On 12/08/2016 09:01 PM, Ingo Molnar wrote: >> > - Handle opt-in wider address space for userspace. >> > >> > Not all userspace is ready to handle addresses wider than current >> > 47-bits. At least some JIT compiler make use of upper bits to encode >> > their info. >> > >> > We need to have an interface to opt-in wider addresses from userspace >> > to avoid regressions. >> > >> > For now, I've included testing-only patch which bumps TASK_SIZE to >> > 56-bits. This can be handy for testing to see what breaks if we max-out >> > size of virtual address space. > So this is just a detail - but it sounds a bit limiting to me to provide an 'opt > in' flag for something that will work just fine on the vast majority of 64-bit > software. MPX is going to be a real pain here. It is relatively transparent to applications that use it, and old MPX binaries are entirely incompatible with the new address space size, so an opt-out wouldn't be friendly. Because the top-level MPX bounds table is indexed by the virtual address, a growth in vaddr space is going to require the table to grow (or change somehow). The solution baked into the hardware spec is to just make the top-level table 512x larger to accommodate the 512x increase in vaddr space. (This behavior is controlled by a new MSR, btw...) So, either we disable MPX on all old MPX binaries by returning an error when the prctl() tries to enable MPX and 5-level paging is on, or we go with some form of an opt-in. New MPX binaries will opt-in to the larger address space since they know to allocate the new, larger table. -- 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>