On Tue, Apr 28, 2015 at 11:02:46AM +0800, Hillf Danton wrote: > > > > With new refcounting we will be able map the same compound page with > > PTEs and PMDs. It requires adjustment to conditions when we can reuse > > the page on write-protection fault. > > > > For PTE fault we can't reuse the page if it's part of huge page. > > > > For PMD we can only reuse the page if nobody else maps the huge page or > > it's part. We can do it by checking page_mapcount() on each sub-page, > > but it's expensive. > > > > The cheaper way is to check page_count() to be equal 1: every mapcount > > takes page reference, so this way we can guarantee, that the PMD is the > > only mapping. > > > > This approach can give false negative if somebody pinned the page, but > > that doesn't affect correctness. > > > Then we have to try more to allocate THP if pinned? > Are we adding new cost? Yes we do. But that shouldn't be often. Alternatively, we could iterate over all sub-pages and check their mapcount. -- Kirill A. Shutemov -- 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>