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

Vlastimil Babka
2023-12-04T19:35:10Z

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@suse.cz>
---
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