Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse the page on WP fault

["Kirill A. Shutemov" <kirill@shutemov.name>]

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

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