> > 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?
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