linux-kernel - Re: linux page table

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Message-ID: <5045C06F.4000507@gmail.com>
Date:	Tue, 04 Sep 2012 16:48:47 +0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Jiri Kosina <jkosina@...e.cz>
CC:	Xin Tong <xerox.time.tech@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: linux page table

On 09/03/2012 02:26 AM, Jiri Kosina wrote:
> On Sun, 2 Sep 2012, Xin Tong wrote:
>
>>>> 3. can two different processes have their CR3 being the same value
>>>> even though they have different first level page tables ?
>>>
>>> Yes, if they are created by clone(CLONE_VM). In such case they share the
>>> same mm_struct, and therefore mm_struct->pgd (which is exactly what is
>>> loaded into cr3 in switch_mm()) is the same.
>>>
>>
>> Is this the COW mechanism in linux. what if the cloned process need to
>> have set of its own pages later. do the CR3s for the 2 processes
>> become different at that point ?
>
> That is a different story. COW is applied on fork() (i.e. spawning new
> process), not on clone(CLONE_VM) (i.e. spawning new thread).
>

Yeah, and unshare(2) does not implement the flag that reverses the 
effects of CLONE_VM.

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