Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX_VADDR

[Catalin Marinas]

On Tue, Feb 21, 2017 at 01:47:36PM +0300, Kirill A. Shutemov wrote:
> On Tue, Feb 21, 2017 at 10:34:02AM +0000, Catalin Marinas wrote:
> > On Fri, Feb 17, 2017 at 03:21:27PM -0800, Linus Torvalds wrote:
> > > On Feb 17, 2017 3:02 PM, "Andy Lutomirski" <luto@xxxxxxxxxxxxxx> wrote:
> > > >   What I'm trying to say is: if we're going to do the route of 48-bit
> > > >   limit unless a specific mmap call requests otherwise, can we at least
> > > >   have an interface that doesn't suck?
> > > 
> > > No, I'm not suggesting specific mmap calls at all. I'm suggesting the complete
> > > opposite: not having some magical "max address" at all in the VM layer. Keep
> > > all the existing TASK_SIZE defines as-is, and just make those be the new 56-bit
> > > limit.
> > > 
> > > But to then not make most processes use it, just make the default x86
> > > arch_get_free_area() return an address limited to the old 47-bit limit. So
> > > effectively all legacy programs work exactly the same way they always did.
> > 
> > arch_get_unmapped_area() changes would not cover STACK_TOP which is
> > currently defined as TASK_SIZE (on both x86 and arm64). I don't think it
> > matters much (normally such upper bits tricks are done on heap objects)
> > but you may find some weird user program that passes pointers to the
> > stack around and expects bits 48-63 to be masked out. If that's a real
> > issue, we could also limit STACK_TOP to 47-bit (48-bit on arm64).
> 
> I've limited STACK_TOP to 47-bit in my implementation of Linus' proposal:
> 
> http://lkml.kernel.org/r/20170220131515.GA9502@xxxxxxxxxxxxxxxxxx

Ah, sorry for the noise then (still catching up with this thread; at
some point we'll need to add 52-bit VA support to arm64, though with 4
levels only).

-- 
Catalin