On Friday, December 9, 2016 6:01:30 AM CET 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. > > Please make this an opt out compatibility flag instead: similar to how we handle > address space layout limitations/quirks ABI details, such as ADDR_LIMIT_32BIT, > ADDR_LIMIT_3GB, ADDR_COMPAT_LAYOUT, READ_IMPLIES_EXEC, etc. We've had a similar discussion about JIT software on ARM64, which has a wide range of supported page table layouts and some software wants to limit that to a specific number. I don't remember the outcome of that discussion, but I'm adding a few people to Cc that might remember. There have also been some discussions in the past to make the depth of the page table a per-task decision on s390, since you may have some tasks that run just fine with two or three levels of paging while another task actually wants the full 64-bit address space. I wonder how much extra work this would be on top of the boot-time option. Arnd -- 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>