On Wed, Aug 29, 2018 at 10:17:54AM +0800, Baoquan He wrote: > Vmemmap area has different base and size depending on paging mode. > Now we just hardcode its size as 1TB in memory KASLR, it's not > right for 5-level paging mode. > > Adjust it according to paging mode and use it during memory KASLR. >
I think 512TiB is wasteful for 5-level paging. We don't need that much. 1TiB limit with 4-level paging is required to fit struct pages for all 64TiB of physical memory, assuming each struct page is 64 bytes. With 5-level paging the limit on physical memory is not 512-times bigger: we cap at 52-bit physical address space. So it's just 64 times bigger and we need only 64TiB in worst case. I think we can limit it further by taking into account memory_tb. Most of machines will be fine with 1TiB there and we save few more bits from KASLR. -- Kirill A. Shutemov
