mm: combine first three unions in struct page

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.

Link: http://lkml.kernel.org/r/20180518194519.3820-8-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0b0c0e224011b84fed1355cbd1762cffe790e91c..158f9693f8652798887bbe57a8e55da9a6313f61 100644 (file)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -70,46 +70,46 @@ struct hmm;
 #endif
 
 struct page {
-       /* First double word block */
        unsigned long flags;            /* Atomic flags, some possibly
                                         * updated asynchronously */
+       /* Three words (12/24 bytes) are available in this union. */
        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 */
+               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; /* not slob */
+                       /* Double-word boundary */
+                       void *freelist;         /* first free object */
+                       union {
+                               void *s_mem;    /* slab: first object */
+                               unsigned long counters;         /* SLUB */
+                               struct {                        /* SLUB */
+                                       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 */
-               unsigned long counters;         /* SLUB */
-               struct {                        /* SLUB */
-                       unsigned inuse:16;
-                       unsigned objects:15;
-                       unsigned frozen:1;
                };
        };