diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index c0623f106d2..ccf78ffe0cb 100644
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -132,6 +132,7 @@ typedef struct AllocSetContext
 	Size		maxBlockSize;	/* maximum block size */
 	Size		nextBlockSize;	/* next block size to allocate */
 	Size		allocChunkLimit;	/* effective chunk size limit */
+	Size		memAllocated;	/* track memory allocated for this context */
 	AllocBlock	keeper;			/* keep this block over resets */
 	/* freelist this context could be put in, or -1 if not a candidate: */
 	int			freeListIndex;	/* index in context_freelists[], or -1 */
@@ -272,6 +273,7 @@ static void *AllocSetRealloc(MemoryContext context, void *pointer, Size size);
 static void AllocSetReset(MemoryContext context);
 static void AllocSetDelete(MemoryContext context);
 static Size AllocSetGetChunkSpace(MemoryContext context, void *pointer);
+static Size AllocSetMemAllocated(MemoryContext context);
 static bool AllocSetIsEmpty(MemoryContext context);
 static void AllocSetStats(MemoryContext context,
 						  MemoryStatsPrintFunc printfunc, void *passthru,
@@ -291,6 +293,7 @@ static const MemoryContextMethods AllocSetMethods = {
 	AllocSetReset,
 	AllocSetDelete,
 	AllocSetGetChunkSpace,
+	AllocSetMemAllocated,
 	AllocSetIsEmpty,
 	AllocSetStats
 #ifdef MEMORY_CONTEXT_CHECKING
@@ -464,8 +467,7 @@ AllocSetContextCreateInternal(MemoryContext parent,
 								parent,
 								name);
 
-			((MemoryContext) set)->mem_allocated =
-				set->keeper->endptr - ((char *) set);
+			set->memAllocated = set->keeper->endptr - ((char *) set);
 
 			return (MemoryContext) set;
 		}
@@ -555,7 +557,7 @@ AllocSetContextCreateInternal(MemoryContext parent,
 						parent,
 						name);
 
-	((MemoryContext) set)->mem_allocated = firstBlockSize;
+	set->memAllocated = firstBlockSize;
 
 	return (MemoryContext) set;
 }
@@ -617,7 +619,7 @@ AllocSetReset(MemoryContext context)
 		else
 		{
 			/* Normal case, release the block */
-			context->mem_allocated -= block->endptr - ((char*) block);
+			set->memAllocated -= block->endptr - ((char*) block);
 
 #ifdef CLOBBER_FREED_MEMORY
 			wipe_mem(block, block->freeptr - ((char *) block));
@@ -627,7 +629,7 @@ AllocSetReset(MemoryContext context)
 		block = next;
 	}
 
-	Assert(context->mem_allocated == keepersize);
+	Assert(set->memAllocated == keepersize);
 
 	/* Reset block size allocation sequence, too */
 	set->nextBlockSize = set->initBlockSize;
@@ -703,7 +705,7 @@ AllocSetDelete(MemoryContext context)
 		AllocBlock	next = block->next;
 
 		if (block != set->keeper)
-			context->mem_allocated -= block->endptr - ((char *) block);
+			set->memAllocated -= block->endptr - ((char *) block);
 
 #ifdef CLOBBER_FREED_MEMORY
 		wipe_mem(block, block->freeptr - ((char *) block));
@@ -715,7 +717,7 @@ AllocSetDelete(MemoryContext context)
 		block = next;
 	}
 
-	Assert(context->mem_allocated == keepersize);
+	Assert(set->memAllocated == keepersize);
 
 	/* Finally, free the context header, including the keeper block */
 	free(set);
@@ -758,7 +760,7 @@ AllocSetAlloc(MemoryContext context, Size size)
 		if (block == NULL)
 			return NULL;
 
-		context->mem_allocated += blksize;
+		set->memAllocated += blksize;
 
 		block->aset = set;
 		block->freeptr = block->endptr = ((char *) block) + blksize;
@@ -955,7 +957,7 @@ AllocSetAlloc(MemoryContext context, Size size)
 		if (block == NULL)
 			return NULL;
 
-		context->mem_allocated += blksize;
+		set->memAllocated += blksize;
 
 		block->aset = set;
 		block->freeptr = ((char *) block) + ALLOC_BLOCKHDRSZ;
@@ -1058,7 +1060,7 @@ AllocSetFree(MemoryContext context, void *pointer)
 		if (block->next)
 			block->next->prev = block->prev;
 
-		context->mem_allocated -= block->endptr - ((char*) block);
+		set->memAllocated -= block->endptr - ((char*) block);
 
 #ifdef CLOBBER_FREED_MEMORY
 		wipe_mem(block, block->freeptr - ((char *) block));
@@ -1161,8 +1163,8 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
 		}
 
 		/* updated separately, not to underflow when (oldblksize > blksize) */
-		context->mem_allocated -= oldblksize;
-		context->mem_allocated += blksize;
+		set->memAllocated -= oldblksize;
+		set->memAllocated += blksize;
 
 		block->freeptr = block->endptr = ((char *) block) + blksize;
 
@@ -1337,6 +1339,24 @@ AllocSetGetChunkSpace(MemoryContext context, void *pointer)
 	return result;
 }
 
+/*
+ * All memory currently allocated for this context (including fragmentation
+ * and freed chunks).
+ *
+ * Allocation sizes double (up to maxBlockSize), so the current block may
+ * represent half of the total space allocated to the context. Subtract away
+ * the free space at the tail of the current block, which may never have been
+ * touched.
+ */
+static Size
+AllocSetMemAllocated(MemoryContext context)
+{
+	AllocSet set = (AllocSet) context;
+	AllocBlock currentBlock = set->blocks;
+	Size tailSpace = currentBlock->endptr - currentBlock->freeptr;
+	return set->memAllocated - tailSpace;
+}
+
 /*
  * AllocSetIsEmpty
  *		Is an allocset empty of any allocated space?
@@ -1538,7 +1558,7 @@ AllocSetCheck(MemoryContext context)
 				 name, block);
 	}
 
-	Assert(total_allocated == context->mem_allocated);
+	Assert(total_allocated == set->memAllocated);
 }
 
 #endif							/* MEMORY_CONTEXT_CHECKING */
diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c
index 56651d06931..f0ef540a7c5 100644
--- a/src/backend/utils/mmgr/generation.c
+++ b/src/backend/utils/mmgr/generation.c
@@ -61,6 +61,7 @@ typedef struct GenerationContext
 
 	/* Generational context parameters */
 	Size		blockSize;		/* standard block size */
+	Size		memAllocated;	/* track memory allocated for this context */
 
 	GenerationBlock *block;		/* current (most recently allocated) block */
 	dlist_head	blocks;			/* list of blocks */
@@ -152,6 +153,7 @@ static void *GenerationRealloc(MemoryContext context, void *pointer, Size size);
 static void GenerationReset(MemoryContext context);
 static void GenerationDelete(MemoryContext context);
 static Size GenerationGetChunkSpace(MemoryContext context, void *pointer);
+static Size GenerationMemAllocated(MemoryContext context);
 static bool GenerationIsEmpty(MemoryContext context);
 static void GenerationStats(MemoryContext context,
 							MemoryStatsPrintFunc printfunc, void *passthru,
@@ -171,6 +173,7 @@ static const MemoryContextMethods GenerationMethods = {
 	GenerationReset,
 	GenerationDelete,
 	GenerationGetChunkSpace,
+	GenerationMemAllocated,
 	GenerationIsEmpty,
 	GenerationStats
 #ifdef MEMORY_CONTEXT_CHECKING
@@ -258,6 +261,7 @@ GenerationContextCreate(MemoryContext parent,
 
 	/* Fill in GenerationContext-specific header fields */
 	set->blockSize = blockSize;
+	set->memAllocated = 0;
 	set->block = NULL;
 	dlist_init(&set->blocks);
 
@@ -297,7 +301,7 @@ GenerationReset(MemoryContext context)
 
 		dlist_delete(miter.cur);
 
-		context->mem_allocated -= block->blksize;
+		set->memAllocated -= block->blksize;
 
 #ifdef CLOBBER_FREED_MEMORY
 		wipe_mem(block, block->blksize);
@@ -354,7 +358,7 @@ GenerationAlloc(MemoryContext context, Size size)
 		if (block == NULL)
 			return NULL;
 
-		context->mem_allocated += blksize;
+		set->memAllocated += blksize;
 
 		/* block with a single (used) chunk */
 		block->blksize = blksize;
@@ -411,7 +415,7 @@ GenerationAlloc(MemoryContext context, Size size)
 		if (block == NULL)
 			return NULL;
 
-		context->mem_allocated += blksize;
+		set->memAllocated += blksize;
 
 		block->blksize = blksize;
 		block->nchunks = 0;
@@ -528,7 +532,7 @@ GenerationFree(MemoryContext context, void *pointer)
 	if (set->block == block)
 		set->block = NULL;
 
-	context->mem_allocated -= block->blksize;
+	set->memAllocated -= block->blksize;
 	free(block);
 }
 
@@ -666,6 +670,17 @@ GenerationGetChunkSpace(MemoryContext context, void *pointer)
 	return result;
 }
 
+/*
+ * All memory currently allocated for this context (including fragmentation
+ * and freed chunks).
+ */
+static Size
+GenerationMemAllocated(MemoryContext context)
+{
+	GenerationContext *set = (GenerationContext *) context;
+	return set->memAllocated;
+}
+
 /*
  * GenerationIsEmpty
  *		Is a GenerationContext empty of any allocated space?
@@ -844,7 +859,7 @@ GenerationCheck(MemoryContext context)
 				 name, nfree, block, block->nfree);
 	}
 
-	Assert(total_allocated == context->mem_allocated);
+	Assert(total_allocated == gen->memAllocated);
 }
 
 #endif							/* MEMORY_CONTEXT_CHECKING */
diff --git a/src/backend/utils/mmgr/mcxt.c b/src/backend/utils/mmgr/mcxt.c
index 9e24fec72d6..e32e279c340 100644
--- a/src/backend/utils/mmgr/mcxt.c
+++ b/src/backend/utils/mmgr/mcxt.c
@@ -469,7 +469,7 @@ MemoryContextIsEmpty(MemoryContext context)
 Size
 MemoryContextMemAllocated(MemoryContext context, bool recurse)
 {
-	Size	total = context->mem_allocated;
+	Size	total = context->methods->mem_allocated(context);
 
 	AssertArg(MemoryContextIsValid(context));
 
@@ -760,7 +760,6 @@ MemoryContextCreate(MemoryContext node,
 	node->methods = methods;
 	node->parent = parent;
 	node->firstchild = NULL;
-	node->mem_allocated = 0;
 	node->prevchild = NULL;
 	node->name = name;
 	node->ident = NULL;
diff --git a/src/backend/utils/mmgr/slab.c b/src/backend/utils/mmgr/slab.c
index c928476c479..63750fbc81f 100644
--- a/src/backend/utils/mmgr/slab.c
+++ b/src/backend/utils/mmgr/slab.c
@@ -67,6 +67,7 @@ typedef struct SlabContext
 	Size		fullChunkSize;	/* chunk size including header and alignment */
 	Size		blockSize;		/* block size */
 	Size		headerSize;		/* allocated size of context header */
+	Size		memAllocated;	/* track memory allocated for this context */
 	int			chunksPerBlock; /* number of chunks per block */
 	int			minFreeChunks;	/* min number of free chunks in any block */
 	int			nblocks;		/* number of blocks allocated */
@@ -132,6 +133,7 @@ static void *SlabRealloc(MemoryContext context, void *pointer, Size size);
 static void SlabReset(MemoryContext context);
 static void SlabDelete(MemoryContext context);
 static Size SlabGetChunkSpace(MemoryContext context, void *pointer);
+static Size SlabMemAllocated(MemoryContext context);
 static bool SlabIsEmpty(MemoryContext context);
 static void SlabStats(MemoryContext context,
 					  MemoryStatsPrintFunc printfunc, void *passthru,
@@ -150,6 +152,7 @@ static const MemoryContextMethods SlabMethods = {
 	SlabReset,
 	SlabDelete,
 	SlabGetChunkSpace,
+	SlabMemAllocated,
 	SlabIsEmpty,
 	SlabStats
 #ifdef MEMORY_CONTEXT_CHECKING
@@ -262,6 +265,7 @@ SlabContextCreate(MemoryContext parent,
 	slab->fullChunkSize = fullChunkSize;
 	slab->blockSize = blockSize;
 	slab->headerSize = headerSize;
+	slab->memAllocated = 0;
 	slab->chunksPerBlock = chunksPerBlock;
 	slab->minFreeChunks = 0;
 	slab->nblocks = 0;
@@ -286,6 +290,17 @@ SlabContextCreate(MemoryContext parent,
 	return (MemoryContext) slab;
 }
 
+/*
+ * All memory currently allocated for this context (including fragmentation
+ * and freed chunks).
+ */
+static Size
+SlabMemAllocated(MemoryContext context)
+{
+	SlabContext *slab = (SlabContext *) context;
+	return slab->memAllocated;
+}
+
 /*
  * SlabReset
  *		Frees all memory which is allocated in the given set.
@@ -322,14 +337,14 @@ SlabReset(MemoryContext context)
 #endif
 			free(block);
 			slab->nblocks--;
-			context->mem_allocated -= slab->blockSize;
+			slab->memAllocated -= slab->blockSize;
 		}
 	}
 
 	slab->minFreeChunks = 0;
 
 	Assert(slab->nblocks == 0);
-	Assert(context->mem_allocated == 0);
+	Assert(slab->memAllocated == 0);
 }
 
 /*
@@ -407,7 +422,7 @@ SlabAlloc(MemoryContext context, Size size)
 
 		slab->minFreeChunks = slab->chunksPerBlock;
 		slab->nblocks += 1;
-		context->mem_allocated += slab->blockSize;
+		slab->memAllocated += slab->blockSize;
 	}
 
 	/* grab the block from the freelist (even the new block is there) */
@@ -501,7 +516,7 @@ SlabAlloc(MemoryContext context, Size size)
 
 	SlabAllocInfo(slab, chunk);
 
-	Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
+	Assert(slab->nblocks * slab->blockSize == slab->memAllocated);
 
 	return SlabChunkGetPointer(chunk);
 }
@@ -578,13 +593,13 @@ SlabFree(MemoryContext context, void *pointer)
 	{
 		free(block);
 		slab->nblocks--;
-		context->mem_allocated -= slab->blockSize;
+		slab->memAllocated -= slab->blockSize;
 	}
 	else
 		dlist_push_head(&slab->freelist[block->nfree], &block->node);
 
 	Assert(slab->nblocks >= 0);
-	Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
+	Assert(slab->nblocks * slab->blockSize == slab->memAllocated);
 }
 
 /*
@@ -804,7 +819,7 @@ SlabCheck(MemoryContext context)
 		}
 	}
 
-	Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
+	Assert(slab->nblocks * slab->blockSize == slab->memAllocated);
 }
 
 #endif							/* MEMORY_CONTEXT_CHECKING */
diff --git a/src/include/nodes/memnodes.h b/src/include/nodes/memnodes.h
index c9f2bbcb367..b6296af7d5f 100644
--- a/src/include/nodes/memnodes.h
+++ b/src/include/nodes/memnodes.h
@@ -63,6 +63,7 @@ typedef struct MemoryContextMethods
 	void		(*reset) (MemoryContext context);
 	void		(*delete_context) (MemoryContext context);
 	Size		(*get_chunk_space) (MemoryContext context, void *pointer);
+	Size		(*mem_allocated) (MemoryContext context);
 	bool		(*is_empty) (MemoryContext context);
 	void		(*stats) (MemoryContext context,
 						  MemoryStatsPrintFunc printfunc, void *passthru,
@@ -79,7 +80,6 @@ typedef struct MemoryContextData
 	/* these two fields are placed here to minimize alignment wastage: */
 	bool		isReset;		/* T = no space alloced since last reset */
 	bool		allowInCritSection; /* allow palloc in critical section */
-	Size		mem_allocated;	/* track memory allocated for this context */
 	const MemoryContextMethods *methods;	/* virtual function table */
 	MemoryContext parent;		/* NULL if no parent (toplevel context) */
 	MemoryContext firstchild;	/* head of linked list of children */