On 03/24/2017 12:33 AM, John Hubbard wrote: > There might be some additional information you are using to come up with > that conclusion, that is not obvious to me. Any thoughts there? These > calls use the same underlying page allocator (and I thought that both > were subject to the same constraints on defragmentation, as a result of > that). So I am not seeing any way that kmalloc could possibly be a > less-fragmenting call than vmalloc.
You guys are having quite a discussion over a very small point. But, Ying is right. Let's say we have a two-page data structure. vmalloc() takes two effectively random order-0 pages, probably from two different 2M pages and pins them. That "kills" two 2M pages. kmalloc(), allocating two *contiguous* pages, is very unlikely to cross a 2M boundary (it theoretically could). That means it will only "kill" the possibility of a single 2M page. More 2M pages == less fragmentation.
