linux-kernel - Re: [PATCH 0/5] make *_gate_vma accept mm_struct instead of task

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Message-ID: <20110310164022.GA6242@fibrous.localdomain>
Date:	Thu, 10 Mar 2011 11:40:22 -0500
From:	Stephen Wilson <wilsons@...rt.ca>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux390@...ibm.com, Paul Mundt <lethal@...ux-sh.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] make *_gate_vma accept mm_struct instead of
	task_struct


On Thu, Mar 10, 2011 at 08:00:32AM -0800, Andi Kleen wrote:
> On Tue, Mar 08, 2011 at 07:31:56PM -0500, Stephen Wilson wrote:
> > 
> > Morally, the question of whether an address lies in a gate vma should be asked
> > with respect to an mm, not a particular task.
> > 
> > Practically, dropping the dependency on task_struct will help make current and
> > future operations on mm's more flexible and convenient.  In particular, it
> > allows some code paths to avoid the need to hold task_lock.
> > 
> > The only architecture this change impacts in any significant way is x86_64.
> > The principle change on that architecture is to mirror TIF_IA32 via
> > a new flag in mm_context_t. 
> 
> The problem is -- you're adding a likely cache miss on mm_struct for
> every 32bit compat syscall now, even if they don't need mm_struct
> currently (and a lot of them do not) Unless there's a very good
> justification to make up for this performance issue elsewhere
> (including numbers) this seems like a bad idea.

I do not think this will result in cache misses on the scale you
suggest.  I am simply mirroring the *state* of the TIF_IA32 flag in
mm_struct, not testing/accessing it in the same way.

The only place where this flag is accessed (outside the exec() syscall
path) is in x86/mm/init_64.c, get_gate_vma(),  which in turn is needed
by a few, relatively heavy weight, page locking/pinning routines on the
mm side (get_user_pages, for example).  Patches 3 and 4 in the series
show the extent of the change.

Or am I missing something?


> > /proc/pid/mem.  I will be posting the second series to lkml shortly.  These
> 
> Making every syscall slower for /proc/pid/mem doesn't seem like a good
> tradeoff to me. Please solve this in some other way.
> 
> -Andi

-- 
steve

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