Re: [PATCH] zsmalloc: keep first object offset in struct page

[Minchan Kim <minchan@kernel.org>]

Andrew,

Please fold it to zsmalloc: page migration support.

Thanks.

On Mon, Jun 13, 2016 at 12:20:15PM +0900, Minchan Kim wrote:
> In early draft of zspage migration, we couldn't use page._mapcount
> because it was used for storing movable flag so we added runtime
> calculation to get first object offset in a page but it causes rather
> many instruction and even bug.
> 
> Since then, we don't use page._mapcount as page flag any more so now
> there is no problem to use the field to store first object offset.
> 
> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/zsmalloc.c | 44 ++++++++++++++++----------------------------
>  1 file changed, 16 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 6a58edc9a015..4b70fcbfb69b 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -512,6 +512,16 @@ static inline struct page *get_first_page(struct zspage *zspage)
>  	return first_page;
>  }
>  
> +static inline int get_first_obj_offset(struct page *page)
> +{
> +	return page->units;
> +}
> +
> +static inline void set_first_obj_offset(struct page *page, int offset)
> +{
> +	page->units = offset;
> +}
> +
>  static inline unsigned int get_freeobj(struct zspage *zspage)
>  {
>  	return zspage->freeobj;
> @@ -872,31 +882,6 @@ static struct page *get_next_page(struct page *page)
>  	return page->freelist;
>  }
>  
> -/* Get byte offset of first object in the @page */
> -static int get_first_obj_offset(struct size_class *class,
> -				struct page *first_page, struct page *page)
> -{
> -	int pos;
> -	int page_idx = 0;
> -	int ofs = 0;
> -	struct page *cursor = first_page;
> -
> -	if (first_page == page)
> -		goto out;
> -
> -	while (page != cursor) {
> -		page_idx++;
> -		cursor = get_next_page(cursor);
> -	}
> -
> -	pos = class->objs_per_zspage * class->size *
> -		page_idx / class->pages_per_zspage;
> -
> -	ofs = (pos + class->size) % PAGE_SIZE;
> -out:
> -	return ofs;
> -}
> -
>  /**
>   * obj_to_location - get (<page>, <obj_idx>) from encoded object value
>   * @page: page object resides in zspage
> @@ -966,6 +951,7 @@ static void reset_page(struct page *page)
>  	clear_bit(PG_private, &page->flags);
>  	clear_bit(PG_private_2, &page->flags);
>  	set_page_private(page, 0);
> +	page_mapcount_reset(page);
>  	ClearPageHugeObject(page);
>  	page->freelist = NULL;
>  }
> @@ -1064,6 +1050,8 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
>  		struct link_free *link;
>  		void *vaddr;
>  
> +		set_first_obj_offset(page, off);
> +
>  		vaddr = kmap_atomic(page);
>  		link = (struct link_free *)vaddr + off / sizeof(*link);
>  
> @@ -1762,9 +1750,8 @@ static unsigned long find_alloced_obj(struct size_class *class,
>  	int offset = 0;
>  	unsigned long handle = 0;
>  	void *addr = kmap_atomic(page);
> -	struct zspage *zspage = get_zspage(page);
>  
> -	offset = get_first_obj_offset(class, get_first_page(zspage), page);
> +	offset = get_first_obj_offset(page);
>  	offset += class->size * index;
>  
>  	while (offset < PAGE_SIZE) {
> @@ -1976,6 +1963,7 @@ static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>  	} while ((page = get_next_page(page)) != NULL);
>  
>  	create_page_chain(class, zspage, pages);
> +	set_first_obj_offset(newpage, get_first_obj_offset(oldpage));
>  	if (unlikely(PageHugeObject(oldpage)))
>  		newpage->index = oldpage->index;
>  	__SetPageMovable(newpage, page_mapping(oldpage));
> @@ -2062,7 +2050,7 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  	get_zspage_mapping(zspage, &class_idx, &fullness);
>  	pool = mapping->private_data;
>  	class = pool->size_class[class_idx];
> -	offset = get_first_obj_offset(class, get_first_page(zspage), page);
> +	offset = get_first_obj_offset(page);
>  
>  	spin_lock(&class->lock);
>  	if (!get_zspage_inuse(zspage)) {
> -- 
> 1.9.1
> 

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