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

From: Kirill A. Shutemov
Date: Tue Feb 21 2017 - 05:47:47 EST

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:

Kirill A. Shutemov