[PATCH v3 08/14] mm: Combine first three unions in struct page

[Vlastimil Babka <vbabka@suse.cz>]

On 04/18/2018 08:49 PM, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@microsoft.com>
> 
> By combining these three one-word unions into one three-word union,
> we make it easier for users to add their own multi-word fields to struct
> page, as well as making it obvious that SLUB needs to keep its double-word
> alignment for its freelist & counters.
> 
> No field moves position; verified with pahole.
> 
> Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
> ---
>  include/linux/mm_types.h | 65 ++++++++++++++++++++--------------------
>  1 file changed, 32 insertions(+), 33 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 04d9dc442029..39521b8385c1 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -70,45 +70,44 @@ struct hmm;
>  #endif
>  
>  struct page {
> -	/* First double word block */
>  	unsigned long flags;		/* Atomic flags, some possibly
>  					 * updated asynchronously */
> -	union {
> -		/* See page-flags.h for the definition of PAGE_MAPPING_FLAGS */
> -		struct address_space *mapping;
> -
> -		struct kmem_cache *slab_cache;	/* SL[AU]B: Pointer to slab */
> +	union {		/* This union is three words (12/24 bytes) in size */
> +		struct {	/* Page cache and anonymous pages */
> +			/* See page-flags.h for PAGE_MAPPING_FLAGS */
> +			struct address_space *mapping;
> +			pgoff_t index;		/* Our offset within mapping. */
> +			/**
> +			 * @private: Mapping-private opaque data.
> +			 * Usually used for buffer_heads if PagePrivate.
> +			 * Used for swp_entry_t if PageSwapCache.
> +			 * Indicates order in the buddy system if PageBuddy.
> +			 */
> +			unsigned long private;
> +		};
> +		struct {	/* slab and slob */
> +			struct kmem_cache *slab_cache;
> +			void *freelist;		/* first free object */
> +			void *s_mem;		/* first object */
> +		};
> +		struct {	/* slub also uses some of the slab fields */
> +			struct kmem_cache *slub_cache;
> +			/* Double-word boundary */
> +			void *slub_freelist;

Is slub going to switch to use those? Or maybe this is an overkill and
we could merge the two sl*b structs and just have an union for s_mem and
the 3 counters?

> +			unsigned inuse:16;
> +			unsigned objects:15;
> +			unsigned frozen:1;
> +		};