On Fri, Dec 09, 2016 at 11:24:12AM +0100, Arnd Bergmann wrote: > 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. The arm64 kernel supports several user VA space configurations (though commonly 39 and 48-bit) and has had these from the initial port. We realised that certain JITs (e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=1143022) and IIRC LLVM assume a 47-bit user VA but AFAICT, most have been fixed. ARMv8.1 also supports 52-bit VA (though only with 64K pages and we haven't added support for it yet). However, it's likely that if we make a 52-bit TASK_SIZE this the default, we will break some user assumptions. While arguably that's not necessarily ABI, if user relies on a 47 or 48-bit VA the kernel shouldn't break it. So I'm strongly inclined to make the 52-bit TASK_SIZE an opt-in on arm64. -- Catalin -- 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>