"Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> writes: > "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 ? Considering that we have stack at the top (around 128TB) we may not be able to get a free area for such a request. But I guess the idea here is that if hint address is below 128TB, we behave as though our TASK_SIZE is 128TB ? Is that correct ? -aneesh
