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Date: Thu, 19 Jan 2012 02:49:56 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: david@lang.hm
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
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        Vasiliy Kulikov <segoon@openwall.com>,
        Andrew Morton <akpm@linux-foundation.org>, Valdis.Kletnieks@vt.edu
Subject: Re: [RFC] syscalls, x86: Add __NR_kcmp syscall
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On Wed, Jan 18, 2012 at 02:05:31PM -0800, david@lang.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