linux-kernel - Re: A question about using a private anonymous mmap

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Date:	Fri, 5 Jun 2009 12:06:18 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Julian Phillips <julian@...ntumfyre.co.uk>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: A question about using a private anonymous mmap

On Thu, 4 Jun 2009 17:48:29 +0100 (BST)
Julian Phillips <julian@...ntumfyre.co.uk> wrote:

> I have a program which creates a reasonably large private anonymous map. 
> The program then writes into a few places in the map, but ends up reading 
> from all of them.
> 
> When I run this program on a system running 2.6.20.7 the process only ever 
> seems to use enough memory to hold the data that has actually been written 
> (well - in units of PAGE_SIZE).  When I run the program on a system 
> running 2.6.24.5 then as it reads the map the amount of memory used 
> continues to increase until the complete map has actually been allocated 
> (and since the total size is greater than the physically available RAM 
> causes swapping).  Basically I seem to be seeing copy-on-read instead of 
> copy-on-write type behaviour.
> 
> Is this an expected change, and is there any option I can tweak to get the 
> old behaviour back?
> 

It comes from removal of ZERO_PAGE.

IMHO, What you can do is mmap(/dev/zero, MAP_PRIVATE) instead of ANON.

Thanks,
-Kame

> TIA,
> 
> -- 
> Julian
> 
>   ---
> I can't die until the government finds a safe place to bury my liver.
>  		-- Phil Harris
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