slab: fix slab accounting imbalance due to defer_deactivate_slab()

Since commit af92793e52c3 ("slab: Introduce kmalloc_nolock() and
kfree_nolock().") there's a possibility in alloc_single_from_new_slab()
that we discard the newly allocated slab if we can't spin and we fail to
trylock. As a result we don't perform inc_slabs_node() later in the
function. Instead we perform a deferred deactivate_slab() which can
either put the unacounted slab on partial list, or discard it
immediately while performing dec_slabs_node(). Either way will cause an
accounting imbalance.

Fix this by not marking the slab as frozen, and using free_slab()
instead of deactivate_slab() for non-frozen slabs in
free_deferred_objects(). For CONFIG_SLUB_TINY, that's the only possible
case. By not using discard_slab() we avoid dec_slabs_node().

Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Link: https://patch.msgid.link/20251023-fix-slab-accounting-v2-1-0e62d50986ea@suse.cz
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slub.c b/mm/slub.c
index 23d8f54e94866dd8dbda866419af709d4111eaf9..87a1d2f9de0d5def25a40ae57b122bb0e95e736b 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3422,7 +3422,6 @@ static void *alloc_single_from_new_slab(struct kmem_cache *s, struct slab *slab,
 
        if (!allow_spin && !spin_trylock_irqsave(&n->list_lock, flags)) {
                /* Unlucky, discard newly allocated slab */
-               slab->frozen = 1;
                defer_deactivate_slab(slab, NULL);
                return NULL;
        }
@@ -6471,9 +6470,12 @@ static void free_deferred_objects(struct irq_work *work)
                struct slab *slab = container_of(pos, struct slab, llnode);
 
 #ifdef CONFIG_SLUB_TINY
-               discard_slab(slab->slab_cache, slab);
+               free_slab(slab->slab_cache, slab);
 #else
-               deactivate_slab(slab->slab_cache, slab, slab->flush_freelist);
+               if (slab->frozen)
+                       deactivate_slab(slab->slab_cache, slab, slab->flush_freelist);
+               else
+                       free_slab(slab->slab_cache, slab);
 #endif
        }
 }