"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 -- 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>
