[PATCH 4/4] mm/slub: free KFENCE objects in slab_free_hook()

Vlastimil Babka
2023-12-04T19:35:06Z

When freeing an object that was allocated from KFENCE, we do that in the
slowpath __slab_free(), relying on the fact that KFENCE "slab" cannot be
the cpu slab, so the fastpath has to fallback to the slowpath.

This optimization doesn't help much though, because is_kfence_address()
is checked earlier anyway during the free hook processing or detached
freelist building. Thus we can simplify the code by making the
slab_free_hook() free the KFENCE object immediately, similarly to KASAN
quarantine.

In slab_free_hook() we can place kfence_free() above init processing, as
callers have been making sure to set init to false for KFENCE objects.
This simplifies slab_free(). This places it also above kasan_slab_free()
which is ok as that skips KFENCE objects anyway.

While at it also determine the init value in slab_free_freelist_hook()
outside of the loop.

This change will also make introducing per cpu array caches easier.

Tested-by: Marco Elver <elver@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/slub.c | 22 ++++++++++------------
1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index ed2fa92e914c..e38c2b712f6c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2039,7 +2039,7 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
* 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.
+ * was delayed by KASAN quarantine, or it was returned to KFENCE.
*/
static __always_inline
bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
@@ -2057,6 +2057,9 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
__kcsan_check_access(x, s->object_size,
KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);

+ if (kfence_free(kasan_reset_tag(x)))
+ return false;
+
/*
* As memory initialization might be integrated into KASAN,
* kasan_slab_free and initialization memset's must be
@@ -2086,23 +2089,25 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
void *object;
void *next = *head;
void *old_tail = *tail;
+ bool init;

if (is_kfence_address(next)) {
slab_free_hook(s, next, false);
- return true;
+ return false;
}

/* Head and tail of the reconstructed freelist */
*head = NULL;
*tail = NULL;

+ init = slab_want_init_on_free(s);
+
do {
object = next;
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, init))) {
/* Move object to the new freelist */
set_freepointer(s, object, *head);
*head = object;
@@ -4103,9 +4108,6 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,

stat(s, FREE_SLOWPATH);

- if (kfence_free(head))
- return;
-
if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
free_to_partial_list(s, slab, head, tail, cnt, addr);
return;
@@ -4290,13 +4292,9 @@ 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)))
+ if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
do_slab_free(s, slab, object, object, 1, addr);
}

--
2.43.0