> On Jul 28, 2016, at 12:21 PM, Maxime Villard <[email protected]> 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.
