linux-kernel - Re: [PATCH 26/26] x86/mm: allow to have userspace mappings above 47-bits

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Date:   Fri, 24 Mar 2017 11:59:31 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     hpa@...or.com
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Michal Hocko <mhocko@...e.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
        x86@...nel.org, Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 26/26] x86/mm: allow to have userspace mappings above
 47-bits

On Mon, Mar 20, 2017 at 11:08:41AM -0700, hpa@...or.com wrote:
> This *better* be conditional on some kind of settable limit.  Having a
> barrier in the middle of the address space for no apparent reason to
> "clean" software is insane.

I had the same argument (on your side) before, but if you look on numbers
it's far from the middle of address space. The barrier is around 0.2% from
the start 56-bit address space.

And it's we have vdso/vvar/stack just below the barier anyway.

I don't think we would loose much if wouldn't not allow VMA to sit
across it.

-- 
 Kirill A. Shutemov