Re: [PATCH 06/19] mm: store mapcount for compound page separate

[Kirill A. Shutemov]

On Wed, Nov 19, 2014 at 11:51:09AM +0100, Jerome Marchand wrote:
> On 11/05/2014 03:49 PM, Kirill A. Shutemov wrote:
> > We're going to allow mapping of individual 4k pages of THP compound and
> > we need a cheap way to find out how many time the compound page is
> > mapped with PMD -- compound_mapcount() does this.
> > 
> > page_mapcount() counts both: PTE and PMD mappings of the page.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/mm.h   | 17 +++++++++++++++--
> >  include/linux/rmap.h |  4 ++--
> >  mm/huge_memory.c     | 23 ++++++++++++++---------
> >  mm/hugetlb.c         |  4 ++--
> >  mm/memory.c          |  2 +-
> >  mm/migrate.c         |  2 +-
> >  mm/page_alloc.c      | 13 ++++++++++---
> >  mm/rmap.c            | 50 +++++++++++++++++++++++++++++++++++++++++++-------
> >  8 files changed, 88 insertions(+), 27 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 1825c468f158..aef03acff228 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -435,6 +435,19 @@ static inline struct page *compound_head(struct page *page)
> >  	return page;
> >  }
> >  
> > +static inline atomic_t *compound_mapcount_ptr(struct page *page)
> > +{
> > +	return (atomic_t *)&page[1].mapping;
> > +}
> 
> IIUC your patch overloads the unused mapping field of the first tail
> page to store the PMD mapcount. That's a non obvious trick. Why not make
> it more explicit by adding a new field (say compound_mapcount - and the
> appropriate comment of course) to the union to which mapping already belong?

I don't think we want to bloat struct page description: nobody outside of
helpers should use it direcly. And it's exactly what we did to store
compound page destructor and compound page order.

> The patch description would benefit from more explanation too.

Agreed.

-- 
 Kirill A. Shutemov
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