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

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Date:	Thu, 10 Mar 2011 08:00:32 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Stephen Wilson <wilsons@...rt.ca>
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 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.

> This is the first of a two part series that implements safe writes to
> /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
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