linux-kernel - Re: [PATCH] nommu arch dont zero the anonymous mapping by adding UNINITIALIZE flag

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Message-ID: <20070330155635.6a6c55d2@the-village.bc.nu>
Date:	Fri, 30 Mar 2007 15:56:35 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"Aubrey Li" <aubreylee@...il.com>
Cc:	"David Howells" <dhowells@...hat.com>, vapier.adi@...il.com,
	jie.zhang@...log.com, bryan.wu@...log.com,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nommu arch dont zero the anonymous mapping by adding
 UNINITIALIZE flag

> I can't find mmap must give zeroed memory in the mmap manual.
> Is there any reason relying on anon mmap() giving zerod memory?

Its how all Unix/Linux like systems behave. You have to clear the memory
to something to deal with security on any kind of real system, and zero
is a good a value as any

> > Personally, I'd prefer to maintain compatibility with MMU-mode wherever
> > possible, but I'm happy with overrides like the MAP_UNINITIALISED flag
> > suggested.
> >
> Not necessary IMHO.

mmap() for anonymous memory pools should not normally be a hot path,
because the C library malloc is supposed to show some brains. If you need
special behaviour (eg for performance hacks) then create yourself
a /dev/zero like device which just maps uncleared pages. Its not much
code and it keeps special cases out of the core kernel.
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