diff --git a/src/backend/access/rmgrdesc/mxactdesc.c b/src/backend/access/rmgrdesc/mxactdesc.c
index 3ca0582db364..052dd0a4ce56 100644
--- a/src/backend/access/rmgrdesc/mxactdesc.c
+++ b/src/backend/access/rmgrdesc/mxactdesc.c
@@ -65,7 +65,7 @@ multixact_desc(StringInfo buf, XLogReaderState *record)
 		xl_multixact_create *xlrec = (xl_multixact_create *) rec;
 		int			i;
 
-		appendStringInfo(buf, "%u offset %u nmembers %d: ", xlrec->mid,
+		appendStringInfo(buf, "%u offset %" PRIu64 " nmembers %d: ", xlrec->mid,
 						 xlrec->moff, xlrec->nmembers);
 		for (i = 0; i < xlrec->nmembers; i++)
 			out_member(buf, &xlrec->members[i]);
@@ -74,7 +74,7 @@ multixact_desc(StringInfo buf, XLogReaderState *record)
 	{
 		xl_multixact_truncate *xlrec = (xl_multixact_truncate *) rec;
 
-		appendStringInfo(buf, "offsets [%u, %u), members [%u, %u)",
+		appendStringInfo(buf, "offsets [%u, %u), members [%" PRIu64 ", %" PRIu64 ")",
 						 xlrec->startTruncOff, xlrec->endTruncOff,
 						 xlrec->startTruncMemb, xlrec->endTruncMemb);
 	}
diff --git a/src/backend/access/rmgrdesc/xlogdesc.c b/src/backend/access/rmgrdesc/xlogdesc.c
index 58040f28656f..3c42aa3e39c0 100644
--- a/src/backend/access/rmgrdesc/xlogdesc.c
+++ b/src/backend/access/rmgrdesc/xlogdesc.c
@@ -66,7 +66,7 @@ xlog_desc(StringInfo buf, XLogReaderState *record)
 		CheckPoint *checkpoint = (CheckPoint *) rec;
 
 		appendStringInfo(buf, "redo %X/%X; "
-						 "tli %u; prev tli %u; fpw %s; wal_level %s; xid %u:%u; oid %u; multi %u; offset %u; "
+						 "tli %u; prev tli %u; fpw %s; wal_level %s; xid %u:%u; oid %u; multi %u; offset %" PRIu64 "; "
 						 "oldest xid %u in DB %u; oldest multi %u in DB %u; "
 						 "oldest/newest commit timestamp xid: %u/%u; "
 						 "oldest running xid %u; %s",
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 3c06ac45532f..059a72f1067c 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -96,14 +96,6 @@
 /*
  * Defines for MultiXactOffset page sizes.  A page is the same BLCKSZ as is
  * used everywhere else in Postgres.
- *
- * Note: because MultiXactOffsets are 32 bits and wrap around at 0xFFFFFFFF,
- * MultiXact page numbering also wraps around at
- * 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE, and segment numbering at
- * 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need
- * take no explicit notice of that fact in this module, except when comparing
- * segment and page numbers in TruncateMultiXact (see
- * MultiXactOffsetPagePrecedes).
  */
 
 /* We need four bytes per offset */
@@ -212,10 +204,14 @@ MXOffsetToMemberOffset(MultiXactOffset offset)
 		member_in_group * sizeof(TransactionId);
 }
 
-/* Multixact members wraparound thresholds. */
-#define MULTIXACT_MEMBER_SAFE_THRESHOLD		(MaxMultiXactOffset / 2)
-#define MULTIXACT_MEMBER_DANGER_THRESHOLD	\
-	(MaxMultiXactOffset - MaxMultiXactOffset / 4)
+/*
+ * Multixact members warning threshold.
+ *
+ * If difference bettween nextOffset and oldestOffset exceed this value, we
+ * trigger autovacuumin order to release the disk space, reduce table bloat if
+ * possible.
+ */
+#define MULTIXACT_MEMBER_AUTOVAC_THRESHOLD		UINT64CONST(0xFFFFFFFF)
 
 static inline MultiXactId
 PreviousMultiXactId(MultiXactId multi)
@@ -272,9 +268,6 @@ typedef struct MultiXactStateData
 	MultiXactId multiStopLimit;
 	MultiXactId multiWrapLimit;
 
-	/* support for members anti-wraparound measures */
-	MultiXactOffset offsetStopLimit;	/* known if oldestOffsetKnown */
-
 	/*
 	 * This is used to sleep until a multixact offset is written when we want
 	 * to create the next one.
@@ -409,8 +402,6 @@ static bool MultiXactOffsetPrecedes(MultiXactOffset offset1,
 									MultiXactOffset offset2);
 static void ExtendMultiXactOffset(MultiXactId multi);
 static void ExtendMultiXactMember(MultiXactOffset offset, int nmembers);
-static bool MultiXactOffsetWouldWrap(MultiXactOffset boundary,
-									 MultiXactOffset start, uint32 distance);
 static bool SetOffsetVacuumLimit(bool is_startup);
 static bool find_multixact_start(MultiXactId multi, MultiXactOffset *result);
 static void WriteMZeroPageXlogRec(int64 pageno, uint8 info);
@@ -1164,78 +1155,6 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 	else
 		*offset = nextOffset;
 
-	/*----------
-	 * Protect against overrun of the members space as well, with the
-	 * following rules:
-	 *
-	 * If we're past offsetStopLimit, refuse to generate more multis.
-	 * If we're close to offsetStopLimit, emit a warning.
-	 *
-	 * Arbitrarily, we start emitting warnings when we're 20 segments or less
-	 * from offsetStopLimit.
-	 *
-	 * Note we haven't updated the shared state yet, so if we fail at this
-	 * point, the multixact ID we grabbed can still be used by the next guy.
-	 *
-	 * Note that there is no point in forcing autovacuum runs here: the
-	 * multixact freeze settings would have to be reduced for that to have any
-	 * effect.
-	 *----------
-	 */
-#define OFFSET_WARN_SEGMENTS	20
-	if (MultiXactState->oldestOffsetKnown &&
-		MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
-								 nmembers))
-	{
-		/* see comment in the corresponding offsets wraparound case */
-		SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
-
-		ereport(ERROR,
-				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
-				 errmsg("multixact \"members\" limit exceeded"),
-				 errdetail_plural("This command would create a multixact with %u members, but the remaining space is only enough for %u member.",
-								  "This command would create a multixact with %u members, but the remaining space is only enough for %u members.",
-								  MultiXactState->offsetStopLimit - nextOffset - 1,
-								  nmembers,
-								  MultiXactState->offsetStopLimit - nextOffset - 1),
-				 errhint("Execute a database-wide VACUUM in database with OID %u with reduced \"vacuum_multixact_freeze_min_age\" and \"vacuum_multixact_freeze_table_age\" settings.",
-						 MultiXactState->oldestMultiXactDB)));
-	}
-
-	/*
-	 * Check whether we should kick autovacuum into action, to prevent members
-	 * wraparound. NB we use a much larger window to trigger autovacuum than
-	 * just the warning limit. The warning is just a measure of last resort -
-	 * this is in line with GetNewTransactionId's behaviour.
-	 */
-	if (!MultiXactState->oldestOffsetKnown ||
-		(MultiXactState->nextOffset - MultiXactState->oldestOffset
-		 > MULTIXACT_MEMBER_SAFE_THRESHOLD))
-	{
-		/*
-		 * To avoid swamping the postmaster with signals, we issue the autovac
-		 * request only when crossing a segment boundary. With default
-		 * compilation settings that's roughly after 50k members.  This still
-		 * gives plenty of chances before we get into real trouble.
-		 */
-		if ((MXOffsetToMemberPage(nextOffset) / SLRU_PAGES_PER_SEGMENT) !=
-			(MXOffsetToMemberPage(nextOffset + nmembers) / SLRU_PAGES_PER_SEGMENT))
-			SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
-	}
-
-	if (MultiXactState->oldestOffsetKnown &&
-		MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
-								 nextOffset,
-								 nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
-		ereport(WARNING,
-				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
-				 errmsg_plural("database with OID %u must be vacuumed before %d more multixact member is used",
-							   "database with OID %u must be vacuumed before %d more multixact members are used",
-							   MultiXactState->offsetStopLimit - nextOffset + nmembers,
-							   MultiXactState->oldestMultiXactDB,
-							   MultiXactState->offsetStopLimit - nextOffset + nmembers),
-				 errhint("Execute a database-wide VACUUM in that database with reduced \"vacuum_multixact_freeze_min_age\" and \"vacuum_multixact_freeze_table_age\" settings.")));
-
 	ExtendMultiXactMember(nextOffset, nmembers);
 
 	/*
@@ -1264,7 +1183,8 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 
 	LWLockRelease(MultiXactGenLock);
 
-	debug_elog4(DEBUG2, "GetNew: returning %u offset %u", result, *offset);
+	debug_elog4(DEBUG2, "GetNew: returning %u offset %" PRIu64, result,
+				*offset);
 	return result;
 }
 
@@ -2293,7 +2213,7 @@ MultiXactGetCheckptMulti(bool is_shutdown,
 	LWLockRelease(MultiXactGenLock);
 
 	debug_elog6(DEBUG2,
-				"MultiXact: checkpoint is nextMulti %u, nextOffset %u, oldestMulti %u in DB %u",
+				"MultiXact: checkpoint is nextMulti %u, nextOffset %" PRIu64 ", oldestMulti %u in DB %u",
 				*nextMulti, *nextMultiOffset, *oldestMulti, *oldestMultiDB);
 }
 
@@ -2328,7 +2248,7 @@ void
 MultiXactSetNextMXact(MultiXactId nextMulti,
 					  MultiXactOffset nextMultiOffset)
 {
-	debug_elog4(DEBUG2, "MultiXact: setting next multi to %u offset %u",
+	debug_elog4(DEBUG2, "MultiXact: setting next multi to %u offset %" PRIu64,
 				nextMulti, nextMultiOffset);
 	LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 	MultiXactState->nextMXact = nextMulti;
@@ -2519,7 +2439,7 @@ MultiXactAdvanceNextMXact(MultiXactId minMulti,
 	}
 	if (MultiXactOffsetPrecedes(MultiXactState->nextOffset, minMultiOffset))
 	{
-		debug_elog3(DEBUG2, "MultiXact: setting next offset to %u",
+		debug_elog3(DEBUG2, "MultiXact: setting next offset to %" PRIU64,
 					minMultiOffset);
 		MultiXactState->nextOffset = minMultiOffset;
 	}
@@ -2699,15 +2619,13 @@ GetOldestMultiXactId(void)
 }
 
 /*
- * Determine how aggressively we need to vacuum in order to prevent member
- * wraparound.
+ * Determine if we need to vacuum for member or not.
  *
  * To do so determine what's the oldest member offset and install the limit
  * info in MultiXactState, where it can be used to prevent overrun of old data
  * in the members SLRU area.
  *
- * The return value is true if emergency autovacuum is required and false
- * otherwise.
+ * The return value is true if autovacuum is required and false otherwise.
  */
 static bool
 SetOffsetVacuumLimit(bool is_startup)
@@ -2719,8 +2637,6 @@ SetOffsetVacuumLimit(bool is_startup)
 	MultiXactOffset nextOffset;
 	bool		oldestOffsetKnown = false;
 	bool		prevOldestOffsetKnown;
-	MultiXactOffset offsetStopLimit = 0;
-	MultiXactOffset prevOffsetStopLimit;
 
 	/*
 	 * NB: Have to prevent concurrent truncation, we might otherwise try to
@@ -2735,7 +2651,6 @@ SetOffsetVacuumLimit(bool is_startup)
 	nextOffset = MultiXactState->nextOffset;
 	prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
 	prevOldestOffset = MultiXactState->oldestOffset;
-	prevOffsetStopLimit = MultiXactState->offsetStopLimit;
 	Assert(MultiXactState->finishedStartup);
 	LWLockRelease(MultiXactGenLock);
 
@@ -2766,11 +2681,7 @@ SetOffsetVacuumLimit(bool is_startup)
 		oldestOffsetKnown =
 			find_multixact_start(oldestMultiXactId, &oldestOffset);
 
-		if (oldestOffsetKnown)
-			ereport(DEBUG1,
-					(errmsg_internal("oldest MultiXactId member is at offset %u",
-									 oldestOffset)));
-		else
+		if (!oldestOffsetKnown)
 			ereport(LOG,
 					(errmsg("MultiXact member wraparound protections are disabled because oldest checkpointed MultiXact %u does not exist on disk",
 							oldestMultiXactId)));
@@ -2783,24 +2694,7 @@ SetOffsetVacuumLimit(bool is_startup)
 	 * overrun of old data in the members SLRU area. We can only do so if the
 	 * oldest offset is known though.
 	 */
-	if (oldestOffsetKnown)
-	{
-		/* move back to start of the corresponding segment */
-		offsetStopLimit = oldestOffset - (oldestOffset %
-										  (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT));
-
-		/* always leave one segment before the wraparound point */
-		offsetStopLimit -= (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
-
-		if (!prevOldestOffsetKnown && !is_startup)
-			ereport(LOG,
-					(errmsg("MultiXact member wraparound protections are now enabled")));
-
-		ereport(DEBUG1,
-				(errmsg_internal("MultiXact member stop limit is now %u based on MultiXact %u",
-								 offsetStopLimit, oldestMultiXactId)));
-	}
-	else if (prevOldestOffsetKnown)
+	if (prevOldestOffsetKnown)
 	{
 		/*
 		 * If we failed to get the oldest offset this time, but we have a
@@ -2810,69 +2704,19 @@ SetOffsetVacuumLimit(bool is_startup)
 		 */
 		oldestOffset = prevOldestOffset;
 		oldestOffsetKnown = true;
-		offsetStopLimit = prevOffsetStopLimit;
 	}
 
 	/* Install the computed values */
 	LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 	MultiXactState->oldestOffset = oldestOffset;
 	MultiXactState->oldestOffsetKnown = oldestOffsetKnown;
-	MultiXactState->offsetStopLimit = offsetStopLimit;
 	LWLockRelease(MultiXactGenLock);
 
 	/*
-	 * Do we need an emergency autovacuum?	If we're not sure, assume yes.
+	 * Do we need autovacuum?	If we're not sure, assume yes.
 	 */
 	return !oldestOffsetKnown ||
-		(nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD);
-}
-
-/*
- * Return whether adding "distance" to "start" would move past "boundary".
- *
- * We use this to determine whether the addition is "wrapping around" the
- * boundary point, hence the name.  The reason we don't want to use the regular
- * 2^31-modulo arithmetic here is that we want to be able to use the whole of
- * the 2^32-1 space here, allowing for more multixacts than would fit
- * otherwise.
- */
-static bool
-MultiXactOffsetWouldWrap(MultiXactOffset boundary, MultiXactOffset start,
-						 uint32 distance)
-{
-	MultiXactOffset finish;
-
-	/*
-	 * Note that offset number 0 is not used (see GetMultiXactIdMembers), so
-	 * if the addition wraps around the UINT_MAX boundary, skip that value.
-	 */
-	finish = start + distance;
-	if (finish < start)
-		finish++;
-
-	/*-----------------------------------------------------------------------
-	 * When the boundary is numerically greater than the starting point, any
-	 * value numerically between the two is not wrapped:
-	 *
-	 *	<----S----B---->
-	 *	[---)			 = F wrapped past B (and UINT_MAX)
-	 *		 [---)		 = F not wrapped
-	 *			  [----] = F wrapped past B
-	 *
-	 * When the boundary is numerically less than the starting point (i.e. the
-	 * UINT_MAX wraparound occurs somewhere in between) then all values in
-	 * between are wrapped:
-	 *
-	 *	<----B----S---->
-	 *	[---)			 = F not wrapped past B (but wrapped past UINT_MAX)
-	 *		 [---)		 = F wrapped past B (and UINT_MAX)
-	 *			  [----] = F not wrapped
-	 *-----------------------------------------------------------------------
-	 */
-	if (start < boundary)
-		return finish >= boundary || finish < start;
-	else
-		return finish >= boundary && finish < start;
+		(nextOffset - oldestOffset > MULTIXACT_MEMBER_AUTOVAC_THRESHOLD);
 }
 
 /*
@@ -2918,100 +2762,6 @@ find_multixact_start(MultiXactId multi, MultiXactOffset *result)
 	return true;
 }
 
-/*
- * Determine how many multixacts, and how many multixact members, currently
- * exist.  Return false if unable to determine.
- */
-static bool
-ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
-{
-	MultiXactOffset nextOffset;
-	MultiXactOffset oldestOffset;
-	MultiXactId oldestMultiXactId;
-	MultiXactId nextMultiXactId;
-	bool		oldestOffsetKnown;
-
-	LWLockAcquire(MultiXactGenLock, LW_SHARED);
-	nextOffset = MultiXactState->nextOffset;
-	oldestMultiXactId = MultiXactState->oldestMultiXactId;
-	nextMultiXactId = MultiXactState->nextMXact;
-	oldestOffset = MultiXactState->oldestOffset;
-	oldestOffsetKnown = MultiXactState->oldestOffsetKnown;
-	LWLockRelease(MultiXactGenLock);
-
-	if (!oldestOffsetKnown)
-		return false;
-
-	*members = nextOffset - oldestOffset;
-	*multixacts = nextMultiXactId - oldestMultiXactId;
-	return true;
-}
-
-/*
- * Multixact members can be removed once the multixacts that refer to them
- * are older than every datminmxid.  autovacuum_multixact_freeze_max_age and
- * vacuum_multixact_freeze_table_age work together to make sure we never have
- * too many multixacts; we hope that, at least under normal circumstances,
- * this will also be sufficient to keep us from using too many offsets.
- * However, if the average multixact has many members, we might exhaust the
- * members space while still using few enough members that these limits fail
- * to trigger relminmxid advancement by VACUUM.  At that point, we'd have no
- * choice but to start failing multixact-creating operations with an error.
- *
- * To prevent that, if more than a threshold portion of the members space is
- * used, we effectively reduce autovacuum_multixact_freeze_max_age and
- * to a value just less than the number of multixacts in use.  We hope that
- * this will quickly trigger autovacuuming on the table or tables with the
- * oldest relminmxid, thus allowing datminmxid values to advance and removing
- * some members.
- *
- * As the fraction of the member space currently in use grows, we become
- * more aggressive in clamping this value.  That not only causes autovacuum
- * to ramp up, but also makes any manual vacuums the user issues more
- * aggressive.  This happens because vacuum_get_cutoffs() will clamp the
- * freeze table and the minimum freeze age cutoffs based on the effective
- * autovacuum_multixact_freeze_max_age this function returns.  In the worst
- * case, we'll claim the freeze_max_age to zero, and every vacuum of any
- * table will freeze every multixact.
- */
-int
-MultiXactMemberFreezeThreshold(void)
-{
-	MultiXactOffset members;
-	uint32		multixacts;
-	uint32		victim_multixacts;
-	double		fraction;
-	int			result;
-
-	/* If we can't determine member space utilization, assume the worst. */
-	if (!ReadMultiXactCounts(&multixacts, &members))
-		return 0;
-
-	/* If member space utilization is low, no special action is required. */
-	if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
-		return autovacuum_multixact_freeze_max_age;
-
-	/*
-	 * Compute a target for relminmxid advancement.  The number of multixacts
-	 * we try to eliminate from the system is based on how far we are past
-	 * MULTIXACT_MEMBER_SAFE_THRESHOLD.
-	 */
-	fraction = (double) (members - MULTIXACT_MEMBER_SAFE_THRESHOLD) /
-		(MULTIXACT_MEMBER_DANGER_THRESHOLD - MULTIXACT_MEMBER_SAFE_THRESHOLD);
-	victim_multixacts = multixacts * fraction;
-
-	/* fraction could be > 1.0, but lowest possible freeze age is zero */
-	if (victim_multixacts > multixacts)
-		return 0;
-	result = multixacts - victim_multixacts;
-
-	/*
-	 * Clamp to autovacuum_multixact_freeze_max_age, so that we never make
-	 * autovacuum less aggressive than it would otherwise be.
-	 */
-	return Min(result, autovacuum_multixact_freeze_max_age);
-}
-
 typedef struct mxtruncinfo
 {
 	int64		earliestExistingPage;
@@ -3211,7 +2961,7 @@ TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
 
 	elog(DEBUG1, "performing multixact truncation: "
 		 "offsets [%u, %u), offsets segments [%" PRIx64 ", %" PRIx64 "), "
-		 "members [%u, %u), members segments [%" PRIx64 ", %" PRIx64 ")",
+		 "members [%" PRIu64 ", %" PRIu64 "), members segments [%" PRIx64 ", %" PRIx64 ")",
 		 oldestMulti, newOldestMulti,
 		 MultiXactIdToOffsetSegment(oldestMulti),
 		 MultiXactIdToOffsetSegment(newOldestMulti),
@@ -3342,7 +3092,7 @@ MultiXactIdPrecedesOrEquals(MultiXactId multi1, MultiXactId multi2)
 static bool
 MultiXactOffsetPrecedes(MultiXactOffset offset1, MultiXactOffset offset2)
 {
-	int32		diff = (int32) (offset1 - offset2);
+	int64		diff = (int64) (offset1 - offset2);
 
 	return (diff < 0);
 }
@@ -3471,7 +3221,7 @@ multixact_redo(XLogReaderState *record)
 
 		elog(DEBUG1, "replaying multixact truncation: "
 			 "offsets [%u, %u), offsets segments [%" PRIx64 ", %" PRIx64 "), "
-			 "members [%u, %u), members segments [%" PRIx64 ", %" PRIx64 ")",
+			 "members [%" PRIu64 ", %" PRIu64 "), members segments [%" PRIx64 ", %" PRIx64 ")",
 			 xlrec.startTruncOff, xlrec.endTruncOff,
 			 MultiXactIdToOffsetSegment(xlrec.startTruncOff),
 			 MultiXactIdToOffsetSegment(xlrec.endTruncOff),
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 6ce979f2d8bc..27b6059291c6 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -884,7 +884,7 @@ InitWalRecovery(ControlFileData *ControlFile, bool *wasShutdown_ptr,
 							 U64FromFullTransactionId(checkPoint.nextXid),
 							 checkPoint.nextOid)));
 	ereport(DEBUG1,
-			(errmsg_internal("next MultiXactId: %u; next MultiXactOffset: %u",
+			(errmsg_internal("next MultiXactId: %u; next MultiXactOffset: %" PRIu64,
 							 checkPoint.nextMulti, checkPoint.nextMultiOffset)));
 	ereport(DEBUG1,
 			(errmsg_internal("oldest unfrozen transaction ID: %u, in database %u",
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 33a33bf6b1cf..0fcfc08a443e 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -1154,7 +1154,7 @@ vacuum_get_cutoffs(Relation rel, const VacuumParams *params,
 	 * normally autovacuum_multixact_freeze_max_age, but may be less if we are
 	 * short of multixact member space.
 	 */
-	effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold();
+	effective_multixact_freeze_max_age = autovacuum_multixact_freeze_max_age;
 
 	/*
 	 * Almost ready to set freeze output parameters; check if OldestXmin or
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 981be42e3afc..f1c04a57f95a 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -1137,7 +1137,7 @@ do_start_worker(void)
 
 	/* Also determine the oldest datminmxid we will consider. */
 	recentMulti = ReadNextMultiXactId();
-	multiForceLimit = recentMulti - MultiXactMemberFreezeThreshold();
+	multiForceLimit = recentMulti - autovacuum_multixact_freeze_max_age;
 	if (multiForceLimit < FirstMultiXactId)
 		multiForceLimit -= FirstMultiXactId;
 
@@ -1925,7 +1925,7 @@ do_autovacuum(void)
 	 * normally autovacuum_multixact_freeze_max_age, but may be less if we are
 	 * short of multixact member space.
 	 */
-	effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold();
+	effective_multixact_freeze_max_age = autovacuum_multixact_freeze_max_age;
 
 	/*
 	 * Find the pg_database entry and select the default freeze ages. We use
diff --git a/src/bin/pg_controldata/pg_controldata.c b/src/bin/pg_controldata/pg_controldata.c
index 7bb801bb8861..b195806699b2 100644
--- a/src/bin/pg_controldata/pg_controldata.c
+++ b/src/bin/pg_controldata/pg_controldata.c
@@ -264,7 +264,7 @@ main(int argc, char *argv[])
 		   ControlFile->checkPointCopy.nextOid);
 	printf(_("Latest checkpoint's NextMultiXactId:  %u\n"),
 		   ControlFile->checkPointCopy.nextMulti);
-	printf(_("Latest checkpoint's NextMultiOffset:  %u\n"),
+	printf(_("Latest checkpoint's NextMultiOffset:  %" PRIu64 "\n"),
 		   ControlFile->checkPointCopy.nextMultiOffset);
 	printf(_("Latest checkpoint's oldestXID:        %u\n"),
 		   ControlFile->checkPointCopy.oldestXid);
diff --git a/src/bin/pg_resetwal/pg_resetwal.c b/src/bin/pg_resetwal/pg_resetwal.c
index e876f35f38ed..925216028047 100644
--- a/src/bin/pg_resetwal/pg_resetwal.c
+++ b/src/bin/pg_resetwal/pg_resetwal.c
@@ -266,7 +266,7 @@ main(int argc, char *argv[])
 
 			case 'O':
 				errno = 0;
-				set_mxoff = strtoul(optarg, &endptr, 0);
+				set_mxoff = strtou64(optarg, &endptr, 0);
 				if (endptr == optarg || *endptr != '\0' || errno != 0)
 				{
 					pg_log_error("invalid argument for option %s", "-O");
@@ -759,7 +759,7 @@ PrintControlValues(bool guessed)
 		   ControlFile.checkPointCopy.nextOid);
 	printf(_("Latest checkpoint's NextMultiXactId:  %u\n"),
 		   ControlFile.checkPointCopy.nextMulti);
-	printf(_("Latest checkpoint's NextMultiOffset:  %u\n"),
+	printf(_("Latest checkpoint's NextMultiOffset:  %" PRIu64 "\n"),
 		   ControlFile.checkPointCopy.nextMultiOffset);
 	printf(_("Latest checkpoint's oldestXID:        %u\n"),
 		   ControlFile.checkPointCopy.oldestXid);
@@ -833,7 +833,7 @@ PrintNewControlValues(void)
 
 	if (set_mxoff != -1)
 	{
-		printf(_("NextMultiOffset:                      %u\n"),
+		printf(_("NextMultiOffset:                      %" PRIu64 "\n"),
 			   ControlFile.checkPointCopy.nextMultiOffset);
 	}
 
diff --git a/src/bin/pg_resetwal/t/001_basic.pl b/src/bin/pg_resetwal/t/001_basic.pl
index d6bbbd0ceda3..cc89e0764aea 100644
--- a/src/bin/pg_resetwal/t/001_basic.pl
+++ b/src/bin/pg_resetwal/t/001_basic.pl
@@ -213,7 +213,7 @@ sub get_slru_files
   sprintf("%d,%d", hex($files[0]) == 0 ? 3 : hex($files[0]), hex($files[-1]));
 
 @files = get_slru_files('pg_multixact/offsets');
-$mult = 32 * $blcksz / 4;
+$mult = 32 * $blcksz / 8;
 # --multixact-ids argument is "new,old"
 push @cmd,
   '--multixact-ids' => sprintf("%d,%d",
diff --git a/src/bin/pg_upgrade/Makefile b/src/bin/pg_upgrade/Makefile
index f83d2b5d3095..70908d63a314 100644
--- a/src/bin/pg_upgrade/Makefile
+++ b/src/bin/pg_upgrade/Makefile
@@ -21,6 +21,7 @@ OBJS = \
 	info.o \
 	option.o \
 	parallel.o \
+	segresize.o \
 	pg_upgrade.o \
 	relfilenumber.o \
 	server.o \
diff --git a/src/bin/pg_upgrade/meson.build b/src/bin/pg_upgrade/meson.build
index ac992f0d14b1..a8b1d77c2d58 100644
--- a/src/bin/pg_upgrade/meson.build
+++ b/src/bin/pg_upgrade/meson.build
@@ -10,6 +10,7 @@ pg_upgrade_sources = files(
   'info.c',
   'option.c',
   'parallel.c',
+  'segresize.c',
   'pg_upgrade.c',
   'relfilenumber.c',
   'server.c',
diff --git a/src/bin/pg_upgrade/pg_upgrade.c b/src/bin/pg_upgrade/pg_upgrade.c
index 536e49d26168..7fae1730164a 100644
--- a/src/bin/pg_upgrade/pg_upgrade.c
+++ b/src/bin/pg_upgrade/pg_upgrade.c
@@ -793,8 +793,42 @@ copy_xact_xlog_xid(void)
 	if (old_cluster.controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER &&
 		new_cluster.controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER)
 	{
-		copy_subdir_files("pg_multixact/offsets", "pg_multixact/offsets");
-		copy_subdir_files("pg_multixact/members", "pg_multixact/members");
+		/*
+		 * If the old server is before the MULTIXACTOFFSET_FORMATCHANGE_CAT_VER
+		 * it must have 32-bit multixid offsets, thus it should be converted.
+		 */
+		if (old_cluster.controldata.cat_ver < MULTIXACTOFFSET_FORMATCHANGE_CAT_VER &&
+			new_cluster.controldata.cat_ver >= MULTIXACTOFFSET_FORMATCHANGE_CAT_VER)
+		{
+			MultiXactOffset		oldest_offset,
+								next_offset;
+
+			remove_new_subdir("pg_multixact/offsets", false);
+			prep_status("Converting pg_multixact/offsets to 64-bit");
+			oldest_offset = convert_multixact_offsets();
+			check_ok();
+
+			remove_new_subdir("pg_multixact/members", false);
+			prep_status("Converting pg_multixact/members");
+			convert_multixact_members(oldest_offset);
+			check_ok();
+
+			next_offset = old_cluster.controldata.chkpnt_nxtmxoff;
+			if (oldest_offset)
+			{
+				if (next_offset < oldest_offset)
+					next_offset += ((MultiXactOffset) 1 << 32) - 1;
+
+				next_offset -= oldest_offset - 1;
+
+				old_cluster.controldata.chkpnt_nxtmxoff = next_offset;
+			}
+		}
+		else
+		{
+			copy_subdir_files("pg_multixact/offsets", "pg_multixact/offsets");
+			copy_subdir_files("pg_multixact/members", "pg_multixact/members");
+		}
 
 		prep_status("Setting next multixact ID and offset for new cluster");
 
@@ -803,7 +837,7 @@ copy_xact_xlog_xid(void)
 		 * counters here and the oldest multi present on system.
 		 */
 		exec_prog(UTILITY_LOG_FILE, NULL, true, true,
-				  "\"%s/pg_resetwal\" -O %u -m %u,%u \"%s\"",
+				  "\"%s/pg_resetwal\" -O %" PRIu64 " -m %u,%u \"%s\"",
 				  new_cluster.bindir,
 				  old_cluster.controldata.chkpnt_nxtmxoff,
 				  old_cluster.controldata.chkpnt_nxtmulti,
diff --git a/src/bin/pg_upgrade/pg_upgrade.h b/src/bin/pg_upgrade/pg_upgrade.h
index 69c965bb7d09..2d4f1d39e55c 100644
--- a/src/bin/pg_upgrade/pg_upgrade.h
+++ b/src/bin/pg_upgrade/pg_upgrade.h
@@ -114,6 +114,13 @@ extern char *output_files[];
  */
 #define MULTIXACT_FORMATCHANGE_CAT_VER 201301231
 
+/*
+ * Swicth from 32-bit to 64-bit for multixid offsets.
+ *
+ * XXX: should be changed to the actual CATALOG_VERSION_NO on commit.
+ */
+#define MULTIXACTOFFSET_FORMATCHANGE_CAT_VER 202409041
+
 /*
  * large object chunk size added to pg_controldata,
  * commit 5f93c37805e7485488480916b4585e098d3cc883
@@ -235,7 +242,7 @@ typedef struct
 	uint32		chkpnt_nxtepoch;
 	uint32		chkpnt_nxtoid;
 	uint32		chkpnt_nxtmulti;
-	uint32		chkpnt_nxtmxoff;
+	uint64		chkpnt_nxtmxoff;
 	uint32		chkpnt_oldstMulti;
 	uint32		chkpnt_oldstxid;
 	uint32		align;
@@ -526,3 +533,8 @@ typedef struct
 	FILE	   *file;
 	char		path[MAXPGPATH];
 } UpgradeTaskReport;
+
+/* segresize.c */
+
+MultiXactOffset		convert_multixact_offsets(void);
+void				convert_multixact_members(MultiXactOffset oldest_offset);
diff --git a/src/bin/pg_upgrade/segresize.c b/src/bin/pg_upgrade/segresize.c
new file mode 100644
index 000000000000..7f80d0652ac0
--- /dev/null
+++ b/src/bin/pg_upgrade/segresize.c
@@ -0,0 +1,527 @@
+/*
+ *	segresize.c
+ *
+ *	SLRU segment resize utility
+ *
+ *	Copyright (c) 2024, PostgreSQL Global Development Group
+ *	src/bin/pg_upgrade/segresize.c
+ */
+
+#include "postgres_fe.h"
+
+#include "pg_upgrade.h"
+#include "access/multixact.h"
+
+/* See slru.h */
+#define SLRU_PAGES_PER_SEGMENT		32
+
+/*
+ * Some kind of iterator associated with a particular SLRU segment.  The idea is
+ * to specify the segment and page number and then move through the pages.
+ */
+typedef struct SlruSegState
+{
+	char	   *dir;
+	char	   *fn;
+	FILE	   *file;
+	int64		segno;
+	uint64		pageno;
+	bool		leading_gap;
+} SlruSegState;
+
+/*
+ * Mirrors the SlruFileName from slru.c
+ */
+static inline char *
+SlruFileName(SlruSegState *state)
+{
+	Assert(state->segno >= 0 && state->segno <= INT64CONST(0xFFFFFF));
+	return psprintf("%s/%04X", state->dir, (unsigned int) state->segno);
+}
+
+/*
+ * Create new SLRU segment file.
+ */
+static void
+create_segment(SlruSegState *state)
+{
+	Assert(state->fn == NULL);
+	Assert(state->file == NULL);
+
+	state->fn = SlruFileName(state);
+	state->file = fopen(state->fn, "wb");
+	if (!state->file)
+		pg_fatal("could not create file \"%s\": %m", state->fn);
+}
+
+/*
+ * Open existing SLRU segment file.
+ */
+static void
+open_segment(SlruSegState *state)
+{
+	Assert(state->fn == NULL);
+	Assert(state->file == NULL);
+
+	state->fn = SlruFileName(state);
+	state->file = fopen(state->fn, "rb");
+	if (!state->file)
+		pg_fatal("could not open file \"%s\": %m", state->fn);
+}
+
+/*
+ * Close SLRU segment file.
+ */
+static void
+close_segment(SlruSegState *state)
+{
+	if (state->file)
+	{
+		fclose(state->file);
+		state->file = NULL;
+	}
+
+	if (state->fn)
+	{
+		pfree(state->fn);
+		state->fn = NULL;
+	}
+}
+
+/*
+ * Read next page from the old 32-bit offset segment file.
+ */
+static int
+read_old_segment_page(SlruSegState *state, void *buf, bool *empty)
+{
+	int		len;
+
+	/* Open next segment file, if needed. */
+	if (!state->fn)
+	{
+		if (!state->segno)
+			state->leading_gap = true;
+
+		open_segment(state);
+
+		/* Set position to the needed page. */
+		if (state->pageno > 0 &&
+			fseek(state->file, state->pageno * BLCKSZ, SEEK_SET))
+		{
+			close_segment(state);
+		}
+	}
+
+	if (state->file)
+	{
+		/* Segment file do exists, read page from it. */
+		state->leading_gap = false;
+
+		len = fread(buf, sizeof(char), BLCKSZ, state->file);
+
+		/* Are we done or was there an error? */
+		if (len <= 0)
+		{
+			if (ferror(state->file))
+				pg_fatal("error reading file \"%s\": %m", state->fn);
+
+			if (feof(state->file))
+			{
+				*empty = true;
+				len = -1;
+
+				close_segment(state);
+			}
+		}
+		else
+			*empty = false;
+	}
+	else if (!state->leading_gap)
+	{
+		/* We reached the last segment. */
+		len = -1;
+		*empty = true;
+	}
+	else
+	{
+		/* Skip few first segments if they were frozen and removed. */
+		len = BLCKSZ;
+		*empty = true;
+	}
+
+	if (++state->pageno >= SLRU_PAGES_PER_SEGMENT)
+	{
+		/* Start a new segment. */
+		state->segno++;
+		state->pageno = 0;
+
+		close_segment(state);
+	}
+
+	return len;
+}
+
+/*
+ * Write next page to the new 64-bit offset segment file.
+ */
+static void
+write_new_segment_page(SlruSegState *state, void *buf)
+{
+	/*
+	 * Create a new segment file if we still didn't.  Creation is
+	 * postponed until the first non-empty page is found.  This helps
+	 * not to create completely empty segments.
+	 */
+	if (!state->file)
+	{
+		create_segment(state);
+
+		/* Write zeroes to the previously skipped prefix. */
+		if (state->pageno > 0)
+		{
+			char		zerobuf[BLCKSZ] = {0};
+
+			for (int64 i = 0; i < state->pageno; i++)
+			{
+				if (fwrite(zerobuf, sizeof(char), BLCKSZ, state->file) != BLCKSZ)
+					pg_fatal("could not write file \"%s\": %m", state->fn);
+			}
+		}
+	}
+
+	/* Write page to the new segment (if it was created). */
+	if (state->file)
+	{
+		if (fwrite(buf, sizeof(char), BLCKSZ, state->file) != BLCKSZ)
+			pg_fatal("could not write file \"%s\": %m", state->fn);
+	}
+
+	/*
+	 * Did we reach the maximum page number?  Then close segment file
+	 * and create a new one on the next iteration.
+	 */
+	if (++state->pageno >= SLRU_PAGES_PER_SEGMENT)
+	{
+		/* Start a new segment. */
+		state->segno++;
+		state->pageno = 0;
+
+		close_segment(state);
+	}
+}
+
+typedef uint32 MultiXactOffsetOld;
+
+#define MaxMultiXactOffsetOld	((MultiXactOffsetOld) 0xFFFFFFFF)
+
+#define MULTIXACT_OFFSETS_PER_PAGE_OLD (BLCKSZ / sizeof(MultiXactOffsetOld))
+#define MULTIXACT_OFFSETS_PER_PAGE_NEW (BLCKSZ / sizeof(MultiXactOffset))
+
+/*
+ * Convert pg_multixact/offsets segments and return oldest multi offset.
+ */
+MultiXactOffset
+convert_multixact_offsets(void)
+{
+	SlruSegState		oldseg = {0},
+						newseg = {0};
+	MultiXactOffsetOld	oldbuf[MULTIXACT_OFFSETS_PER_PAGE_OLD] = {0};
+	MultiXactOffset		newbuf[MULTIXACT_OFFSETS_PER_PAGE_NEW] = {0},
+						oldest_offset = 0;
+	uint64				oldest_multi = old_cluster.controldata.chkpnt_oldstMulti,
+						next_multi = old_cluster.controldata.chkpnt_nxtmulti,
+						multi,
+						old_entry,
+						new_entry;
+	bool				oldest_offset_known = false;
+
+	oldseg.dir = psprintf("%s/pg_multixact/offsets", old_cluster.pgdata);
+	newseg.dir = psprintf("%s/pg_multixact/offsets", new_cluster.pgdata);
+
+	old_entry = oldest_multi % MULTIXACT_OFFSETS_PER_PAGE_OLD;
+	oldseg.pageno = oldest_multi / MULTIXACT_OFFSETS_PER_PAGE_OLD;
+	oldseg.segno = oldseg.pageno / SLRU_PAGES_PER_SEGMENT;
+	oldseg.pageno %= SLRU_PAGES_PER_SEGMENT;
+
+	new_entry = oldest_multi % MULTIXACT_OFFSETS_PER_PAGE_NEW;
+	newseg.pageno = oldest_multi / MULTIXACT_OFFSETS_PER_PAGE_NEW;
+	newseg.segno = newseg.pageno / SLRU_PAGES_PER_SEGMENT;
+	newseg.pageno %= SLRU_PAGES_PER_SEGMENT;
+
+	if (next_multi < oldest_multi)
+		next_multi += (uint64) 1 << 32;	/* wraparound */
+
+	/* Copy multi offsets reading only needed segment pages */
+	for (multi = oldest_multi; multi < next_multi; old_entry = 0)
+	{
+		int		oldlen;
+		bool	empty;
+
+		/* Handle possible segment wraparound */
+#define OLD_OFFSET_SEGNO_MAX	\
+	(MaxMultiXactId / MULTIXACT_OFFSETS_PER_PAGE_OLD / SLRU_PAGES_PER_SEGMENT)
+		if (oldseg.segno > OLD_OFFSET_SEGNO_MAX)
+		{
+			oldseg.segno = 0;
+			oldseg.pageno = 0;
+		}
+
+		oldlen = read_old_segment_page(&oldseg, oldbuf, &empty);
+		if (empty || oldlen != BLCKSZ)
+			pg_fatal("cannot read page %" PRIu64 " from file \"%s\": %m",
+					 oldseg.pageno, oldseg.fn);
+
+		/* Save oldest multi offset */
+		if (!oldest_offset_known)
+		{
+			oldest_offset = oldbuf[old_entry];
+			oldest_offset_known = true;
+		}
+
+		/* Skip wrapped-around invalid MultiXactIds */
+		if (multi == (uint64) 1 << 32)
+		{
+			Assert(oldseg.segno == 0);
+			Assert(oldseg.pageno == 1);
+			Assert(old_entry == 0);
+			Assert(new_entry == 0);
+
+			multi += FirstMultiXactId;
+			old_entry = FirstMultiXactId;
+			new_entry = FirstMultiXactId;
+		}
+
+		/* Copy entries to the new page */
+		for (; multi < next_multi && old_entry < MULTIXACT_OFFSETS_PER_PAGE_OLD;
+			 multi++, old_entry++)
+		{
+			MultiXactOffset offset = oldbuf[old_entry];
+
+			/* Handle possible offset wraparound (1 becomes 2^32) */
+			if (offset < oldest_offset)
+				offset += ((uint64) 1 << 32) - 1;
+
+			/* Subtract oldest_offset, so new offsets will start from 1 */
+			newbuf[new_entry++] = offset - oldest_offset + 1;
+
+			if (new_entry >= MULTIXACT_OFFSETS_PER_PAGE_NEW)
+			{
+				/* Handle possible segment wraparound */
+#define NEW_OFFSET_SEGNO_MAX	\
+	(MaxMultiXactId / MULTIXACT_OFFSETS_PER_PAGE_NEW / SLRU_PAGES_PER_SEGMENT)
+				if (newseg.segno > NEW_OFFSET_SEGNO_MAX)
+				{
+					newseg.segno = 0;
+					newseg.pageno = 0;
+				}
+
+				/* Write new page */
+				write_new_segment_page(&newseg, newbuf);
+				new_entry = 0;
+			}
+		}
+	}
+
+	/* Write the last incomplete page */
+	if (new_entry > 0 || oldest_multi == next_multi)
+	{
+		memset(&newbuf[new_entry], 0,
+			   sizeof(newbuf[0]) * (MULTIXACT_OFFSETS_PER_PAGE_NEW - new_entry));
+		write_new_segment_page(&newseg, newbuf);
+	}
+
+	/* Use next_offset as oldest_offset, if oldest_multi == next_multi */
+	if (!oldest_offset_known)
+	{
+		Assert(oldest_multi == next_multi);
+		oldest_offset = (MultiXactOffset) old_cluster.controldata.chkpnt_nxtmxoff;
+	}
+
+	/* Release resources */
+	close_segment(&oldseg);
+	close_segment(&newseg);
+
+	pfree(oldseg.dir);
+	pfree(newseg.dir);
+
+	return oldest_offset;
+}
+
+#define MXACT_MEMBERS_FLAG_BYTES			1
+
+#define MULTIXACT_MEMBERS_PER_GROUP			4
+#define MULTIXACT_MEMBERGROUP_SIZE			\
+	(MULTIXACT_MEMBERS_PER_GROUP * (sizeof(TransactionId) + MXACT_MEMBERS_FLAG_BYTES))
+#define MULTIXACT_MEMBERGROUPS_PER_PAGE		\
+	(BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
+
+#define MULTIXACT_MEMBERS_PER_PAGE				\
+	(MULTIXACT_MEMBERS_PER_GROUP * MULTIXACT_MEMBERGROUPS_PER_PAGE)
+#define MULTIXACT_MEMBER_FLAG_BYTES_PER_GROUP	\
+	(MXACT_MEMBERS_FLAG_BYTES * MULTIXACT_MEMBERS_PER_GROUP)
+
+typedef struct MultiXactMembersCtx
+{
+	SlruSegState	seg;
+	char			buf[BLCKSZ];
+	int				group;
+	int				member;
+	char		   *flag;
+	TransactionId  *xid;
+} MultiXactMembersCtx;
+
+static void
+MultiXactMembersCtxInit(MultiXactMembersCtx *ctx)
+{
+	ctx->seg.dir = psprintf("%s/pg_multixact/members", new_cluster.pgdata);
+
+	ctx->group = 0;
+	ctx->member = 1;		/* skip invalid zero offset */
+
+	ctx->flag = (char *) ctx->buf + ctx->group * MULTIXACT_MEMBERGROUP_SIZE;
+	ctx->xid = (TransactionId *)(ctx->flag + MXACT_MEMBERS_FLAG_BYTES * MULTIXACT_MEMBERS_PER_GROUP);
+
+	ctx->flag += ctx->member;
+	ctx->xid += ctx->member;
+}
+
+static void
+MultiXactMembersCtxAdd(MultiXactMembersCtx *ctx, char flag, TransactionId xid)
+{
+	/* Copy member's xid and flags to the new page */
+	*ctx->flag++ = flag;
+	*ctx->xid++ = xid;
+
+	if (++ctx->member < MULTIXACT_MEMBERS_PER_GROUP)
+		return;
+
+	/* Start next member group */
+	ctx->member = 0;
+
+	if (++ctx->group >= MULTIXACT_MEMBERGROUPS_PER_PAGE)
+	{
+		/* Write current page and start new */
+		write_new_segment_page(&ctx->seg, ctx->buf);
+
+		ctx->group = 0;
+		memset(ctx->buf, 0, BLCKSZ);
+	}
+
+	ctx->flag = (char *) ctx->buf + ctx->group * MULTIXACT_MEMBERGROUP_SIZE;
+	ctx->xid = (TransactionId *)(ctx->flag + MXACT_MEMBERS_FLAG_BYTES * MULTIXACT_MEMBERS_PER_GROUP);
+}
+
+static void
+MultiXactMembersCtxFinit(MultiXactMembersCtx *ctx)
+{
+	if (ctx->flag > (char *) ctx->buf)
+		write_new_segment_page(&ctx->seg, ctx->buf);
+
+	close_segment(&ctx->seg);
+
+	pfree(ctx->seg.dir);
+}
+
+/*
+ * Convert pg_multixact/members segments, offsets will start from 1.
+ *
+ */
+void
+convert_multixact_members(MultiXactOffset oldest_offset)
+{
+	MultiXactOffset			next_offset,
+							offset;
+	SlruSegState			oldseg = {0};
+	char					oldbuf[BLCKSZ] = {0};
+	int						oldidx;
+	MultiXactMembersCtx		newctx = {0};
+
+	oldseg.dir = psprintf("%s/pg_multixact/members", old_cluster.pgdata);
+
+	next_offset = (MultiXactOffset) old_cluster.controldata.chkpnt_nxtmxoff;
+	if (next_offset < oldest_offset)
+		next_offset += ((uint64) 1 << 32) - 1;
+
+	/* Initialize the old starting position */
+	oldseg.pageno = oldest_offset / MULTIXACT_MEMBERS_PER_PAGE;
+	oldseg.segno = oldseg.pageno / SLRU_PAGES_PER_SEGMENT;
+	oldseg.pageno %= SLRU_PAGES_PER_SEGMENT;
+
+	/* Initialize new starting position */
+	MultiXactMembersCtxInit(&newctx);
+
+	/* Iterate through the original directory */
+	oldidx = oldest_offset % MULTIXACT_MEMBERS_PER_PAGE;
+	for (offset = oldest_offset; offset < next_offset;)
+	{
+		bool	empty;
+		int		oldlen;
+		int		ngroups;
+		int		oldgroup;
+		int		oldmember;
+
+		oldlen = read_old_segment_page(&oldseg, oldbuf, &empty);
+		if (empty || oldlen != BLCKSZ)
+			pg_fatal("cannot read page %" PRIu64 " from file \"%s\": %m",
+					 oldseg.pageno, oldseg.fn);
+
+		/* Iterate through the old member groups */
+		ngroups = oldlen / MULTIXACT_MEMBERGROUP_SIZE;
+		oldmember = oldidx % MULTIXACT_MEMBERS_PER_GROUP;
+		oldgroup = oldidx / MULTIXACT_MEMBERS_PER_GROUP;
+		while (oldgroup < ngroups && offset < next_offset)
+		{
+			char		   *oldflag;
+			TransactionId  *oldxid;
+			int				i;
+
+			oldflag = (char *) oldbuf + oldgroup * MULTIXACT_MEMBERGROUP_SIZE;
+			oldxid = (TransactionId *)(oldflag + MULTIXACT_MEMBER_FLAG_BYTES_PER_GROUP);
+
+			oldxid += oldmember;
+			oldflag += oldmember;
+
+			/* Iterate through the old members */
+			for (i = oldmember;
+				 i < MULTIXACT_MEMBERS_PER_GROUP && offset < next_offset;
+				 i++)
+			{
+				MultiXactMembersCtxAdd(&newctx, *oldflag++, *oldxid++);
+
+				if (++offset == (uint64) 1 << 32)
+				{
+					Assert(i == MaxMultiXactOffsetOld % MULTIXACT_MEMBERS_PER_GROUP);
+					goto wraparound;
+				}
+			}
+
+			oldgroup++;
+			oldmember = 0;
+		}
+
+		oldidx = 0;
+
+		continue;
+
+wraparound:
+#define SEGNO_MAX	MaxMultiXactOffsetOld / MULTIXACT_MEMBERS_PER_PAGE / SLRU_PAGES_PER_SEGMENT
+#define PAGENO_MAX	MaxMultiXactOffsetOld / MULTIXACT_MEMBERS_PER_PAGE % SLRU_PAGES_PER_SEGMENT
+		Assert((oldseg.segno == SEGNO_MAX && oldseg.pageno == PAGENO_MAX + 1) ||
+			   (oldseg.segno == SEGNO_MAX + 1 && oldseg.pageno == 0));
+
+		/* Switch to segment 0000 */
+		close_segment(&oldseg);
+		oldseg.segno = 0;
+		oldseg.pageno = 0;
+
+		/* skip invalid zero multi offset */
+		oldidx = 1;
+	}
+
+	MultiXactMembersCtxFinit(&newctx);
+
+	/* Release resources */
+	close_segment(&oldseg);
+
+	pfree(oldseg.dir);
+}
diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index 4e6b0eec2ff4..5ee632dfe691 100644
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -27,7 +27,7 @@
 
 #define MultiXactIdIsValid(multi) ((multi) != InvalidMultiXactId)
 
-#define MaxMultiXactOffset	((MultiXactOffset) 0xFFFFFFFF)
+#define MaxMultiXactOffset	UINT64CONST(0xFFFFFFFFFFFFFFFF)
 
 /*
  * Possible multixact lock modes ("status").  The first four modes are for
@@ -143,7 +143,6 @@ extern void MultiXactSetNextMXact(MultiXactId nextMulti,
 extern void MultiXactAdvanceNextMXact(MultiXactId minMulti,
 									  MultiXactOffset minMultiOffset);
 extern void MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB);
-extern int	MultiXactMemberFreezeThreshold(void);
 
 extern void multixact_twophase_recover(TransactionId xid, uint16 info,
 									   void *recdata, uint32 len);
diff --git a/src/include/c.h b/src/include/c.h
index 8cdc16a0f4a9..45ba4c523fe8 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -632,7 +632,7 @@ typedef uint32 SubTransactionId;
 /* MultiXactId must be equivalent to TransactionId, to fit in t_xmax */
 typedef TransactionId MultiXactId;
 
-typedef uint32 MultiXactOffset;
+typedef uint64 MultiXactOffset;
 
 typedef uint32 CommandId;