To: PostgreSQL Developers <pgsql-hackers@postgresql.org>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Subject: bug in SlabAlloc / VALGRIND_MAKE_MEM_DEFINED
Message-ID: <a6543b6d-6015-99b1-63ef-3ed55a76a730@2ndquadrant.com>
Date: Tue, 4 Apr 2017 22:42:39 +0200
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Hi,

Andres nagged to me about valgrind runs taking much longer since 
9fab40ad introduced the SlabContext into reorderbuffer.c. And by 
"longer" I mean hours instead of minutes.

After a bit of investigation I stumbled on this line in slab.c:

   VALGRIND_MAKE_MEM_DEFINED(chunk, SlabChunkGetPointer(chunk));

which is wrong, because the second parameter should be size and not 
another pointer. This essentially marks a lot of memory as defined, 
which results in the extreme test runtime.

Instead, the line should be:

   VALGRIND_MAKE_MEM_DEFINED(SlabChunkGetPointer(chunk), sizeof(int32));

Patch attached.

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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diff --git a/src/backend/utils/mmgr/slab.c b/src/backend/utils/mmgr/slab.c
index d6be4fe..a542017 100644
--- a/src/backend/utils/mmgr/slab.c
+++ b/src/backend/utils/mmgr/slab.c
@@ -403,7 +403,7 @@ SlabAlloc(MemoryContext context, Size size)
 	 * Remove the chunk from the freelist head. The index of the next free
 	 * chunk is stored in the chunk itself.
 	 */
-	VALGRIND_MAKE_MEM_DEFINED(chunk, SlabChunkGetPointer(chunk));
+	VALGRIND_MAKE_MEM_DEFINED(SlabChunkGetPointer(chunk), sizeof(int32));
 	block->firstFreeChunk = *(int32 *) SlabChunkGetPointer(chunk);
 
 	Assert(block->firstFreeChunk >= 0);

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