Re: [PATCH] mm: avoid tail page refcounting on non-THP compound pages

[Hugh Dickins]

On Wed, 25 Mar 2015, Hugh Dickins wrote:
> On Thu, 26 Mar 2015, Kirill A. Shutemov wrote:
> 
> > THP uses tail page refcounting to be able to split huge page at any
> > time. Tail page refcounting is not needed for rest users of compound
> > pages and it's harmful because of overhead.
> > 
> > We try to exclude non-THP pages from tail page refcounting using
> > __compound_tail_refcounted() check. It excludes most common non-THP
> > compound pages: SL*B and hugetlb, but it doesn't catch rest of
> > __GFP_COMP users -- drivers.
> > 
> > And it's not only about overhead.
> > 
> > Drivers might want to use compound pages to get refcounting semantics
> > suitable for mapping high-order pages to userspace. But tail page
> > refcounting breaks it.
> > 
> > Tail page refcounting uses ->_mapcount in tail pages to store GUP pins
> > on them. It means GUP pins would affect page_mapcount() for tail pages.
> > It's not a problem for THP, because it never maps tail pages. But unlike
> > THP, drivers map parts of compound pages with PTEs and it makes
> > page_mapcount() be called for tail pages.
> > 
> > In particular, GUP pins would shift PSS up and affect /proc/kpagecount
> > for such pages. But, I'm not aware about anything which can lead to
> > crash or other serious misbehaviour.
> > 
> > Since currently all THP pages are anonymous and all drivers pages are
> > not, we can fix the __compound_tail_refcounted() check by requiring
> > PageAnon() to enable tail page refcounting.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

Oh, hold on a moment: does this actually build in a tree without your
page-flags.h consolidation?  It didn't when I tried to add a PageAnon
test there for my series against v3.19, has something changed in v4.0?

> 
> > ---
> >  include/linux/mm.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 4a3a38522ab4..16fe322b66ea 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -456,7 +456,7 @@ static inline int page_count(struct page *page)
> >  
> >  static inline bool __compound_tail_refcounted(struct page *page)
> >  {
> > -	return !PageSlab(page) && !PageHeadHuge(page);
> > +	return PageAnon(page) && !PageSlab(page) && !PageHeadHuge(page);
> >  }
> >  
> >  /*
> > -- 
> > 2.1.4
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