linux-kernel - Re: nommu: handling anonymous mmap clearing in userspace rather than kernel

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Date:	Wed, 02 Apr 2008 15:20:14 +0100
From:	David Howells <dhowells@...hat.com>
To:	"Mike Frysinger" <vapier.adi@...il.com>
Cc:	dhowells@...hat.com, "Greg Ungerer" <gerg@...pgear.com>,
	"David McCullough" <David_Mccullough@...urecomputing.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Bryan Wu" <Bryan.Wu@...log.com>,
	"Bernd Schmidt" <bernds_cb1@...nline.de>,
	"Robin Getz" <rgetz@...ckfin.uclinux.org>
Subject: Re: nommu: handling anonymous mmap clearing in userspace rather than kernel

Mike Frysinger <vapier.adi@...il.com> wrote:

> a workaround: introduce a new no-mmu-only mmap flag MAP_UNINITIALIZE
> to signal to the kernel that it should skip the memset().  this way,
> userspace malloc() can do mmap(MAP_ANONYMOUS|MAP_UNINITIALIZE) to get
> large chunks of memory without affecting any other anonymous mmap()
> call.

I think that's reasonable for NOMMU.  It's not like the process accessing the
uninitialised memory is prevented from accessing anything it wants to anyway.

I would vote that the memset() should only be skipped if requested as there
may be programs that call mmap(MAP_ANONYMOUS) expecting the memory they're
given to be zeroed out.

David


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