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    SubjectRe: [PATCH 26/26] x86/mm: allow to have userspace mappings above 47-bits
    Date
    "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 ?

    -aneesh

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