linux-kernel - Re: missing madvise functionality

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Message-Id: <200704040007.16916.arnd@arndb.de>
Date:	Wed, 4 Apr 2007 00:07:16 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: missing madvise functionality

On Tuesday 03 April 2007, Ulrich Drepper wrote:
> The problem is glibc has to work around kernel limitations.  If the
> malloc implementation detects that a large chunk of previously allocated
> memory is now free and unused it wants to return the memory to the
> system.  What we currently have to do is this:
> 
>   to free:      mmap(PROT_NONE) over the area
>   to reuse:     mprotect(PROT_READ|PROT_WRITE)
> 
> Yep, that's expensive, both operations need to get locks preventing
> other threads from doing the same.

I thought this is what the read_zero_pagealigned hack [1] was used
for (read from /dev/zero replaces target pages with empty_zero_page).
Now if read_zero_pagealigned does not solve _this_ scenario, is it
good for anything else then? 
Can we simply kill that function as a misfeature and avoid future
pain arising from it?

	Arnd <><

[1] http://lkml.org/lkml/1997/1/16/49
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