linux-kernel - Re: [PATCH 1/4] mm: introduce compaction and migration for virtio ballooned pages

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Date:	Wed, 27 Jun 2012 10:42:13 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 1/4] mm: introduce compaction and migration for virtio
 ballooned pages

On Tue, Jun 26, 2012 at 10:34:00PM +0200, Andi Kleen wrote:
> > How is the compiler meant to optimise away "cond" if it's a function
> > call?
> 
> Inlines can be optimized away.  These tests are usually inlines.
> 
> > What did I miss? If nothing, then I will revert this particular change
> > and Rafael will need to be sure his patch is not using VM_BUG_ON to call
> > a function with side-effects.
> 
> Do you have an example where the code is actually different,
> or are you just speculating?
> 
> FWIW for my config both generates the same code:
> 
> size vmlinux-andi-vmbug vmlinux-vmbug-nothing 
>    text    data     bss     dec     hex filename
> 11809704        1457352 1159168 14426224         dc2070 vmlinux-andi-vmbug
> 11809704        1457352 1159168 14426224         dc2070 vmlinux-vmbug-nothing
> 

They are the same size because CONFIG_DEBUG_VM and !CONFIG_DEBUG_VM
generate the same code! I applied the patch below again 3.4 and got the
following results

   text	   data	    bss	    dec	    hex	        filename
6918617	1795640	2260992	10975249	 a77811 vmlinux-default-no-vmdebug
6916633	1795640	2260992	10973265	 a77051 vmlinux-patched-no-vmdebug

That's almost 2K of text!

I see now that in 3.5 this was already spotted and fixed by Konstantin
in commit [02602a18: bug: completely remove code generated by disabled
VM_BUG_ON()]. That patch restores the rule that VM_BUG_ON() cannot call
anything with side-effects. So Rafael, watch your use of VM_BUG_ON or
you'll find that the your patches work in 3.4 and leak in 3.5.

diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index c04ecfe..ee24ef8 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -4,7 +4,7 @@
 #ifdef CONFIG_DEBUG_VM
 #define VM_BUG_ON(cond) BUG_ON(cond)
 #else
-#define VM_BUG_ON(cond) do { (void)(cond); } while (0)
+#define VM_BUG_ON(cond) do { } while (0)
 #endif
 
 #ifdef CONFIG_DEBUG_VIRTUAL
--
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