On Fri, Jun 10, 2022 at 05:35:27PM +0300, Kirill A. Shutemov wrote: > LAM_U48 allows to encode 15 bits of tags into address. > > LAM_U48 steals bits above 47-bit for tags and makes it impossible for > userspace to use full address space on 5-level paging machine. > > Make these features mutually exclusive: whichever gets enabled first > blocks the other one. This patch is broken in that it doesn't fix untag_pointer() *If* you really want to continue down this road; you'll need something like: #define untagged_addr(mm, addr) ({ \ u64 __addr = (__force u64)(addr); \ s64 sign = (s64)__addr >> 63; \ __addr ^= sign; \ __addr &= (mm)->context.untag_mask[sign & 1]; \ __addr ^= sign; \ (__force __typeof__(addr))__addr; \ }) Which uses a different mask for kernel and user pointers. Anyway, without this U48 patch on, the mask could be a constant, no need to keep this variable, we can unconditionally unmask U57. Let me go reply to that other mail too.