Messages in this thread | | | Date | Fri, 17 Mar 2017 20:57:14 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH 26/26] x86/mm: allow to have userspace mappings above 47-bits |
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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.
-- Kirill A. Shutemov
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