[RFC PATCH V3 4/7] mm/slab: use stride to access slabobj_ext

[Harry Yoo <harry.yoo@oracle.com>]

Use a configurable stride value when accessing slab object extension
metadata instead of assuming a fixed sizeof(struct slabobj_ext).

Store stride value in free bits of slab->counters field. This allows
for flexibility in cases where the extension is embedded within
slab objects.

Since these free bits exist only on 64-bit, any future optimizations
that need to change stride value cannot be enabled on 32-bit architectures.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/slab.h | 37 +++++++++++++++++++++++++++++++++----
 mm/slub.c |  2 ++
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index df2c987d950d..22ee28cb55e1 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -83,6 +83,14 @@ struct slab {
 							 * that the slab was corrupted
 							 */
 							unsigned frozen:1;
+#ifdef CONFIG_64BIT
+							/*
+							 * Some optimizations use free bits in 'counters' field
+							 * to save memory. In case ->stride field is not available,
+							 * such optimizations are disabled.
+							 */
+							unsigned short stride;
+#endif
 						};
 					};
 				};
@@ -550,6 +558,26 @@ static inline unsigned long slab_obj_exts(struct slab *slab)
 	return obj_exts & ~OBJEXTS_FLAGS_MASK;
 }
 
+#ifdef CONFIG_64BIT
+static inline void slab_set_stride(struct slab *slab, unsigned short stride)
+{
+	slab->stride = stride;
+}
+static inline unsigned short slab_get_stride(struct slab *slab)
+{
+	return slab->stride;
+}
+#else
+static inline void slab_set_stride(struct slab *slab, unsigned short stride)
+{
+	VM_WARN_ON_ONCE(stride != sizeof(struct slabobj_ext));
+}
+static inline unsigned short slab_get_stride(struct slab *slab)
+{
+	return sizeof(struct slabobj_ext);
+}
+#endif
+
 /*
  * slab_obj_ext - get the pointer to the slab object extension metadata
  * associated with an object in a slab.
@@ -563,13 +591,10 @@ static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
 					       unsigned long obj_exts,
 					       unsigned int index)
 {
-	struct slabobj_ext *obj_ext;
-
 	VM_WARN_ON_ONCE(!slab_obj_exts(slab));
 	VM_WARN_ON_ONCE(obj_exts != slab_obj_exts(slab));
 
-	obj_ext = (struct slabobj_ext *)obj_exts;
-	return &obj_ext[index];
+	return (struct slabobj_ext *)(obj_exts + slab_get_stride(slab) * index);
 }
 
 int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
@@ -588,6 +613,10 @@ static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
 	return NULL;
 }
 
+static inline void slab_set_stride(struct slab *slab, unsigned int stride) { }
+static inline unsigned int slab_get_stride(struct slab *slab) { return 0; }
+
+
 #endif /* CONFIG_SLAB_OBJ_EXT */
 
 static inline enum node_stat_item cache_vmstat_idx(struct kmem_cache *s)
diff --git a/mm/slub.c b/mm/slub.c
index ae73403f8c29..4383740a4d34 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2134,6 +2134,8 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 #endif
 	old_exts = READ_ONCE(slab->obj_exts);
 	handle_failed_objexts_alloc(old_exts, vec, objects);
+	slab_set_stride(slab, sizeof(struct slabobj_ext));
+
 	if (new_slab) {
 		/*
 		 * If the slab is brand new and nobody can yet access its
-- 
2.43.0