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

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Date:   Fri, 17 Mar 2017 20:57:14 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...e.com>, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 26/26] x86/mm: allow to have userspace mappings above
 47-bits

On Fri, Mar 17, 2017 at 11:23:54PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> writes:
> 
> > On x86, 5-level paging enables 56-bit userspace virtual address space.
> > Not all user space is ready to handle wide addresses. It's known that
> > at least some JIT compilers use higher bits in pointers to encode their
> > information. It collides with valid pointers with 5-level paging and
> > leads to crashes.
> >
> > To mitigate this, we are not going to allocate virtual address space
> > above 47-bit by default.
> >
> > But userspace can ask for allocation from full address space by
> > specifying hint address (with or without MAP_FIXED) above 47-bits.
> >
> > If hint address set above 47-bit, but MAP_FIXED is not specified, we try
> > to look for unmapped area by specified address. If it's already
> > occupied, we look for unmapped area in *full* address space, rather than
> > from 47-bit window.
> >
> > This approach helps to easily make application's memory allocator aware
> > about large address space without manually tracking allocated virtual
> > address space.
> >
> 
> So if I have done a successful mmap which returned > 128TB what should a
> following mmap(0,...) return ? Should that now search the *full* address
> space or below 128TB ?

No, I don't think so. And this implementation doesn't do this.

It's safer this way: if an library can't handle high addresses, it's
better not to switch it automagically to full address space if other part
of the process requested high address.

-- 
 Kirill A. Shutemov