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>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
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..f0ccb699641d 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, slob and slub */
+ struct kmem_cache *slab_cache; /* (slub) */
+ void *freelist; /* first free object (slub) */
+ void *s_mem; /* first object */
+ };
+ struct { /* slub */
+ struct kmem_cache *slub_cache; /* shared with slab */
+ /* Double-word boundary */
+ void *slub_freelist; /* shared with slab */
+ unsigned inuse:16;
+ unsigned objects:15;
+ unsigned frozen:1;
+ };
atomic_t compound_mapcount; /* first tail page */
- /* page_deferred_list().next -- second tail page */
- };
-
- /* Second double word */
- union {
- pgoff_t index; /* Our offset within mapping. */
- void *freelist; /* sl[aou]b first free object */
- /* page_deferred_list().prev -- second tail page */
- };
-
- union {
- /*
- * 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;
-#if USE_SPLIT_PTE_PTLOCKS
+ struct list_head deferred_list; /* second tail page */
+ struct { /* Page table pages */
+ unsigned long _pt_pad_2; /* mapping */
+ unsigned long _pt_pad_3;
#if ALLOC_SPLIT_PTLOCKS
- spinlock_t *ptl;
+ spinlock_t *ptl;
#else
- spinlock_t ptl;
-#endif
+ spinlock_t ptl;
#endif
- void *s_mem; /* slab first object */
- struct { /* SLUB */
- unsigned inuse:16;
- unsigned objects:15;
- unsigned frozen:1;
};
};
--
2.17.0
