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Date: Wed, 18 Jan 2012 14:05:31 -0800 (PST)
From: david@lang.hm
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To: "Eric W. Biederman" <ebiederm@xmission.com>
cc: "H. Peter Anvin" <hpa@zytor.com>, Cyrill Gorcunov <gorcunov@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
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        Glauber Costa <glommer@parallels.com>,
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        Eric Dumazet <eric.dumazet@gmail.com>,
        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 Tue, 17 Jan 2012, Eric W. Biederman wrote:

> "H. Peter Anvin" <hpa@zytor.com> writes:
>
>> On 01/17/2012 06:44 AM, Cyrill Gorcunov wrote:
>>> On Tue, Jan 17, 2012 at 04:38:14PM +0200, Alexey Dobriyan wrote:
>>>> On 1/17/12, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>>>>> +#define KCMP_EQ		0
>>>>> +#define KCMP_LT		1
>>>>> +#define KCMP_GT		2
>>>>
>>>> LT and GT are meaningless.
>>>>
>>>
>>> I found symbolic names better than open-coded values. But sure,
>>> if this is problem it could be dropped.
>>>
>>> Or you mean that in general anything but 'equal' is useless?
>>>
>>
>> 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?

David Lang