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

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Date:	Wed, 02 Apr 2008 17:03:07 +0200
From:	Bernd Schmidt <bernds_cb1@...nline.de>
To:	Mike Frysinger <vapier.adi@...il.com>
CC:	David Howells <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>,
	Robin Getz <rgetz@...ckfin.uclinux.org>
Subject: Re: nommu: handling anonymous mmap clearing in userspace rather than
 kernel

Mike Frysinger wrote:
> On Wed, Apr 2, 2008 at 10:20 AM, David Howells <dhowells@...hat.com> wrote:
>> 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.
> 
> in the second proposal, the C library would be expected to do this, so
> no programs would be broken.  but you're right that any program that
> invokes the mmap() syscall directly would not get zeroed memory ...
> but is anyone doing such a crazy thing, let alone on embedded ?

It's not a guarantee we should break.  What's wrong with just using the 
MAP_UNINITIALIZE code we have?


Bernd
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