[PATCH RFC v3 4/9] mm/slub: free KFENCE objects in slab_free_hook()

[Vlastimil Babka]

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.

Signed-off-by: Vlastimil Babka <vbabka at suse.cz>
---
 mm/slub.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 7d23f10d42e6..59912a376c6d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1772,7 +1772,7 @@ static bool freelist_corrupted(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)
@@ -1790,6 +1790,9 @@ 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
@@ -1819,22 +1822,25 @@ 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 (slab_free_hook(s, object, slab_want_init_on_free(s))) {
+		if (slab_free_hook(s, object, init)) {
 			/* Move object to the new freelist */
 			set_freepointer(s, object, *head);
 			*head = object;
@@ -3619,9 +3625,6 @@ 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;
@@ -3806,13 +3809,9 @@ 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