mm/slub: free KFENCE objects in slab_free_hook()

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>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slub.c b/mm/slub.c
index af8c8fc..ccd5763 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2053,7 +2053,7 @@ void memcg_slab_alloc_error_hook(struct kmem_cache *s, int objects,
  * 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)
@@ -2071,6 +2071,9 @@ 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(x))
+               return false;
+
        /*
         * As memory initialization might be integrated into KASAN,
         * kasan_slab_free and initialization memset's must be
@@ -2100,23 +2103,25 @@ 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;
@@ -4117,9 +4122,6 @@ 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;
@@ -4304,13 +4306,9 @@ 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);
 }