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SubjectRe: [PATCH 06/19] mm: store mapcount for compound page separate
On Fri, Nov 21, 2014 at 11:42:51AM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:
>
> > 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@linux.intel.com>
> > ---
> > 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;
> > +}
> > +
> > +static inline int compound_mapcount(struct page *page)
> > +{
> > + if (!PageCompound(page))
> > + return 0;
> > + page = compound_head(page);
> > + return atomic_read(compound_mapcount_ptr(page)) + 1;
> > +}
>
>
> How about
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6e0b286649f1..59c9cf3d8510 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -46,6 +46,11 @@ struct page {
> unsigned long flags; /* Atomic flags, some possibly
> * updated asynchronously */
> union {
> + /*
> + * For THP we use this to track the compound
> + * page mapcount.
> + */
> + atomic_t _compound_mapcount;
> struct address_space *mapping; /* If low bit clear, points to
> * inode address_space, or NULL.
> * If page mapped as anonymous
>
> and
>
> static inline atomic_t *compound_mapcount_ptr(struct page *page)
> {
> return (atomic_t *)&page[1]._compound_mapcount;
> }

Cast is redundant ;)

See answer to Christoph.

--
Kirill A. Shutemov


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