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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
