /* not the segment we're looking for */
        if (pagesegno != segno)
            continue;
 
        /* If page is clean, just change state to EMPTY (expected case). */
        if (shared->page_status[slotno] == SLRU_PAGE_VALID &&
            !shared->page_dirty[slotno])
        {
            shared->page_status[slotno] = SLRU_PAGE_EMPTY;
            continue;
        }
 
        /* Same logic as SimpleLruTruncate() */
        if (shared->page_status[slotno] == SLRU_PAGE_VALID)
            SlruInternalWritePage(ctl, slotno, NULL);
        else
            SimpleLruWaitIO(ctl, slotno);
 
        did_write = true;
    }
 
    /*
     * Be extra careful and re-check. The IO functions release the control
     * lock, so new pages could have been read in.
     */
    if (did_write)
        goto restart;
 
    SlruInternalDeleteSegment(ctl, segno);
 
    LWLockRelease(&shared->bank_locks[prevbank].lock);
}
 
/*
 * Determine whether a segment is okay to delete.
 *
 * segpage is the first page of the segment, and cutoffPage is the oldest (in
 * PagePrecedes order) page in the SLRU containing still-useful data.  Since
 * every core PagePrecedes callback implements "wrap around", check the
 * segment's first and last pages:
 *
 * first<cutoff  && last<cutoff:  yes
 * first<cutoff  && last>=cutoff: no; cutoff falls inside this segment
 * first>=cutoff && last<cutoff:  no; wrap point falls inside this segment
 * first>=cutoff && last>=cutoff: no; every page of this segment is too young
 */
static bool
SlruMayDeleteSegment(SlruCtl ctl, int64 segpage, int64 cutoffPage)
{
    int64       seg_last_page = segpage + SLRU_PAGES_PER_SEGMENT - 1;
 
    Assert(segpage % SLRU_PAGES_PER_SEGMENT == 0);
 
    return (ctl->PagePrecedes(segpage, cutoffPage) &&
            ctl->PagePrecedes(seg_last_page, cutoffPage));
}
 
#ifdef USE_ASSERT_CHECKING
static void
SlruPagePrecedesTestOffset(SlruCtl ctl, int per_page, uint32 offset)
{
    TransactionId lhs,
                rhs;
    int64       newestPage,
                oldestPage;
    TransactionId newestXact,
                oldestXact;
 
    /*
     * Compare an XID pair having undefined order (see RFC 1982), a pair at
     * "opposite ends" of the XID space.  TransactionIdPrecedes() treats each
     * as preceding the other.  If RHS is oldestXact, LHS is the first XID we
     * must not assign.
     */
    lhs = per_page + offset;    /* skip first page to avoid non-normal XIDs */
    rhs = lhs + (1U << 31);
    Assert(TransactionIdPrecedes(lhs, rhs));
    Assert(TransactionIdPrecedes(rhs, lhs));
    Assert(!TransactionIdPrecedes(lhs - 1, rhs));
    Assert(TransactionIdPrecedes(rhs, lhs - 1));
    Assert(TransactionIdPrecedes(lhs + 1, rhs));
    Assert(!TransactionIdPrecedes(rhs, lhs + 1));
    Assert(!TransactionIdFollowsOrEquals(lhs, rhs));
    Assert(!TransactionIdFollowsOrEquals(rhs, lhs));
    Assert(!ctl->PagePrecedes(lhs / per_page, lhs / per_page));
    Assert(!ctl->PagePrecedes(lhs / per_page, rhs / per_page));
    Assert(!ctl->PagePrecedes(rhs / per_page, lhs / per_page));
    Assert(!ctl->PagePrecedes((lhs - per_page) / per_page, rhs / per_page));
    Assert(ctl->PagePrecedes(rhs / per_page, (lhs - 3 * per_page) / per_page));
    Assert(ctl->PagePrecedes(rhs / per_page, (lhs - 2 * per_page) / per_page));
    Assert(ctl->PagePrecedes(rhs / per_page, (lhs - 1 * per_page) / per_page)
           || (1U << 31) % per_page != 0);  /* See CommitTsPagePrecedes() */
    Assert(ctl->PagePrecedes((lhs + 1 * per_page) / per_page, rhs / per_page)
           || (1U << 31) % per_page != 0);
    Assert(ctl->PagePrecedes((lhs + 2 * per_page) / per_page, rhs / per_page));
    Assert(ctl->PagePrecedes((lhs + 3 * per_page) / per_page, rhs / per_page));
    Assert(!ctl->PagePrecedes(rhs / per_page, (lhs + per_page) / per_page));
 
    /*
     * GetNewTransactionId() has assigned the last XID it can safely use, and
     * that XID is in the *LAST* page of the second segment.  We must not
     * delete that segment.
     */
    newestPage = 2 * SLRU_PAGES_PER_SEGMENT - 1;
    newestXact = newestPage * per_page + offset;
    Assert(newestXact / per_page == newestPage);
    oldestXact = newestXact + 1;
    oldestXact -= 1U << 31;
    oldestPage = oldestXact / per_page;
    Assert(!SlruMayDeleteSegment(ctl,
                                 (newestPage -
                                  newestPage % SLRU_PAGES_PER_SEGMENT),
                                 oldestPage));
 
    /*
     * GetNewTransactionId() has assigned the last XID it can safely use, and
     * that XID is in the *FIRST* page of the second segment.  We must not
     * delete that segment.
     */
    newestPage = SLRU_PAGES_PER_SEGMENT;
    newestXact = newestPage * per_page + offset;
    Assert(newestXact / per_page == newestPage);
    oldestXact = newestXact + 1;
    oldestXact -= 1U << 31;
    oldestPage = oldestXact / per_page;
    Assert(!SlruMayDeleteSegment(ctl,
                                 (newestPage -
                                  newestPage % SLRU_PAGES_PER_SEGMENT),
                                 oldestPage));
}
 
/*
 * Unit-test a PagePrecedes function.
 *
 * This assumes every uint32 >= FirstNormalTransactionId is a valid key.  It
 * assumes each value occupies a contiguous, fixed-size region of SLRU bytes.
 * (MultiXactMemberCtl separates flags from XIDs.  NotifyCtl has
 * variable-length entries, no keys, and no random access.  These unit tests
 * do not apply to them.)
 */
void
SlruPagePrecedesUnitTests(SlruCtl ctl, int per_page)
{
    /* Test first, middle and last entries of a page. */
    SlruPagePrecedesTestOffset(ctl, per_page, 0);
    SlruPagePrecedesTestOffset(ctl, per_page, per_page / 2);
    SlruPagePrecedesTestOffset(ctl, per_page, per_page - 1);
}
#endif
 
/*
 * SlruScanDirectory callback
 *      This callback reports true if there's any segment wholly prior to the
 *      one containing the page passed as "data".
 */
bool
SlruScanDirCbReportPresence(SlruCtl ctl, char *filename, int64 segpage,
                            void *data)
{
    int64       cutoffPage = *(int64 *) data;
 
    if (SlruMayDeleteSegment(ctl, segpage, cutoffPage))
        return true;            /* found one; don't iterate any more */
 
    return false;               /* keep going */
}
 
/*
 * SlruScanDirectory callback.
 *      This callback deletes segments prior to the one passed in as "data".
 */
static bool
SlruScanDirCbDeleteCutoff(SlruCtl ctl, char *filename, int64 segpage,
                          void *data)
{
    int64       cutoffPage = *(int64 *) data;
 
    if (SlruMayDeleteSegment(ctl, segpage, cutoffPage))
        SlruInternalDeleteSegment(ctl, segpage / SLRU_PAGES_PER_SEGMENT);
 
    return false;               /* keep going */
}
 
/*
 * SlruScanDirectory callback.
 *      This callback deletes all segments.
 */
bool
SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int64 segpage, void *data)
{
    SlruInternalDeleteSegment(ctl, segpage / SLRU_PAGES_PER_SEGMENT);
 
    return false;               /* keep going */
}
 
/*
 * An internal function used by SlruScanDirectory().
 *
 * Returns true if a file with a name of a given length may be a correct
 * SLRU segment.
 */
static inline bool
SlruCorrectSegmentFilenameLength(SlruCtl ctl, size_t len)
{
    if (ctl->long_segment_names)
        return (len == 15);     /* see SlruFileName() */
    else
 
        /*
         * Commit 638cf09e76d allowed 5-character lengths. Later commit
         * 73c986adde5 allowed 6-character length.
         *
         * Note: There is an ongoing plan to migrate all SLRUs to 64-bit page
         * numbers, and the corresponding 15-character file names, which may
         * eventually deprecate the support for 4, 5, and 6-character names.
         */
        return (len == 4 || len == 5 || len == 6);
}
 
/*
 * Scan the SimpleLru directory and apply a callback to each file found in it.
 *
 * If the callback returns true, the scan is stopped.  The last return value
 * from the callback is returned.
 *
 * The callback receives the following arguments: 1. the SlruCtl struct for the
 * slru being truncated; 2. the filename being considered; 3. the page number
 * for the first page of that file; 4. a pointer to the opaque data given to us
 * by the caller.
 *
 * Note that the ordering in which the directory is scanned is not guaranteed.
 *
 * Note that no locking is applied.
 */
bool
SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
{
    bool        retval = false;
    DIR        *cldir;
    struct dirent *clde;
    int64       segno;
    int64       segpage;
 
    cldir = AllocateDir(ctl->Dir);
    while ((clde = ReadDir(cldir, ctl->Dir)) != NULL)
    {
        size_t      len;
 
        len = strlen(clde->d_name);
 
        if (SlruCorrectSegmentFilenameLength(ctl, len) &&
            strspn(clde->d_name, "0123456789ABCDEF") == len)
        {
            segno = strtoi64(clde->d_name, NULL, 16);
            segpage = segno * SLRU_PAGES_PER_SEGMENT;
 
            elog(DEBUG2, "SlruScanDirectory invoking callback on %s/%s",
                 ctl->Dir, clde->d_name);
            retval = callback(ctl, clde->d_name, segpage, data);
            if (retval)
                break;
        }
    }
    FreeDir(cldir);
 
    return retval;
}
 
/*
 * Individual SLRUs (clog, ...) have to provide a sync.c handler function so
 * that they can provide the correct "SlruCtl" (otherwise we don't know how to
 * build the path), but they just forward to this common implementation that
 * performs the fsync.
 */
int
SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path)
{
    int         fd;
    int         save_errno;
    int         result;
 
    SlruFileName(ctl, path, ftag->segno);
 
    fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
    if (fd < 0)
        return -1;
 
    pgstat_report_wait_start(WAIT_EVENT_SLRU_FLUSH_SYNC);
    result = pg_fsync(fd);
    pgstat_report_wait_end();
    save_errno = errno;
 
    CloseTransientFile(fd);
 
    errno = save_errno;
    return result;
}