Re: UVM and the NULL page

[Matt Thomas <matt%3am-software.com@localhost>]

> On Jul 28, 2016, at 12:21 PM, Maxime Villard <max%M00nBSD.net@localhost> wrote:
> 
> Le 28/07/2016 à 19:45, Eduardo Horvath a écrit :
>> On Thu, 28 Jul 2016, Maxime Villard wrote:
>> 
>>> Currently, there is no real way to make sure a userland process won't be
>>> able to allocate the NULL page. There is this attempt [1], but it has two
>>> major issues.
>> 
>> I don't think this is a good idea.  You should leave this to the pmap
>> layer rather than polluting UVM.  There are some architectures that need
>> to have page zero mapped in for various reasons.
>> 
> 
> No. Quite on the contrary. UVM should handle that, instead of polluting
> pmap.
> 
> You are saying that some architectures need the NULL page. That's fine,
> since #define __USER_VA0_IS_SAFE is architecture-dependent.
> 
> For your information, __USER_VA0_IS_SAFE is never set, so clearly, no
> one  needs the NULL page.

That is not true.  Older ARM processors use a vector page located at 0 which
has to be mapped into each processes address space.  

Now while the VA is mapped but it does not need to be writeable.