[PATCH net-next V2] net: core: page_pool: add user refcnt and reintroduce page_pool_destroy

[Jesper Dangaard Brouer]

Jesper recently removed page_pool_destroy() (from driver invocation) and
moved shutdown and free of page_pool into xdp_rxq_info_unreg(), in-order to
handle in-flight packets/pages. This created an asymmetry in drivers
create/destroy pairs.

This patch reintroduce page_pool_destroy and add page_pool user refcnt.
This serves the purpose to simplify drivers error handling as driver now
drivers always calls page_pool_destroy() and don't need to track if
xdp_rxq_info_reg_mem_model() was unsuccessful.

This could be used for a special cases where a single RX-queue (with a single
page_pool) provides packets for two net_device'es, and thus needs to register
the same page_pool twice with two xdp_rxq_info structures. This use-case is
explicitly denied in this V2 patch, as it needs more discussion upstream.

This patch is primarily to ease API usage for drivers. The recently merged
netsec driver, actually have a bug in this area, which is solved by this API
change.

This patch is a modified version of Ivan Khoronzhu's original patch.

Link: https://lore.kernel.org/netdev/20190625175948.24771-2-ivan.khoronzhuk@xxxxxxxxxx/
Fixes: 5c67bf0ec4d0 ("net: netsec: Use page_pool API")
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---

I hope we can agree on this first step. Afterwards we can discuss, which is the
best approach for Ivan's two netdev's with one RX-queue API change.

 drivers/net/ethernet/mellanox/mlx5/core/en_main.c |    6 ++---
 drivers/net/ethernet/socionext/netsec.c           |    8 ++----
 include/net/page_pool.h                           |   26 +++++++++++++++++++++
 net/core/page_pool.c                              |    8 ++++++
 net/core/xdp.c                                    |    9 +++++++
 5 files changed, 48 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
index 1085040675ae..ce1c7a449eae 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
@@ -545,10 +545,8 @@ static int mlx5e_alloc_rq(struct mlx5e_channel *c,
 	}
 	err = xdp_rxq_info_reg_mem_model(&rq->xdp_rxq,
 					 MEM_TYPE_PAGE_POOL, rq->page_pool);
-	if (err) {
-		page_pool_free(rq->page_pool);
+	if (err)
 		goto err_free;
-	}
 
 	for (i = 0; i < wq_sz; i++) {
 		if (rq->wq_type == MLX5_WQ_TYPE_LINKED_LIST_STRIDING_RQ) {
@@ -613,6 +611,7 @@ static int mlx5e_alloc_rq(struct mlx5e_channel *c,
 	if (rq->xdp_prog)
 		bpf_prog_put(rq->xdp_prog);
 	xdp_rxq_info_unreg(&rq->xdp_rxq);
+	page_pool_destroy(rq->page_pool);
 	mlx5_wq_destroy(&rq->wq_ctrl);
 
 	return err;
@@ -643,6 +642,7 @@ static void mlx5e_free_rq(struct mlx5e_rq *rq)
 	}
 
 	xdp_rxq_info_unreg(&rq->xdp_rxq);
+	page_pool_destroy(rq->page_pool);
 	mlx5_wq_destroy(&rq->wq_ctrl);
 }
 
diff --git a/drivers/net/ethernet/socionext/netsec.c b/drivers/net/ethernet/socionext/netsec.c
index 5544a722543f..43ab0ce90704 100644
--- a/drivers/net/ethernet/socionext/netsec.c
+++ b/drivers/net/ethernet/socionext/netsec.c
@@ -1210,15 +1210,11 @@ static void netsec_uninit_pkt_dring(struct netsec_priv *priv, int id)
 		}
 	}
 
-	/* Rx is currently using page_pool
-	 * since the pool is created during netsec_setup_rx_dring(), we need to
-	 * free the pool manually if the registration failed
-	 */
+	/* Rx is currently using page_pool */
 	if (id == NETSEC_RING_RX) {
 		if (xdp_rxq_info_is_reg(&dring->xdp_rxq))
 			xdp_rxq_info_unreg(&dring->xdp_rxq);
-		else
-			page_pool_free(dring->page_pool);
+		page_pool_destroy(dring->page_pool);
 	}
 
 	memset(dring->desc, 0, sizeof(struct netsec_desc) * DESC_NUM);
diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index ee9c871d2043..0a0984d8f5d5 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -101,6 +101,12 @@ struct page_pool {
 	struct ptr_ring ring;
 
 	atomic_t pages_state_release_cnt;
+
+	/* A page_pool is strictly tied to a single RX-queue being
+	 * protected by NAPI, due to above pp_alloc_cache.  This
+	 * refcnt serves purpose is to simplify drivers error handling.
+	 */
+	refcount_t user_cnt;
 };
 
 struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp);
@@ -134,6 +140,15 @@ static inline void page_pool_free(struct page_pool *pool)
 #endif
 }
 
+/* Drivers use this instead of page_pool_free */
+static inline void page_pool_destroy(struct page_pool *pool)
+{
+	if (!pool)
+		return;
+
+	page_pool_free(pool);
+}
+
 /* Never call this directly, use helpers below */
 void __page_pool_put_page(struct page_pool *pool,
 			  struct page *page, bool allow_direct);
@@ -201,4 +216,15 @@ static inline bool is_page_pool_compiled_in(void)
 #endif
 }
 
+static inline unsigned int page_pool_get(struct page_pool *pool)
+{
+	refcount_inc(&pool->user_cnt);
+	return refcount_read(&pool->user_cnt);
+}
+
+static inline bool page_pool_put(struct page_pool *pool)
+{
+	return refcount_dec_and_test(&pool->user_cnt);
+}
+
 #endif /* _NET_PAGE_POOL_H */
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index b366f59885c1..3272dc7a8c81 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -49,6 +49,9 @@ static int page_pool_init(struct page_pool *pool,
 
 	atomic_set(&pool->pages_state_release_cnt, 0);
 
+	/* Driver calling page_pool_create() also call page_pool_destroy() */
+	refcount_set(&pool->user_cnt, 1);
+
 	if (pool->p.flags & PP_FLAG_DMA_MAP)
 		get_device(pool->p.dev);
 
@@ -70,6 +73,7 @@ struct page_pool *page_pool_create(const struct page_pool_params *params)
 		kfree(pool);
 		return ERR_PTR(err);
 	}
+
 	return pool;
 }
 EXPORT_SYMBOL(page_pool_create);
@@ -356,6 +360,10 @@ static void __warn_in_flight(struct page_pool *pool)
 
 void __page_pool_free(struct page_pool *pool)
 {
+	/* Only last user actually free/release resources */
+	if (!page_pool_put(pool))
+		return;
+
 	WARN(pool->alloc.count, "API usage violation");
 	WARN(!ptr_ring_empty(&pool->ring), "ptr_ring is not empty");
 
diff --git a/net/core/xdp.c b/net/core/xdp.c
index b29d7b513a18..57c2317d9ce5 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -370,6 +370,15 @@ int xdp_rxq_info_reg_mem_model(struct xdp_rxq_info *xdp_rxq,
 		goto err;
 	}
 
+	if (type == MEM_TYPE_PAGE_POOL) {
+		if (page_pool_get(xdp_alloc->page_pool) > 2) {
+			WARN(1, "API misuse, argue to enable use-case");
+			page_pool_put(xdp_alloc->page_pool);
+			errno = -EINVAL;
+			goto err;
+		}
+	}
+
 	mutex_unlock(&mem_id_lock);
 
 	trace_mem_connect(xdp_alloc, xdp_rxq);