[PATCH v2] slab: fix slab accounting imbalance due to defer_deactivate_slab()

[Vlastimil Babka <vbabka@suse.cz>]

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().")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
Changes in v2:
- Fix the problem differently. Harry pointed out that we can't move
  inc_slabs_node() outside of list_lock protected regions as that would
  reintroduce issues fixed by commit c7323a5ad078
- Link to v1: https://patch.msgid.link/20251022-fix-slab-accounting-v1-1-27870ec363ce@suse.cz
---
 mm/slub.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 23d8f54e9486..87a1d2f9de0d 100644
--- 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
 	}
 }

---
base-commit: 6ed8bfd24ce1cb31742b09a3eb557cd008533eec
change-id: 20251022-fix-slab-accounting-f0abbda8a6ff

Best regards,
-- 
Vlastimil Babka <vbabka@suse.cz>