On 03/24/2017 02:56 PM, Dave Hansen wrote: > 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. Sorry, I know I'm too late for this discussion, just wanted to clarify a bit. > 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). If by "theoretically" you mean we switch kmalloc() from a buddy allocator to something else, then yes. Otherwise, in the buddy allocator, it cannot cross the 2M boundary by design. > That means it will only "kill" > the possibility of a single 2M page. More 2M pages == less fragmentation. IMHO John is right that kmalloc() will reduce the number of high-order pages *in the short term*. But in the long term, vmalloc() will hurt us more due to the scattering of unmovable pages as you describe. As this is AFAIU a long-term allocation, kmalloc() should be preferred. Vlastimil > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
