On Sun, Apr 23, 2017 at 12:53:46PM +0200, John Paul Adrian Glaubitz wrote: > Hi Kirill! > > I recently read the LWN article on your and your colleagues work to > add five-level page table support for x86 to the Linux kernel [1] > and I got your email address from the last patch of the series. > > Since this extends the address space beyond 48-bits, as you may know, > it will cause potential headaches with Javascript engines which use > tagged pointers. On SPARC, the virtual address space already extends > to 52 bits and we are running into these very issues with Javascript > engines on SPARC. > > Now, a possible way to mitigate this problem would be to pass the > "hint" parameter to mmap() in order to tell the kernel not to allocate > memory beyond the 48 bits address space. Unfortunately, on Linux this > will only work when the area pointed to by "hint" is unallocated which > means one cannot simply use a hardcoded "hint" to mitigate this problem. In proposed implementation, we also use hint address, but in different way: by default, if hint address is NULL, kernel would not create mappings above 47-bits, preserving compatibility. If an application wants to have access to larger address space, it has to specify hint addess above 47-bits. See details here: http://lkml.kernel.org/r/20170420162147.86517-10-kirill.shutemov@xxxxxxxxxxxxxxx -- Kirill A. Shutemov
