From 23eead34921a16b8301160ce8bde51be615856f0 Mon Sep 17 00:00:00 2001
From: Tomas Vondra <tomas.vondra@postgresql.org>
Date: Wed, 22 Nov 2023 23:26:08 +0100
Subject: [PATCH v20231124 5/7] hold the vm buffer for IOS prefetching

---
 src/backend/access/index/indexam.c       |  7 ++-----
 src/backend/executor/nodeIndexonlyscan.c | 12 ++++++++++++
 src/include/access/genam.h               |  3 +++
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 7456a69ab34..53948986ada 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -336,6 +336,7 @@ index_beginscan_internal(Relation indexRelation,
 
 		prefetcher->prefetchTarget = 0;
 		prefetcher->prefetchMaxTarget = prefetch_max;
+		prefetcher->vmBuffer = InvalidBuffer;
 
 		scan->xs_prefetch = prefetcher;
 	}
@@ -1278,14 +1279,10 @@ index_prefetch(IndexScanDesc scan, ItemPointer tid, bool skip_all_visible)
 	if (skip_all_visible)
 	{
 		bool	all_visible;
-		Buffer	vmbuffer = InvalidBuffer;
 
 		all_visible = VM_ALL_VISIBLE(scan->heapRelation,
 									 block,
-									 &vmbuffer);
-
-		if (vmbuffer != InvalidBuffer)
-			ReleaseBuffer(vmbuffer);
+									 &prefetch->vmBuffer);
 
 		if (all_visible)
 			return;
diff --git a/src/backend/executor/nodeIndexonlyscan.c b/src/backend/executor/nodeIndexonlyscan.c
index 545046e98ad..60cb772344f 100644
--- a/src/backend/executor/nodeIndexonlyscan.c
+++ b/src/backend/executor/nodeIndexonlyscan.c
@@ -412,6 +412,18 @@ ExecEndIndexOnlyScan(IndexOnlyScanState *node)
 		node->ioss_VMBuffer = InvalidBuffer;
 	}
 
+	/* Release VM buffer pin from prefetcher, if any. */
+	if (indexScanDesc && indexScanDesc->xs_prefetch)
+	{
+		IndexPrefetch indexPrefetch = indexScanDesc->xs_prefetch;
+
+		if (indexPrefetch->vmBuffer != InvalidBuffer)
+		{
+			ReleaseBuffer(indexPrefetch->vmBuffer);
+			indexPrefetch->vmBuffer = InvalidBuffer;
+		}
+	}
+
 	/*
 	 * close the index relation (no-op if we didn't open it)
 	 */
diff --git a/src/include/access/genam.h b/src/include/access/genam.h
index 9e2d77ef23b..8a3be673730 100644
--- a/src/include/access/genam.h
+++ b/src/include/access/genam.h
@@ -270,6 +270,9 @@ typedef struct IndexPrefetchData
 	uint64		countSkipSequential;
 	uint64		countSkipCached;
 
+	/* used when prefetching index-only scans */
+	Buffer		vmBuffer;
+
 	/*
 	 * Queue of TIDs to prefetch.
 	 *
-- 
2.42.0