[PATCH 3/4] mm/slub: handle bulk and single object freeing separately

[Vlastimil Babka]

Currently we have a single function slab_free() handling both single
object freeing and bulk freeing with necessary hooks, the latter case
requiring slab_free_freelist_hook(). It should be however better to
distinguish the two use cases for the following reasons:

- code simpler to follow for the single object case

- better code generation - although inlining should eliminate the
  slab_free_freelist_hook() for single object freeing in case no
  debugging options are enabled, it seems it's not perfect. When e.g.
  KASAN is enabled, we're imposing additional unnecessary overhead for
  single object freeing.

- preparation to add percpu array caches in near future

Therefore, simplify slab_free() for the single object case by dropping
unnecessary parameters and calling only slab_free_hook() instead of
slab_free_freelist_hook(). Rename the bulk variant to slab_free_bulk()
and adjust callers accordingly.

While at it, flip (and document) slab_free_hook() return value so that
it returns true when the freeing can proceed, which matches the logic of
slab_free_freelist_hook() and is not confusingly the opposite.

Additionally we can simplify a bit by changing the tail parameter of
do_slab_free() when freeing a single object - instead of NULL we can set
it equal to head.

bloat-o-meter shows small code reduction with a .config that has KASAN
etc disabled:

add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-118 (-118)
Function                                     old     new   delta
kmem_cache_alloc_bulk                       1203    1196      -7
kmem_cache_free                              861     835     -26
__kmem_cache_free                            741     704     -37
kmem_cache_free_bulk                         911     863     -48

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/slub.c | 59 +++++++++++++++++++++++++++++++++++------------------------
 1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 0742564c4538..ed2fa92e914c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2037,9 +2037,12 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
 /*
  * Hooks for other subsystems that check memory allocations. In a typical
  * production configuration these hooks all should produce no code at all.
+ *
+ * Returns true if freeing of the object can proceed, false if its reuse
+ * was delayed by KASAN quarantine.
  */
-static __always_inline bool slab_free_hook(struct kmem_cache *s,
-						void *x, bool init)
+static __always_inline
+bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
 {
 	kmemleak_free_recursive(x, s->flags);
 	kmsan_slab_free(s, x);
@@ -2072,7 +2075,7 @@ static __always_inline bool slab_free_hook(struct kmem_cache *s,
 		       s->size - s->inuse - rsize);
 	}
 	/* KASAN might put x into memory quarantine, delaying its reuse. */
-	return kasan_slab_free(s, x, init);
+	return !kasan_slab_free(s, x, init);
 }
 
 static inline bool slab_free_freelist_hook(struct kmem_cache *s,
@@ -2082,7 +2085,7 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 
 	void *object;
 	void *next = *head;
-	void *old_tail = *tail ? *tail : *head;
+	void *old_tail = *tail;
 
 	if (is_kfence_address(next)) {
 		slab_free_hook(s, next, false);
@@ -2098,8 +2101,8 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 		next = get_freepointer(s, object);
 
 		/* If object's reuse doesn't have to be delayed */
-		if (likely(!slab_free_hook(s, object,
-					   slab_want_init_on_free(s)))) {
+		if (likely(slab_free_hook(s, object,
+					  slab_want_init_on_free(s)))) {
 			/* Move object to the new freelist */
 			set_freepointer(s, object, *head);
 			*head = object;
@@ -2114,9 +2117,6 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 		}
 	} while (object != old_tail);
 
-	if (*head == *tail)
-		*tail = NULL;
-
 	return *head != NULL;
 }
 
@@ -4227,7 +4227,6 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
 				struct slab *slab, void *head, void *tail,
 				int cnt, unsigned long addr)
 {
-	void *tail_obj = tail ? : head;
 	struct kmem_cache_cpu *c;
 	unsigned long tid;
 	void **freelist;
@@ -4246,14 +4245,14 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
 	barrier();
 
 	if (unlikely(slab != c->slab)) {
-		__slab_free(s, slab, head, tail_obj, cnt, addr);
+		__slab_free(s, slab, head, tail, cnt, addr);
 		return;
 	}
 
 	if (USE_LOCKLESS_FAST_PATH()) {
 		freelist = READ_ONCE(c->freelist);
 
-		set_freepointer(s, tail_obj, freelist);
+		set_freepointer(s, tail, freelist);
 
 		if (unlikely(!__update_cpu_freelist_fast(s, freelist, head, tid))) {
 			note_cmpxchg_failure("slab_free", s, tid);
@@ -4270,7 +4269,7 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
 		tid = c->tid;
 		freelist = c->freelist;
 
-		set_freepointer(s, tail_obj, freelist);
+		set_freepointer(s, tail, freelist);
 		c->freelist = head;
 		c->tid = next_tid(tid);
 
@@ -4283,15 +4282,27 @@ static void do_slab_free(struct kmem_cache *s,
 				struct slab *slab, void *head, void *tail,
 				int cnt, unsigned long addr)
 {
-	void *tail_obj = tail ? : head;
-
-	__slab_free(s, slab, head, tail_obj, cnt, addr);
+	__slab_free(s, slab, head, tail, cnt, addr);
 }
 #endif /* CONFIG_SLUB_TINY */
 
-static __fastpath_inline void slab_free(struct kmem_cache *s, struct slab *slab,
-				      void *head, void *tail, void **p, int cnt,
-				      unsigned long addr)
+static __fastpath_inline
+void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
+	       unsigned long addr)
+{
+	bool init;
+
+	memcg_slab_free_hook(s, slab, &object, 1);
+
+	init = !is_kfence_address(object) && slab_want_init_on_free(s);
+
+	if (likely(slab_free_hook(s, object, init)))
+		do_slab_free(s, slab, object, object, 1, addr);
+}
+
+static __fastpath_inline
+void slab_free_bulk(struct kmem_cache *s, struct slab *slab, void *head,
+		    void *tail, void **p, int cnt, unsigned long addr)
 {
 	memcg_slab_free_hook(s, slab, p, cnt);
 	/*
@@ -4305,7 +4316,7 @@ static __fastpath_inline void slab_free(struct kmem_cache *s, struct slab *slab,
 #ifdef CONFIG_KASAN_GENERIC
 void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
 {
-	do_slab_free(cache, virt_to_slab(x), x, NULL, 1, addr);
+	do_slab_free(cache, virt_to_slab(x), x, x, 1, addr);
 }
 #endif
 
@@ -4349,7 +4360,7 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
 	if (!s)
 		return;
 	trace_kmem_cache_free(_RET_IP_, x, s);
-	slab_free(s, virt_to_slab(x), x, NULL, &x, 1, _RET_IP_);
+	slab_free(s, virt_to_slab(x), x, _RET_IP_);
 }
 EXPORT_SYMBOL(kmem_cache_free);
 
@@ -4395,7 +4406,7 @@ void kfree(const void *object)
 
 	slab = folio_slab(folio);
 	s = slab->slab_cache;
-	slab_free(s, slab, x, NULL, &x, 1, _RET_IP_);
+	slab_free(s, slab, x, _RET_IP_);
 }
 EXPORT_SYMBOL(kfree);
 
@@ -4512,8 +4523,8 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
 		if (!df.slab)
 			continue;
 
-		slab_free(df.s, df.slab, df.freelist, df.tail, &p[size], df.cnt,
-			  _RET_IP_);
+		slab_free_bulk(df.s, df.slab, df.freelist, df.tail, &p[size],
+			       df.cnt, _RET_IP_);
 	} while (likely(size));
 }
 EXPORT_SYMBOL(kmem_cache_free_bulk);

-- 
2.43.0