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

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E9BA9BB93@irsmsx504.ger.corp.intel.com>
Date:	Fri, 24 Apr 2009 09:39:38 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>
CC:	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>
Subject: RE: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Friday, April 24, 2009 10:31 AM
>To: Andrew Morton
>Cc: Metzger, Markus T; a.p.zijlstra@...llo.nl; markus.t.metzger@...il.com; roland@...hat.com;
>eranian@...glemail.com; oleg@...hat.com; Villacis, Juan; 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
>
>
>* Andrew Morton <akpm@...ux-foundation.org> wrote:
>
>> 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.
>
>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.

OK. I'll drop 2/2 and send out 1/2 as a patch, then.

The original suggestion was to use the page allocator and vmap().
I assume you don't want that, either.

thanks,
markus.

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