linux-kernel - Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP

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Date:	Thu, 28 Jun 2007 23:04:10 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Kyle Moffett <mrmacman_g4@....com>
CC:	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Isaacson <adi@...apodia.org>
Subject: Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP_NOZERO early summer
 madness

Kyle Moffett wrote:
> On Jun 28, 2007, at 14:49:24, Davide Libenzi wrote:
>> I was using oprofile to sample some userspace code I am working on, 
>> and I was continuosly noticing  clear_page  in the top three entries 
>> of the oprofile logs.
>>
>> Also, a simple kernel build, in my Dual Opteron with 8GB of RAM, 
>> shows  clear_page  as the first kernel entry, second only to the 
>> userspace the  cc1  and  as. Most of the userspace code uses malloc() 
>> (and anonymous  mappings) in such a way that the memory returned via 
>> kernel->glibc is immediately written soon after. The POSIX malloc() 
>> definition itself also, does not require the returned memory to be 
>> zeroed (as calloc() does).
>>
>> So I implemented a rather quick hack that introduces a new mmap() flag 
>> MAP_NOZERO (only valid for anonymous mappings) and the  vma  
>> counter-part VM_NOZERO. Also, a new sys_brk2() has been introduced to 
>> accept a new flags  parameter. A brief description of the patches 
>> follows in the next emails.
> 
> Hmm, sounds like this would also need a "MAP_NOREUSE" flag of some kind 
> for security sensitive applications.

That wants MAP_PRIVATE so that the kernel can also decide to not
swap these pages out to an unencrypted swap area.

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