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    SubjectRe: [PATCH 06/19] mm: store mapcount for compound page separate
    Date
    "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;
    }


    -aneesh



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