On March 19, 2017 1:26:58 AM PDT, "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: >On Mar 19, 2017 09:25, "Aneesh Kumar K.V" ><aneesh.kumar@xxxxxxxxxxxxxxxxxx> >wrote: > >"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 ? > > >Yes, I believe so. This *better* be conditional on some kind of settable limit. Having a barrier in the middle of the address space for no apparent reason to "clean" software is insane. -- Sent from my Android device with K-9 Mail. Please excuse my brevity. -- 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
