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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
