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

[Kirill A. Shutemov]

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.

-- 
 Kirill A. Shutemov