linux-kernel - Re: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer

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Date:	Fri, 24 Apr 2009 01:13:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Markus Metzger <markus.t.metzger@...el.com>
Cc:	mingo@...e.hu, a.p.zijlstra@...llo.nl, markus.t.metzger@...il.com,
	roland@...hat.com, eranian@...glemail.com, oleg@...hat.com,
	juan.villacis@...el.com, ak@...ux.jf.intel.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, hpa@...or.com
Subject: Re: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer

On Fri, 24 Apr 2009 10:00:55 +0200 Markus Metzger <markus.t.metzger@...el.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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