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From: Mel Gorman <mel@csn.ul.ie>
To: Andi Kleen <andi@firstfloor.org>
Cc: Rafael Aquini <aquini@redhat.com>,
	linux-mm@kvack.org, Rik van Riel <riel@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 1/4] mm: introduce compaction and migration for virtio ballooned pages
Date: Wed, 27 Jun 2012 10:42:13 +0100	[thread overview]
Message-ID: <20120627094213.GK8103@csn.ul.ie> (raw)
In-Reply-To: <20120626203400.GA11413@one.firstfloor.org>

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

WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mel@csn.ul.ie>
To: Andi Kleen <andi@firstfloor.org>
Cc: Rafael Aquini <aquini@redhat.com>,
	linux-mm@kvack.org, Rik van Riel <riel@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 1/4] mm: introduce compaction and migration for virtio ballooned pages
Date: Wed, 27 Jun 2012 10:42:13 +0100	[thread overview]
Message-ID: <20120627094213.GK8103@csn.ul.ie> (raw)
In-Reply-To: <20120626203400.GA11413@one.firstfloor.org>

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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  parent reply	other threads:[~2012-06-27  9:42 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-06-25 23:25 [PATCH 0/4] make balloon pages movable by compaction Rafael Aquini
2012-06-25 23:25 ` Rafael Aquini
2012-06-25 23:25 ` [PATCH 1/4] mm: introduce compaction and migration for virtio ballooned pages Rafael Aquini
2012-06-25 23:25 ` Rafael Aquini
2012-06-25 23:25   ` Rafael Aquini
2012-06-25 23:31   ` Konrad Rzeszutek Wilk
2012-06-25 23:31   ` Konrad Rzeszutek Wilk
2012-06-25 23:31     ` Konrad Rzeszutek Wilk
2012-06-25 23:57     ` Rafael Aquini
2012-06-25 23:57     ` Rafael Aquini
2012-06-25 23:57       ` Rafael Aquini
2012-06-26 10:17   ` Mel Gorman
2012-06-26 10:17     ` Mel Gorman
2012-06-26 16:52     ` Andi Kleen
2012-06-26 16:52       ` Andi Kleen
2012-06-26 16:54       ` Andi Kleen
2012-06-26 16:54         ` Andi Kleen
2012-06-26 16:54         ` Andi Kleen
2012-06-26 20:15       ` Mel Gorman
2012-06-26 20:15         ` Mel Gorman
2012-06-26 20:34         ` Andi Kleen
2012-06-26 20:34         ` Andi Kleen
2012-06-26 20:34           ` Andi Kleen
2012-06-27  9:42           ` Mel Gorman
2012-06-27  9:42           ` Mel Gorman [this message]
2012-06-27  9:42             ` Mel Gorman
2012-06-26 20:15       ` Mel Gorman
2012-06-26 16:52     ` Andi Kleen
2012-06-26 22:01     ` Rafael Aquini
2012-06-26 22:01       ` Rafael Aquini
2012-06-26 22:01       ` Rafael Aquini
2012-06-26 10:17   ` Mel Gorman
2012-06-26 13:17   ` Rik van Riel
2012-06-26 13:17     ` Rik van Riel
2012-06-26 13:17     ` Rik van Riel
2012-06-26 13:20     ` Mel Gorman
2012-06-26 13:20     ` Mel Gorman
2012-06-26 13:20       ` Mel Gorman
2012-06-26 23:57   ` Konrad Rzeszutek Wilk
2012-06-26 23:57     ` Konrad Rzeszutek Wilk
2012-06-27 15:17     ` Rafael Aquini
2012-06-27 15:17       ` Rafael Aquini
2012-06-27 15:30       ` Konrad Rzeszutek Wilk
2012-06-27 15:30         ` Konrad Rzeszutek Wilk
2012-06-27 15:30       ` Konrad Rzeszutek Wilk
2012-06-27 15:17     ` Rafael Aquini
2012-06-26 23:57   ` Konrad Rzeszutek Wilk
2012-06-25 23:25 ` [PATCH 2/4] virtio_balloon: handle concurrent accesses to virtio_balloon struct elements Rafael Aquini
2012-06-25 23:25 ` Rafael Aquini
2012-06-25 23:25   ` Rafael Aquini
2012-06-25 23:25 ` [PATCH 3/4] virtio_balloon: introduce migration primitives to balloon pages Rafael Aquini
2012-06-25 23:25   ` Rafael Aquini
2012-06-25 23:25 ` Rafael Aquini
2012-06-25 23:25 ` [PATCH 4/4] mm: add vm event counters for balloon pages compaction Rafael Aquini
2012-06-25 23:25   ` Rafael Aquini
2012-06-25 23:25 ` Rafael Aquini

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