diff --git a/src/backend/utils/mmgr/slab.c b/src/backend/utils/mmgr/slab.c
index ae1a735b8c..2d70adef09 100644
--- a/src/backend/utils/mmgr/slab.c
+++ b/src/backend/utils/mmgr/slab.c
@@ -58,6 +58,8 @@
 #include "utils/memutils_memorychunk.h"
 #include "utils/memutils_internal.h"
 
+#define Slab_BLOCKHDRSZ	MAXALIGN(sizeof(SlabBlock))
+
 /*
  * SlabContext is a specialized implementation of MemoryContext.
  */
@@ -102,10 +104,10 @@ typedef struct SlabBlock
 #define SlabChunkGetPointer(chk)	\
 	((void *)(((char *)(chk)) + sizeof(MemoryChunk)))
 #define SlabBlockGetChunk(slab, block, idx) \
-	((MemoryChunk *) ((char *) (block) + sizeof(SlabBlock)	\
+	((MemoryChunk *) ((char *) (block) + Slab_BLOCKHDRSZ	\
 					+ (idx * slab->fullChunkSize)))
 #define SlabBlockStart(block)	\
-	((char *) block + sizeof(SlabBlock))
+	((char *) block + Slab_BLOCKHDRSZ)
 #define SlabChunkIndex(slab, block, chunk)	\
 	(((char *) chunk - SlabBlockStart(block)) / slab->fullChunkSize)
 
@@ -146,12 +148,12 @@ SlabContextCreate(MemoryContext parent,
 	fullChunkSize = Slab_CHUNKHDRSZ + MAXALIGN(chunkSize);
 
 	/* Make sure the block can store at least one chunk. */
-	if (blockSize < fullChunkSize + sizeof(SlabBlock))
+	if (blockSize < fullChunkSize + Slab_BLOCKHDRSZ)
 		elog(ERROR, "block size %zu for slab is too small for %zu chunks",
 			 blockSize, chunkSize);
 
 	/* Compute maximum number of chunks per block */
-	chunksPerBlock = (blockSize - sizeof(SlabBlock)) / fullChunkSize;
+	chunksPerBlock = (blockSize - Slab_BLOCKHDRSZ) / fullChunkSize;
 
 	/* The freelist starts with 0, ends with chunksPerBlock. */
 	freelistSize = sizeof(dlist_head) * (chunksPerBlock + 1);
@@ -744,7 +746,7 @@ SlabCheck(MemoryContext context)
 
 					/* there might be sentinel (thanks to alignment) */
 					if (slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ))
-						if (!sentinel_ok(chunk, slab->chunkSize))
+						if (!sentinel_ok(chunk, Slab_CHUNKHDRSZ + slab->chunkSize))
 							elog(WARNING, "problem in slab %s: detected write past chunk end in block %p, chunk %p",
 								 name, block, chunk);
 				}