[PATCH v4 2/4] mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems

[Wenchao Hao <haowenchao22@gmail.com>]

From: Wenchao Hao <haowenchao@xiaomi.com>

With class_idx encoded in obj, zs_free() can locate the size_class
without holding pool->lock on 64-bit systems.  Page migration also
takes class->lock and only rewrites the PFN field of obj, so:

  1. read obj locklessly,
  2. lock the size_class derived from obj's class_idx,
  3. re-read obj under class->lock to get a stable PFN.

This eliminates the rwlock read-side cacheline bouncing between
zs_free() and migration/compaction on multi-core systems.

Annotate handle_to_obj()/record_obj() with READ_ONCE()/WRITE_ONCE() to
prevent load/store tearing on the lockless read path and silence KCSAN
data race reports.

When ZS_OBJ_CLASS_BITS == 0 (32-bit, or 64-bit with obj too narrow to
hold class_idx), zs_free() keeps pool->lock.

Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zsmalloc.c | 75 ++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 60 insertions(+), 15 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index f84258d63917..fe20ab297542 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -21,6 +21,10 @@
  *	pool->lock
  *	class->lock
  *	zspage->lock
+ *
+ * When ZS_OBJ_CLASS_BITS > 0, zs_free() skips pool->lock; it picks
+ * the size_class from obj's encoded class_idx and serializes against
+ * page migration via class->lock.
  */
 
 #include <linux/module.h>
@@ -457,10 +461,13 @@ static void cache_free_zspage(struct zspage *zspage)
 	kmem_cache_free(zspage_cachep, zspage);
 }
 
-/* class->lock(which owns the handle) synchronizes races */
+/*
+ * Pairs with READ_ONCE() in handle_to_obj(): zs_free() may read the
+ * handle locklessly, so prevent store tearing here.
+ */
 static void record_obj(unsigned long handle, unsigned long obj)
 {
-	*(unsigned long *)handle = obj;
+	WRITE_ONCE(*(unsigned long *)handle, obj);
 }
 
 static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc)
@@ -817,7 +824,7 @@ static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx
 
 static unsigned long handle_to_obj(unsigned long handle)
 {
-	return *(unsigned long *)handle;
+	return READ_ONCE(*(unsigned long *)handle);
 }
 
 static inline bool obj_allocated(struct zpdesc *zpdesc, void *obj,
@@ -1451,10 +1458,58 @@ static void obj_free(int class_size, unsigned long obj)
 	mod_zspage_inuse(zspage, -1);
 }
 
+/*
+ * Resolve @handle to its zspage / size_class and acquire class->lock.
+ *
+ * When class_idx is encoded in obj (ZS_OBJ_CLASS_BITS > 0), it is
+ * invariant under page migration, so the handle can be read locklessly
+ * to pick the size_class.  Once class->lock is held migration is
+ * blocked and the handle is re-read to obtain a stable PFN.
+ *
+ * Otherwise (32-bit, or 64-bit fallback paths like UML where the
+ * encoding is disabled), fall back to pool->lock for the lookup.
+ */
+#if ZS_OBJ_CLASS_BITS > 0
+static inline void obj_handle_class_lock(struct zs_pool *pool, unsigned long handle,
+					 unsigned long *objp, struct zspage **zspagep,
+					 struct size_class **classp)
+	__acquires(&(*classp)->lock)
+{
+	struct zpdesc *f_zpdesc;
+	unsigned long obj;
+
+	obj = handle_to_obj(handle);
+	*classp = pool->size_class[obj_to_class_idx(obj)];
+	spin_lock(&(*classp)->lock);
+	/* Re-read under class->lock: PFN is now stable vs migration. */
+	obj = handle_to_obj(handle);
+	obj_to_zpdesc(obj, &f_zpdesc);
+	*zspagep = get_zspage(f_zpdesc);
+	*objp = obj;
+}
+#else
+static inline void obj_handle_class_lock(struct zs_pool *pool, unsigned long handle,
+					 unsigned long *objp, struct zspage **zspagep,
+					 struct size_class **classp)
+	__acquires(&(*classp)->lock)
+{
+	struct zpdesc *f_zpdesc;
+	unsigned long obj;
+
+	read_lock(&pool->lock);
+	obj = handle_to_obj(handle);
+	obj_to_zpdesc(obj, &f_zpdesc);
+	*zspagep = get_zspage(f_zpdesc);
+	*classp = zspage_class(pool, *zspagep);
+	spin_lock(&(*classp)->lock);
+	read_unlock(&pool->lock);
+	*objp = obj;
+}
+#endif
+
 void zs_free(struct zs_pool *pool, unsigned long handle)
 {
 	struct zspage *zspage;
-	struct zpdesc *f_zpdesc;
 	unsigned long obj;
 	struct size_class *class;
 	int fullness;
@@ -1462,17 +1517,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
 	if (IS_ERR_OR_NULL((void *)handle))
 		return;
 
-	/*
-	 * The pool->lock protects the race with zpage's migration
-	 * so it's safe to get the page from handle.
-	 */
-	read_lock(&pool->lock);
-	obj = handle_to_obj(handle);
-	obj_to_zpdesc(obj, &f_zpdesc);
-	zspage = get_zspage(f_zpdesc);
-	class = zspage_class(pool, zspage);
-	spin_lock(&class->lock);
-	read_unlock(&pool->lock);
+	obj_handle_class_lock(pool, handle, &obj, &zspage, &class);
 
 	class_stat_sub(class, ZS_OBJS_INUSE, 1);
 	obj_free(class->size, obj);
-- 
2.34.1