linux-kernel - Re: [RFC] syscalls, x86: Add __NR

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Date:	Thu, 19 Jan 2012 02:49:56 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	david@...g.hm
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Valdis.Kletnieks@...edu
Subject: Re: [RFC] syscalls, x86: Add __NR_kcmp syscall

On Wed, Jan 18, 2012 at 02:05:31PM -0800, david@...g.hm wrote:
...
> >>
> >>Why on Earth would user space need to know which order in memory certain
> >>kernel objects are?
> >
> >For checkpoint restart and for some other kinds of introspection what is
> >needed is a comparison function to see if two processes share the same
> >object.  The most interesting of these objects from a checkpoint restart case
> >are file descriptors, and there can be a lot of file descriptors.
> >
> >The order in memory does not matter.  What does matter is that the
> >comparison function return some ordering between objects.  The algorithm
> >for figuring out of N items which of them are duplicates is O(N^2) if
> >the comparison function can only return equal or not equal.  The
> >algorithm for finding duplications is only O(NlogN) if the comparison
> >function will return an ordering among the objects.
> 
> so what you really want is a syscall that can take a list of objects
> instead of having to do a syscall per object. right?
> 

It doesn't matter. Even if we take a list of objects the kernel either
should return us some ordering info or find duplicates, in any case it
makes things more complex i think. So we wanted to bring some minimum
into kernel leaving the rest of work to user-space.

(and, btw, Eric, I'm really sorry that I didn't mentioned this sorting
 problem at the very first email).

	Cyrill
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