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

[Ingo Molnar <mingo@elte.hu>]

* Andrew Morton <akpm@linux-foundation.org> wrote:

> 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.

Performance does not matter here (this is really a slowpath), but 
fragmentation does matter, especially on 32-bit systems.

I'd not uglify the code via vmap() - and vmap has the same 
fundamental address space limitations on 32-bit as vmalloc().

The existing kmalloc() is fine. We do larger than PAGE_SIZE 
allocations elsewhere too (the kernel stack for example), and this 
is a debug facility, so failing the allocation is not a big problem 
even if it happens.

	Ingo