linux-kernel - Re: [RFC][v6][PATCH 0/9] clone_with

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Message-Id: <1252688404.7126.40.camel@laptop>
Date:	Fri, 11 Sep 2009 19:00:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	Oren Laadan <orenl@...columbia.edu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...l.org>, torvalds@...ux-foundation.org,
	mikew@...gle.com, mingo@...e.hu, hpa@...or.com,
	Nathan Lynch <nathanl@...tin.ibm.com>, container@...ibm.com,
	sukadev@...ibm.com
Subject: Re: [RFC][v6][PATCH 0/9] clone_with_pids() syscall

On Fri, 2009-09-11 at 09:47 -0700, Sukadev Bhattiprolu wrote:
> | Also, if you're passing a struct, why not put nr_pids in there, and
> | replace clone_pid_struct with a simple array? That would give us
> | 
> | struct clone_struct {
> |         u64 flags;
> |         u64 child_stack;
> |         u32 child_tid;
> |         u32 parent_tid;
> | 	u32 nr_pids;
> | 	u32 reserved1;
> | 	u64 reserved2;
> | };
> | 
> | int clone2(struct clone_struct *cs, pid_t *pids);
> 
> My only concern with this approach was the extra copy_from_user() in the
> common case (i.e when not using the extended features).  I assume the
> overhead of copy_from_user() is small enough to be ignored ?

I would think so, esp for small structure, it would be a cache hot copy.
That is, for x86 I doubt it'll show up. No idea what hoops other arch
have to jump through to get copy_from_user() doing what it does.

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