diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index 026f545676..2589941ec4 100644
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -1112,7 +1112,7 @@ AllocSetRealloc(void *pointer, Size size)
 	AllocBlock	block;
 	AllocSet	set;
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
-	Size		oldsize;
+	Size		oldchksize;
 	int			fidx;
 
 	/* Allow access to private part of chunk header. */
@@ -1140,11 +1140,11 @@ AllocSetRealloc(void *pointer, Size size)
 
 		set = block->aset;
 
-		oldsize = block->endptr - (char *) pointer;
+		oldchksize = block->endptr - (char *) pointer;
 
 #ifdef MEMORY_CONTEXT_CHECKING
 		/* Test for someone scribbling on unused space in chunk */
-		Assert(chunk->requested_size < oldsize);
+		Assert(chunk->requested_size < oldchksize);
 		if (!sentinel_ok(pointer, chunk->requested_size))
 			elog(WARNING, "detected write past chunk end in %s %p",
 				 set->header.name, chunk);
@@ -1187,21 +1187,29 @@ AllocSetRealloc(void *pointer, Size size)
 
 #ifdef MEMORY_CONTEXT_CHECKING
 #ifdef RANDOMIZE_ALLOCATED_MEMORY
-		/* We can only fill the extra space if we know the prior request */
+
+		/*
+		 * We can only randomize the extra space if we know the prior request.
+		 * When using Valgrind, randomize_mem() also marks memory UNDEFINED.
+		 */
 		if (size > chunk->requested_size)
 			randomize_mem((char *) pointer + chunk->requested_size,
 						  size - chunk->requested_size);
-#endif
+#else
 
 		/*
-		 * realloc() (or randomize_mem()) will have left any newly-allocated
-		 * part UNDEFINED, but we may need to adjust trailing bytes from the
-		 * old allocation.
+		 * If this is an increase, realloc() will have marked any
+		 * newly-allocated part (from oldchksize to chksize) UNDEFINED, but we
+		 * also need to adjust trailing bytes from the old allocation (from
+		 * chunk->requested_size to oldchksize) as they are marked NOACCESS.
+		 * Make sure not to mark too many bytes in case chunk->requested_size
+		 * < size < oldchksize.
 		 */
 #ifdef USE_VALGRIND
-		if (oldsize > chunk->requested_size)
+		if (Min(size, oldchksize) > chunk->requested_size)
 			VALGRIND_MAKE_MEM_UNDEFINED((char *) pointer + chunk->requested_size,
-										oldsize - chunk->requested_size);
+										Min(size, oldchksize) - chunk->requested_size);
+#endif
 #endif
 
 		chunk->requested_size = size;
@@ -1211,11 +1219,14 @@ AllocSetRealloc(void *pointer, Size size)
 #else							/* !MEMORY_CONTEXT_CHECKING */
 
 		/*
-		 * We don't know how much of the old chunk size was the actual
-		 * allocation; it could have been as small as one byte.  We have to be
-		 * conservative and just mark the entire old portion DEFINED.
+		 * We may need to adjust marking of bytes from the old allocation as
+		 * some of them may be marked NOACCESS.  We don't know how much of the
+		 * old chunk size was the requested size; it could have been as small
+		 * as one byte.  We have to be conservative and just mark the entire
+		 * old portion DEFINED.  Make sure not to mark memory beyond the new
+		 * allocation in case it's smaller than the old one.
 		 */
-		VALGRIND_MAKE_MEM_DEFINED(pointer, oldsize);
+		VALGRIND_MAKE_MEM_DEFINED(pointer, Min(size, oldchksize));
 #endif
 
 		/* Ensure any padding bytes are marked NOACCESS. */
@@ -1240,11 +1251,11 @@ AllocSetRealloc(void *pointer, Size size)
 
 	fidx = MemoryChunkGetValue(chunk);
 	Assert(FreeListIdxIsValid(fidx));
-	oldsize = GetChunkSizeFromFreeListIdx(fidx);
+	oldchksize = GetChunkSizeFromFreeListIdx(fidx);
 
 #ifdef MEMORY_CONTEXT_CHECKING
 	/* Test for someone scribbling on unused space in chunk */
-	if (chunk->requested_size < oldsize)
+	if (chunk->requested_size < oldchksize)
 		if (!sentinel_ok(pointer, chunk->requested_size))
 			elog(WARNING, "detected write past chunk end in %s %p",
 				 set->header.name, chunk);
@@ -1255,7 +1266,7 @@ AllocSetRealloc(void *pointer, Size size)
 	 * allocated area already is >= the new size.  (In particular, we will
 	 * fall out here if the requested size is a decrease.)
 	 */
-	if (oldsize >= size)
+	if (oldchksize >= size)
 	{
 #ifdef MEMORY_CONTEXT_CHECKING
 		Size		oldrequest = chunk->requested_size;
@@ -1278,10 +1289,10 @@ AllocSetRealloc(void *pointer, Size size)
 										size - oldrequest);
 		else
 			VALGRIND_MAKE_MEM_NOACCESS((char *) pointer + size,
-									   oldsize - size);
+									   oldchksize - size);
 
 		/* set mark to catch clobber of "unused" space */
-		if (size < oldsize)
+		if (size < oldchksize)
 			set_sentinel(pointer, size);
 #else							/* !MEMORY_CONTEXT_CHECKING */
 
@@ -1290,7 +1301,7 @@ AllocSetRealloc(void *pointer, Size size)
 		 * the old request or shrinking it, so we conservatively mark the
 		 * entire new allocation DEFINED.
 		 */
-		VALGRIND_MAKE_MEM_NOACCESS(pointer, oldsize);
+		VALGRIND_MAKE_MEM_NOACCESS(pointer, oldchksize);
 		VALGRIND_MAKE_MEM_DEFINED(pointer, size);
 #endif
 
@@ -1313,6 +1324,7 @@ AllocSetRealloc(void *pointer, Size size)
 		 * memory indefinitely.  See pgsql-hackers archives for 2007-08-11.)
 		 */
 		AllocPointer newPointer;
+		Size		oldsize;
 
 		/* allocate new chunk */
 		newPointer = AllocSetAlloc((MemoryContext) set, size);
@@ -1337,6 +1349,7 @@ AllocSetRealloc(void *pointer, Size size)
 #ifdef MEMORY_CONTEXT_CHECKING
 		oldsize = chunk->requested_size;
 #else
+		oldsize = oldchksize;
 		VALGRIND_MAKE_MEM_DEFINED(pointer, oldsize);
 #endif