bpf: Switch to bpf_selem_unlink_nofail in bpf_local_storage_{map_free, destroy}

Take care of rqspinlock error in bpf_local_storage_{map_free, destroy}()
properly by switching to bpf_selem_unlink_nofail().

Both functions iterate their own RCU-protected list of selems and call
bpf_selem_unlink_nofail(). In map_free(), to prevent infinite loop when
both map_free() and destroy() fail to remove a selem from b->list
(extremely unlikely), switch to hlist_for_each_entry_rcu(). In destroy(),
also switch to hlist_for_each_entry_rcu() since we no longer iterate
local_storage->list under local_storage->lock.

bpf_selem_unlink() now becomes dedicated to helpers and syscalls paths
so reuse_now should always be false. Remove it from the argument and
hardcode it.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Co-developed-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260205222916.1788211-12-ameryhung@gmail.com

Author: ameryhung

include/linux/bpf_local_storage.h

@@ -171,7 +171,7 @@ bpf_local_storage_lookup(struct bpf_local_storage *local_storage,
 	return SDATA(selem);
 }
 
-void bpf_local_storage_destroy(struct bpf_local_storage *local_storage);
+u32 bpf_local_storage_destroy(struct bpf_local_storage *local_storage);
 
 void bpf_local_storage_map_free(struct bpf_map *map,
 				struct bpf_local_storage_cache *cache);
@@ -184,7 +184,7 @@ int bpf_local_storage_map_check_btf(const struct bpf_map *map,
 void bpf_selem_link_storage_nolock(struct bpf_local_storage *local_storage,
 				   struct bpf_local_storage_elem *selem);
 
-int bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now);
+int bpf_selem_unlink(struct bpf_local_storage_elem *selem);
 
 int bpf_selem_link_map(struct bpf_local_storage_map *smap,
 		       struct bpf_local_storage *local_storage,

kernel/bpf/bpf_cgrp_storage.c

@@ -89,7 +89,7 @@ static int cgroup_storage_delete(struct cgroup *cgroup, struct bpf_map *map)
 	if (!sdata)
 		return -ENOENT;
 
-	return bpf_selem_unlink(SELEM(sdata), false);
+	return bpf_selem_unlink(SELEM(sdata));
 }
 
 static long bpf_cgrp_storage_delete_elem(struct bpf_map *map, void *key)

kernel/bpf/bpf_inode_storage.c

@@ -110,7 +110,7 @@ static int inode_storage_delete(struct inode *inode, struct bpf_map *map)
 	if (!sdata)
 		return -ENOENT;
 
-	return bpf_selem_unlink(SELEM(sdata), false);
+	return bpf_selem_unlink(SELEM(sdata));
 }
 
 static long bpf_fd_inode_storage_delete_elem(struct bpf_map *map, void *key)

kernel/bpf/bpf_local_storage.c

@@ -377,7 +377,11 @@ static void bpf_selem_link_map_nolock(struct bpf_local_storage_map_bucket *b,
 	hlist_add_head_rcu(&selem->map_node, &b->list);
 }
 
-int bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now)
+/*
+ * Unlink an selem from map and local storage with lock held.
+ * This is the common path used by local storages to delete an selem.
+ */
+int bpf_selem_unlink(struct bpf_local_storage_elem *selem)
 {
 	struct bpf_local_storage *local_storage;
 	bool free_local_storage = false;
@@ -411,10 +415,10 @@ int bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now)
 out:
 	raw_res_spin_unlock_irqrestore(&local_storage->lock, flags);
 
-	bpf_selem_free_list(&selem_free_list, reuse_now);
+	bpf_selem_free_list(&selem_free_list, false);
 
 	if (free_local_storage)
-		bpf_local_storage_free(local_storage, reuse_now);
+		bpf_local_storage_free(local_storage, false);
 
 	return err;
 }
@@ -804,13 +808,13 @@ int bpf_local_storage_map_check_btf(const struct bpf_map *map,
 	return 0;
 }
 
-void bpf_local_storage_destroy(struct bpf_local_storage *local_storage)
+/*
+ * Destroy local storage when the owner is going away. Caller must uncharge memory
+ * if memory charging is used.
+ */
+u32 bpf_local_storage_destroy(struct bpf_local_storage *local_storage)
 {
 	struct bpf_local_storage_elem *selem;
-	bool free_storage = false;
-	HLIST_HEAD(free_selem_list);
-	struct hlist_node *n;
-	unsigned long flags;
 
 	/* Neither the bpf_prog nor the bpf_map's syscall
 	 * could be modifying the local_storage->list now.
@@ -821,32 +825,20 @@ void bpf_local_storage_destroy(struct bpf_local_storage *local_storage)
 	 * when unlinking elem from the local_storage->list and
 	 * the map's bucket->list.
 	 */
-	raw_res_spin_lock_irqsave(&local_storage->lock, flags);
-	hlist_for_each_entry_safe(selem, n, &local_storage->list, snode) {
-		/* Always unlink from map before unlinking from
-		 * local_storage.
-		 */
-		bpf_selem_unlink_map(selem);
-		/* If local_storage list has only one element, the
-		 * bpf_selem_unlink_storage_nolock() will return true.
-		 * Otherwise, it will return false. The current loop iteration
-		 * intends to remove all local storage. So the last iteration
-		 * of the loop will set the free_cgroup_storage to true.
-		 */
-		free_storage = bpf_selem_unlink_storage_nolock(
-			local_storage, selem, &free_selem_list);
-	}
-	raw_res_spin_unlock_irqrestore(&local_storage->lock, flags);
-
-	bpf_selem_free_list(&free_selem_list, true);
-
-	if (free_storage)
-		bpf_local_storage_free(local_storage, true);
+	hlist_for_each_entry_rcu(selem, &local_storage->list, snode)
+		bpf_selem_unlink_nofail(selem, NULL);
 
 	if (!refcount_dec_and_test(&local_storage->owner_refcnt)) {
 		while (refcount_read(&local_storage->owner_refcnt))
 			cpu_relax();
+		/*
+		 * Paired with refcount_dec() in bpf_selem_unlink_nofail()
+		 * to make sure destroy() sees the correct local_storage->mem_charge.
+		 */
+		smp_mb();
 	}
+
+	return local_storage->mem_charge;
 }
 
 u64 bpf_local_storage_map_mem_usage(const struct bpf_map *map)
@@ -940,11 +932,14 @@ void bpf_local_storage_map_free(struct bpf_map *map,
 
 		rcu_read_lock();
 		/* No one is adding to b->list now */
-		while ((selem = hlist_entry_safe(
-				rcu_dereference_raw(hlist_first_rcu(&b->list)),
-				struct bpf_local_storage_elem, map_node))) {
-			bpf_selem_unlink(selem, true);
-			cond_resched_rcu();
+restart:
+		hlist_for_each_entry_rcu(selem, &b->list, map_node) {
+			bpf_selem_unlink_nofail(selem, b);
+
+			if (need_resched()) {
+				cond_resched_rcu();
+				goto restart;
+			}
 		}
 		rcu_read_unlock();
 	}

kernel/bpf/bpf_task_storage.c

@@ -134,7 +134,7 @@ static int task_storage_delete(struct task_struct *task, struct bpf_map *map)
 	if (!sdata)
 		return -ENOENT;
 
-	return bpf_selem_unlink(SELEM(sdata), false);
+	return bpf_selem_unlink(SELEM(sdata));
 }
 
 static long bpf_pid_task_storage_delete_elem(struct bpf_map *map, void *key)

net/core/bpf_sk_storage.c

@@ -40,20 +40,23 @@ static int bpf_sk_storage_del(struct sock *sk, struct bpf_map *map)
 	if (!sdata)
 		return -ENOENT;
 
-	return bpf_selem_unlink(SELEM(sdata), false);
+	return bpf_selem_unlink(SELEM(sdata));
 }
 
 /* Called by __sk_destruct() & bpf_sk_storage_clone() */
 void bpf_sk_storage_free(struct sock *sk)
 {
 	struct bpf_local_storage *sk_storage;
+	u32 uncharge;
 
 	rcu_read_lock_dont_migrate();
 	sk_storage = rcu_dereference(sk->sk_bpf_storage);
 	if (!sk_storage)
 		goto out;
 
-	bpf_local_storage_destroy(sk_storage);
+	uncharge = bpf_local_storage_destroy(sk_storage);
+	if (uncharge)
+		atomic_sub(uncharge, &sk->sk_omem_alloc);
 out:
 	rcu_read_unlock_migrate();
 }