On 04/18/2018 08:49 PM, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> ---
> 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;
> + };
