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

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Date:   Sun, 19 Mar 2017 14:25:08 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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

"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> writes:

> "Kirill A. Shutemov" <kirill@...temov.name> writes:
>
>> 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.
>>
>
> What is the epectation when the hint addr is below 128TB but addr + len >
> 128TB ? Should such mmap request fail ?

Considering that we have stack at the top (around 128TB) we may not be
able to get a free area for such a request. But I guess the idea here is
that if hint address is below 128TB, we behave as though our TASK_SIZE
is 128TB ? Is that correct ?
 
-aneesh