Date: Fri, 24 Apr 2009 01:13:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Markus Metzger <markus.t.metzger@intel.com>
Cc: mingo@elte.hu, a.p.zijlstra@chello.nl, markus.t.metzger@gmail.com,
       roland@redhat.com, eranian@googlemail.com, oleg@redhat.com,
       juan.villacis@intel.com, ak@linux.jf.intel.com,
       linux-kernel@vger.kernel.org, tglx@linutronix.de, hpa@zytor.com
Subject: Re: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer
Message-Id: <20090424011328.b5e949ce.akpm@linux-foundation.org>
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References: <20090424100055.A30408@sedona.ch.intel.com>
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On Fri, 24 Apr 2009 10:00:55 +0200 Markus Metzger <markus.t.metzger@intel.com> wrote:

> Use vmalloc to allocate the branch trace buffer.
> 
> Peter Zijlstra suggested to use vmalloc rather than kmalloc to
> allocate the potentially multi-page branch trace buffer.

The changelog provides no reason for this change.  It should do so.

> Is there a way to have vmalloc allocate a physically non-contiguous
> buffer for test purposes? Ideally, the memory area would have big
> holes in it with sensitive data in between so I would know immediately
> when this is overwritten.

I suppose you could allocate the pages by hand and then vmap() them. 
Allocating 2* the number you need and then freeing every second one
should make them physically holey.

> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -22,6 +22,7 @@
>  #include <linux/seccomp.h>
>  #include <linux/signal.h>
>  #include <linux/workqueue.h>
> +#include <linux/vmalloc.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/pgtable.h>
> @@ -626,7 +627,7 @@ static int alloc_bts_buffer(struct bts_c
>  	if (err < 0)
>  		return err;
>  
> -	buffer = kzalloc(size, GFP_KERNEL);
> +	buffer = vmalloc(size);
>  	if (!buffer)
>  		goto out_refund;
>  
> @@ -646,7 +647,7 @@ static inline void free_bts_buffer(struc
>  	if (!context->buffer)
>  		return;
>  
> -	kfree(context->buffer);
> +	vfree(context->buffer);
>  	context->buffer = NULL;
>  

The patch looks like a regression to me.  vmalloc memory is slower to
allocate, slower to free, slower to access and can exhaust or fragment
the vmalloc arena.  Confused.
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