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

[Aneesh Kumar K.V]

"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:

> On Fri, Mar 17, 2017 at 11:23:54PM +0530, Aneesh Kumar K.V wrote:
>> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> 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 ?

-aneesh