Re: [PATCH 8/8] x86/mm: Allow to have userspace mappings above 47-bits

[hpa@zytor.com]

On April 7, 2017 8:59:45 AM PDT, "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
>On Fri, Apr 07, 2017 at 07:05:26PM +0530, Anshuman Khandual wrote:
>> On 04/06/2017 07:31 PM, Kirill A. Shutemov wrote:
>> > 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.
>> 
>> I am wondering if the commitment of virtual space range to the
>> user space is kind of an API which needs to be maintained there
>> after. If that is the case then we need to have some plans when
>> increasing it from the current level.
>
>I don't think we should ever enable full address space for all
>applications. There's no point.
>
>/bin/true doesn't need more than 64TB of virtual memory.
>And I hope never will.
>
>By increasing virtual address space for everybody we will pay (assuming
>current page table format) at least one extra page per process for
>moving
>stack at very end of address space.
>
>Yes, you can gain something in security by having more bits for ASLR,
>but
>I don't think it worth the cost.
>
>> Will those JIT compilers keep using the higher bit positions of
>> the pointer for ever ? Then it will limit the ability of the
>> kernel to expand the virtual address range later as well. I am
>> not saying we should not increase till the extent it does not
>> affect any *known* user but then we should not increase twice
>> for now, create the hint mechanism to be passed from the user
>> to avail beyond that (which will settle in as a expectation
>> from the kernel later on). Do the same thing again while
>> expanding the address range next time around. I think we need
>> to have a plan for this and particularly around 'hint' mechanism
>> and whether it should be decided per mmap() request or at the
>> task level.
>
>I think the reasonable way for an application to claim it's 63-bit
>clean
>is to make allocations with (void *)-1 as hint address.

You realize that people have said that about just about every memory threshold from 64K onward?
-- 
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