linux-kernel - Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX

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Message-ID: <20170221105412.GB31018@e104818-lin.cambridge.arm.com>
Date:   Tue, 21 Feb 2017 10:54:12 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and
 PR_GET_MAX_VADDR

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@...capital.net> 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@node.shutemov.name

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