Re: [PATCH] mm: add fields for compound destructor and order into struct page

[Jerome Marchand]

On 12/11/2014 02:20 PM, Kirill A. Shutemov wrote:
> Currently, we use lru.next/lru.prev plus cast to access or set
> destructor and order of compound page.
> 
> Let's replace it with explicit fields in struct page.

Thanks! That made everything much clearer: the complexity of page struct
should not be swept under the carpet.

> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Acked-by: Jerome Marchand <jmarchan@xxxxxxxxxx>

> ---
>  include/linux/mm.h       | 9 ++++-----
>  include/linux/mm_types.h | 8 ++++++++
>  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5bfd9b9756fa..a8de6fe11d0a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -525,29 +525,28 @@ int split_free_page(struct page *page);
>   * prototype for that function and accessor functions.
>   * These are _only_ valid on the head of a PG_compound page.
>   */
> -typedef void compound_page_dtor(struct page *);
>  
>  static inline void set_compound_page_dtor(struct page *page,
>  						compound_page_dtor *dtor)
>  {
> -	page[1].lru.next = (void *)dtor;
> +	page[1].compound_dtor = dtor;
>  }
>  
>  static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
>  {
> -	return (compound_page_dtor *)page[1].lru.next;
> +	return page[1].compound_dtor;
>  }
>  
>  static inline int compound_order(struct page *page)
>  {
>  	if (!PageHead(page))
>  		return 0;
> -	return (unsigned long)page[1].lru.prev;
> +	return page[1].compound_order;
>  }
>  
>  static inline void set_compound_order(struct page *page, unsigned long order)
>  {
> -	page[1].lru.prev = (void *)order;
> +	page[1].compound_order = order;
>  }
>  
>  #ifdef CONFIG_MMU
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 03945eef1350..cbc71f32a53c 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -28,6 +28,8 @@ struct mem_cgroup;
>  		IS_ENABLED(CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK))
>  #define ALLOC_SPLIT_PTLOCKS	(SPINLOCK_SIZE > BITS_PER_LONG/8)
>  
> +typedef void compound_page_dtor(struct page *);
> +
>  /*
>   * Each physical page in the system has a struct page associated with
>   * it to keep track of whatever it is we are using the page for at the
> @@ -131,6 +133,12 @@ struct page {
>  		struct rcu_head rcu_head;	/* Used by SLAB
>  						 * when destroying via RCU
>  						 */
> +		/* First tail page of compound page */
> +		struct {
> +			compound_page_dtor *compound_dtor;
> +			unsigned long compound_order;
> +		};
> +
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
>  		pgtable_t pmd_huge_pte; /* protected by page->ptl */
>  #endif
>

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