linux-kernel - Re: [RFC] perf: mmap2 not covering VM

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Message-ID: <CABPqkBRbqnwRpigACZwnV4rSK2wZekZqXi-vfB2+YbSWgfTUKA@mail.gmail.com>
Date:	Thu, 3 Oct 2013 10:55:28 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] perf: mmap2 not covering VM_CLONE regions

On Wed, Oct 2, 2013 at 10:31 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Oct 02, 2013 at 12:38:54PM -0700, Kees Cook wrote:
>> On Wed, Oct 2, 2013 at 12:00 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Wed, Oct 02, 2013 at 11:10:15AM -0700, Kees Cook wrote:
>> >> Seems like a simple enough solution. Surely there must be a catch. :)
>> >
>> > I didn't want to add this to the core mm just for perf..
>>
>> It seems like it would be pretty inexpensive. It might also be
>> valuable in other situations. Not that I can think of any at the
>> moment. Additionally, it could likely be hidden by a CONFIG, so that
>> if perf isn't built in, there's no change?
>
> You optimist, you think you can build a kernel without perf? ;-)
>
> Its just that I would hate to add more completely global state to the
> fork() path. The tasklist_lock might be hard to crack, but at least the
> pid-hash could use per bucket locks (it doesn't apparently).
>
> I suppose people don't really care that much about fork() performance;
> which is sad. KSM and THP also add their own global locks :-(

I don't know the MM code but I assume that that vm_mm struct is
allocated dynamically
and maybe you already grabbing a lock while doing this. Could we
leverage that lock
to increment a global generation number?
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