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

[Aneesh Kumar K.V]

"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> 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@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;
> +}
> +
> +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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