linux-kernel - Re: [patch 2/3] MAP_NOZERO

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Date:	Tue, 26 Jun 2007 20:33:05 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Rik van Riel <riel@...hat.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 2/3] MAP_NOZERO - implement sys_brk2()

On Tue, 26 Jun 2007, Rik van Riel wrote:

> Davide Libenzi wrote:
> > The following patch implements the sys_brk2() syscall, that nothing is
> > other than a sys_brk() with an extra "flags" parameter. This can be used
> > to pass the new MAP_NOZERO bit, to ask the kernel to hand over non-zero
> > pages if possible.
> 
> Since programs can get back free()d memory after a malloc(),
> with the old contents of the memory intact, surely your
> MAP_NONZERO behavior could be the default for programs that
> can get away with it?
> 
> Maybe we could use some magic ELF header, similar to the
> way non-executable stack is handled?

Well, the quick&ugly glibc patch simply uses an environment variable, just 
because I wanted to bench the kernel build with using the same glibc+gcc.
Yes, it can be the default behaviour for the allocator. The patch handles 
calloc() correctly, by forcibly zeroing memory in such calls.
But other software must be taught too, to use MAP_NOZERO when they do not 
need zeroed memory. I did that for the gcc garbage collector.



- Davide


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