[PATCH v3 1/4] mm/zsmalloc: encode class index in obj value for lockless class lookup

[Wenchao Hao <haowenchao22@gmail.com>]

From: Wenchao Hao <haowenchao@xiaomi.com>

Encode the size_class index (class_idx) into the obj value so that
zs_free() can determine the correct size_class without dereferencing
the handle->obj->PFN->zpdesc->zspage->class chain under pool->lock.
class_idx is invariant across page migration (only PFN is rewritten),
so a lockless read of obj always yields a valid class_idx.

The space below the PFN field in obj is over-provisioned on 64-bit
systems, with more bits than obj_idx needs. Split that space into
class_idx and obj_idx subfields:

 |<-- _PFN_BITS -->|<-- ZS_OBJ_CLASS_BITS -->|<-- ZS_OBJ_IDX_BITS -->|
 +-----------------+-------------------------+-----------------------+
 |       PFN       |        class_idx        |        obj_idx        |
 +-----------------+-------------------------+-----------------------+
MSB                ^                                                LSB
                   |
                   +-- ZS_OBJ_PFN_SHIFT

The macro layout changes as follows:

    Before            After               Meaning
    ----------------  ------------------  ----------------------------
    OBJ_INDEX_BITS    ZS_OBJ_IDX_BITS     width of obj_idx subfield
    OBJ_INDEX_MASK    ZS_OBJ_IDX_MASK     mask  of obj_idx subfield
    (n/a)             ZS_OBJ_CLASS_BITS   width of class_idx subfield
    (n/a)             ZS_OBJ_CLASS_MASK   mask  of class_idx subfield
    (n/a)             ZS_OBJ_PFN_SHIFT    bit offset of PFN in obj

On 32-bit systems there is no spare room for class_idx, so the
encoding is disabled (ZS_OBJ_CLASS_BITS = 0) and the obj layout
remains [PFN | obj_idx].

Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zsmalloc.c | 80 ++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 66 insertions(+), 14 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 63128ddb7959..6b0014b43408 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -67,8 +67,8 @@
 #define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
 #else
 /*
- * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
- * be PAGE_SHIFT
+ * If this definition of MAX_PHYSMEM_BITS is used, ZS_OBJ_PFN_SHIFT will
+ * just be PAGE_SHIFT
  */
 #define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
 #endif
@@ -88,8 +88,27 @@
 #define OBJ_TAG_BITS	1
 #define OBJ_TAG_MASK	OBJ_ALLOCATED_TAG
 
-#define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS)
-#define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
+/*
+ * obj is encoded as [PFN | class_idx | obj_idx] within an unsigned long:
+ *
+ *   |<-- _PFN_BITS -->|<-- ZS_OBJ_CLASS_BITS -->|<-- ZS_OBJ_IDX_BITS -->|
+ *   +-----------------+-------------------------+-----------------------+
+ *   |       PFN       |        class_idx        |        obj_idx        |
+ *   +-----------------+-------------------------+-----------------------+
+ *  MSB                ^                                                LSB
+ *                     |
+ *                     +-- ZS_OBJ_PFN_SHIFT
+ *
+ * Encoding class_idx into obj lets zs_free() locate the size_class
+ * without holding pool->lock; class_idx is invariant across page
+ * migration (only PFN changes), so a lockless read of the obj value
+ * always yields a valid class_idx.
+ *
+ * On 32-bit systems there is no spare room for class_idx, so
+ * ZS_OBJ_CLASS_BITS is 0 and the layout collapses to the original
+ * [PFN | obj_idx] without any ifdef in callers.
+ */
+#define ZS_OBJ_PFN_SHIFT	(BITS_PER_LONG - _PFN_BITS)
 
 #define HUGE_BITS	1
 #define FULLNESS_BITS	4
@@ -98,9 +117,29 @@
 
 #define ZS_MAX_PAGES_PER_ZSPAGE	(_AC(CONFIG_ZSMALLOC_CHAIN_SIZE, UL))
 
+/*
+ * Reuse the width that struct zspage already reserves for its
+ * class field (zspage->class:CLASS_BITS + 1) for the class_idx
+ * field encoded in obj.  On 32-bit there is no spare room, so set
+ * it to 0; the encoded class_idx then folds to a constant 0 and
+ * the layout collapses back to [PFN | obj_idx].
+ */
+#if BITS_PER_LONG >= 64
+#define ZS_OBJ_CLASS_BITS	(CLASS_BITS + 1)
+#else
+#define ZS_OBJ_CLASS_BITS	0
+#endif
+#define ZS_OBJ_CLASS_MASK	((_AC(1, UL) << ZS_OBJ_CLASS_BITS) - 1)
+
+#define ZS_OBJ_IDX_BITS		(ZS_OBJ_PFN_SHIFT - ZS_OBJ_CLASS_BITS)
+#define ZS_OBJ_IDX_MASK		((_AC(1, UL) << ZS_OBJ_IDX_BITS) - 1)
+
+static_assert(ZS_OBJ_IDX_BITS > 0,
+	      "zsmalloc: PFN + class_idx leave no room for obj_idx");
+
 /* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
 #define ZS_MIN_ALLOC_SIZE \
-	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
+	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> ZS_OBJ_IDX_BITS))
 /* each chunk includes extra space to keep handle */
 #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
 
@@ -721,26 +760,38 @@ static struct zpdesc *get_next_zpdesc(struct zpdesc *zpdesc)
 static void obj_to_location(unsigned long obj, struct zpdesc **zpdesc,
 				unsigned int *obj_idx)
 {
-	*zpdesc = pfn_zpdesc(obj >> OBJ_INDEX_BITS);
-	*obj_idx = (obj & OBJ_INDEX_MASK);
+	*zpdesc = pfn_zpdesc(obj >> ZS_OBJ_PFN_SHIFT);
+	*obj_idx = (obj & ZS_OBJ_IDX_MASK);
 }
 
 static void obj_to_zpdesc(unsigned long obj, struct zpdesc **zpdesc)
 {
-	*zpdesc = pfn_zpdesc(obj >> OBJ_INDEX_BITS);
+	*zpdesc = pfn_zpdesc(obj >> ZS_OBJ_PFN_SHIFT);
+}
+
+/*
+ * On 32-bit systems ZS_OBJ_CLASS_BITS is 0 and ZS_OBJ_CLASS_MASK is 0,
+ * so this collapses to a constant 0.  No ifdef needed at the call site.
+ */
+static unsigned int obj_to_class_idx(unsigned long obj)
+{
+	return (obj >> ZS_OBJ_IDX_BITS) & ZS_OBJ_CLASS_MASK;
 }
 
 /**
- * location_to_obj - get obj value encoded from (<zpdesc>, <obj_idx>)
+ * location_to_obj - encode (<zpdesc>, <obj_idx>, <class_idx>) into obj value
  * @zpdesc: zpdesc object resides in zspage
  * @obj_idx: object index
+ * @class_idx: size class index; ignored on 32-bit (ZS_OBJ_CLASS_BITS == 0)
  */
-static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx)
+static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx,
+				     unsigned int class_idx)
 {
 	unsigned long obj;
 
-	obj = zpdesc_pfn(zpdesc) << OBJ_INDEX_BITS;
-	obj |= obj_idx & OBJ_INDEX_MASK;
+	obj  = zpdesc_pfn(zpdesc) << ZS_OBJ_PFN_SHIFT;
+	obj |= (unsigned long)(class_idx & ZS_OBJ_CLASS_MASK) << ZS_OBJ_IDX_BITS;
+	obj |= obj_idx & ZS_OBJ_IDX_MASK;
 
 	return obj;
 }
@@ -1276,7 +1327,7 @@ static unsigned long obj_malloc(struct zs_pool *pool,
 	kunmap_local(vaddr);
 	mod_zspage_inuse(zspage, 1);
 
-	obj = location_to_obj(m_zpdesc, obj);
+	obj = location_to_obj(m_zpdesc, obj, zspage->class);
 	record_obj(handle, obj);
 
 	return obj;
@@ -1762,7 +1813,8 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 
 			old_obj = handle_to_obj(handle);
 			obj_to_location(old_obj, &dummy, &obj_idx);
-			new_obj = (unsigned long)location_to_obj(newzpdesc, obj_idx);
+			new_obj = location_to_obj(newzpdesc, obj_idx,
+						  obj_to_class_idx(old_obj));
 			record_obj(handle, new_obj);
 		}
 	}
-- 
2.34.1