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

[hpa@zytor.com]

On March 19, 2017 1:26:58 AM PDT, "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
>On Mar 19, 2017 09:25, "Aneesh Kumar K.V"
><aneesh.kumar@linux.vnet.ibm.com>
>wrote:
>
>"Kirill A. Shutemov" <kirill@shutemov.name> writes:
>
>> On Fri, Mar 17, 2017 at 11:23:54PM +0530, Aneesh Kumar K.V wrote:
>>> "Kirill A. Shutemov" <kirill.shutemov@linux.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 ?
>
>
>Yes, I believe so.

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.
-- 
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