* Linus Torvalds <torva...@linux-foundation.org> wrote: > On Thu, Oct 26, 2017 at 9:02 PM, Ingo Molnar <mi...@kernel.org> wrote: > > > > Well, 'mem=2048M' shouldn't really limit device memory, it's supposed to > > limit > > (trim) 'RAM' and not much else. > > Agreed. You should very much be able to map in IO memory or whatever > above the 2G address even if the high_memory itself might be limited > to 2GB. > > So I think that commit ce56a86e2ade ("x86/mm: Limit mmap() of /dev/mem > to valid physical addresses") is wrong, in that "high_memory" is very > much the wrong thing to test. > > The memory mapping limit might validly be something like > > 1ull << boot_cpu_data.x86_phys_bits > > or similar, but for now I suspect that the right thing to do is to > revert. I'm not convinced that our "x86_phys_bits" value is guaranteed > to be always right, since I think we mainlyjust use it for showing > things, rather than have lots of code that depends on it. > > Ingo?
Yeah, I think a more robust condition would be something like: int valid_phys_addr_range(phys_addr_t addr, size_t count) { return !((addr + count) >> MAX_PHYSMEM_BITS); } ... as we already rely on MAX_PHYSMEM_BITS in a number of other critical places. (Totally untested though.) Thanks, Ingo