Simplify coding in slru.c

[pgsql.git] / src / backend / access / transam / slru.c

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2024, 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_hooks.h"

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/%015llX", ctl->Dir,
                                                (long long) 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).
 * ctllock: LWLock to use to control access to the shared control structure.
 * 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
 */
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->bank_mask = (nslots / SLRU_BANK_SIZE) - 1;
        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
         * ControlLock 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_page_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));
}

/*
 * 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_page_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_page_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->bank_mask;
        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 macro */
                        SlruRecentlyUsed(shared, slotno);

                        /* update the stats counter of pages found in the SLRU */
                        pgstat_count_slru_page_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 buffer written. */
        if (fdata)
                CheckpointStats.ckpt_bufs_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_page_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_page_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->bank_mask;
                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 = 0; slotno < shared->num_slots; 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++)
        {
                int                     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;
}