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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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