mlx4: use order-0 pages for RX

Use of order-3 pages is problematic in some cases.

This patch might add three kinds of regression :

1) a CPU performance regression, but we will add later page
recycling and performance should be back.

2) TCP receiver could grow its receive window slightly slower,
   because skb->len/skb->truesize ratio will decrease.
   This is mostly ok, we prefer being conservative to not risk OOM,
   and eventually tune TCP better in the future.
   This is consistent with other drivers using 2048 per ethernet frame.

3) Because we allocate one page per RX slot, we consume more
   memory for the ring buffers. XDP already had this constraint anyway.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -53,38 +53,26 @@
 static int mlx4_alloc_pages(struct mlx4_en_priv *priv,
 			    struct mlx4_en_rx_alloc *page_alloc,
 			    const struct mlx4_en_frag_info *frag_info,
-			    gfp_t _gfp)
+			    gfp_t gfp)
 {
-	int order;
 	struct page *page;
 	dma_addr_t dma;
 
-	for (order = priv->rx_page_order; ;) {
-		gfp_t gfp = _gfp;
-
-		if (order)
-			gfp |= __GFP_COMP | __GFP_NOWARN | __GFP_NOMEMALLOC;
-		page = alloc_pages(gfp, order);
-		if (likely(page))
-			break;
-		if (--order < 0 ||
-		    ((PAGE_SIZE << order) < frag_info->frag_size))
-			return -ENOMEM;
-	}
-	dma = dma_map_page(priv->ddev, page, 0, PAGE_SIZE << order,
-			   priv->dma_dir);
+	page = alloc_page(gfp);
+	if (unlikely(!page))
+		return -ENOMEM;
+	dma = dma_map_page(priv->ddev, page, 0, PAGE_SIZE, priv->dma_dir);
 	if (unlikely(dma_mapping_error(priv->ddev, dma))) {
 		put_page(page);
 		return -ENOMEM;
 	}
-	page_alloc->page_size = PAGE_SIZE << order;
 	page_alloc->page = page;
 	page_alloc->dma = dma;
 	page_alloc->page_offset = 0;
 	/* Not doing get_page() for each frag is a big win
 	 * on asymetric workloads. Note we can not use atomic_set().
 	 */
-	page_ref_add(page, page_alloc->page_size / frag_info->frag_stride - 1);
+	page_ref_add(page, PAGE_SIZE / frag_info->frag_stride - 1);
 	return 0;
 }
 
@@ -105,7 +93,7 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
 		page_alloc[i].page_offset += frag_info->frag_stride;
 
 		if (page_alloc[i].page_offset + frag_info->frag_stride <=
-		    ring_alloc[i].page_size)
+		    PAGE_SIZE)
 			continue;
 
 		if (unlikely(mlx4_alloc_pages(priv, &page_alloc[i],
@@ -127,11 +115,10 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
 	while (i--) {
 		if (page_alloc[i].page != ring_alloc[i].page) {
 			dma_unmap_page(priv->ddev, page_alloc[i].dma,
-				page_alloc[i].page_size,
-				priv->dma_dir);
+				       PAGE_SIZE, priv->dma_dir);
 			page = page_alloc[i].page;
 			/* Revert changes done by mlx4_alloc_pages */
-			page_ref_sub(page, page_alloc[i].page_size /
+			page_ref_sub(page, PAGE_SIZE /
 					   priv->frag_info[i].frag_stride - 1);
 			put_page(page);
 		}
@@ -147,8 +134,8 @@ static void mlx4_en_free_frag(struct mlx4_en_priv *priv,
 	u32 next_frag_end = frags[i].page_offset + 2 * frag_info->frag_stride;
 
 
-	if (next_frag_end > frags[i].page_size)
-		dma_unmap_page(priv->ddev, frags[i].dma, frags[i].page_size,
+	if (next_frag_end > PAGE_SIZE)
+		dma_unmap_page(priv->ddev, frags[i].dma, PAGE_SIZE,
 			       priv->dma_dir);
 
 	if (frags[i].page)
@@ -168,9 +155,8 @@ static int mlx4_en_init_allocator(struct mlx4_en_priv *priv,
 				     frag_info, GFP_KERNEL | __GFP_COLD))
 			goto out;
 
-		en_dbg(DRV, priv, "  frag %d allocator: - size:%d frags:%d\n",
-		       i, ring->page_alloc[i].page_size,
-		       page_ref_count(ring->page_alloc[i].page));
+		en_dbg(DRV, priv, "  frag %d allocator: - frags:%d\n",
+		       i, page_ref_count(ring->page_alloc[i].page));
 	}
 	return 0;
 
@@ -180,11 +166,10 @@ static int mlx4_en_init_allocator(struct mlx4_en_priv *priv,
 
 		page_alloc = &ring->page_alloc[i];
 		dma_unmap_page(priv->ddev, page_alloc->dma,
-			       page_alloc->page_size,
-			       priv->dma_dir);
+			       PAGE_SIZE, priv->dma_dir);
 		page = page_alloc->page;
 		/* Revert changes done by mlx4_alloc_pages */
-		page_ref_sub(page, page_alloc->page_size /
+		page_ref_sub(page, PAGE_SIZE /
 				   priv->frag_info[i].frag_stride - 1);
 		put_page(page);
 		page_alloc->page = NULL;
@@ -206,9 +191,9 @@ static void mlx4_en_destroy_allocator(struct mlx4_en_priv *priv,
 		       i, page_count(page_alloc->page));
 
 		dma_unmap_page(priv->ddev, page_alloc->dma,
-				page_alloc->page_size, priv->dma_dir);
+			       PAGE_SIZE, priv->dma_dir);
 		while (page_alloc->page_offset + frag_info->frag_stride <
-		       page_alloc->page_size) {
+		       PAGE_SIZE) {
 			put_page(page_alloc->page);
 			page_alloc->page_offset += frag_info->frag_stride;
 		}
@@ -1191,7 +1176,6 @@ void mlx4_en_calc_rx_buf(struct net_device *dev)
 	 * This only works when num_frags == 1.
 	 */
 	if (priv->tx_ring_num[TX_XDP]) {
-		priv->rx_page_order = 0;
 		priv->frag_info[0].frag_size = eff_mtu;
 		/* This will gain efficient xdp frame recycling at the
 		 * expense of more costly truesize accounting
@@ -1201,22 +1185,32 @@ void mlx4_en_calc_rx_buf(struct net_device *dev)
 		priv->rx_headroom = XDP_PACKET_HEADROOM;
 		i = 1;
 	} else {
-		int buf_size = 0;
+		int frag_size_max = 2048, buf_size = 0;
+
+		/* should not happen, right ? */
+		if (eff_mtu > PAGE_SIZE + (MLX4_EN_MAX_RX_FRAGS - 1) * 2048)
+			frag_size_max = PAGE_SIZE;
 
 		while (buf_size < eff_mtu) {
-			int frag_size = eff_mtu - buf_size;
+			int frag_stride, frag_size = eff_mtu - buf_size;
+			int pad, nb;
 
 			if (i < MLX4_EN_MAX_RX_FRAGS - 1)
-				frag_size = min(frag_size, 2048);
+				frag_size = min(frag_size, frag_size_max);
 
 			priv->frag_info[i].frag_size = frag_size;
+			frag_stride = ALIGN(frag_size, SMP_CACHE_BYTES);
+			/* We can only pack 2 1536-bytes frames in on 4K page
+			 * Therefore, each frame would consume more bytes (truesize)
+			 */
+			nb = PAGE_SIZE / frag_stride;
+			pad = (PAGE_SIZE - nb * frag_stride) / nb;
+			pad &= ~(SMP_CACHE_BYTES - 1);
+			priv->frag_info[i].frag_stride = frag_stride + pad;
 
-			priv->frag_info[i].frag_stride = ALIGN(frag_size,
-							       SMP_CACHE_BYTES);
 			buf_size += frag_size;
 			i++;
 		}
-		priv->rx_page_order = MLX4_EN_ALLOC_PREFER_ORDER;
 		priv->dma_dir = PCI_DMA_FROMDEVICE;
 		priv->rx_headroom = 0;
 	}

drivers/net/ethernet/mellanox/mlx4/mlx4_en.h

@@ -102,9 +102,6 @@
 /* Use the maximum between 16384 and a single page */
 #define MLX4_EN_ALLOC_SIZE	PAGE_ALIGN(16384)
 
-#define MLX4_EN_ALLOC_PREFER_ORDER min_t(int, get_order(32768),		\
-					 PAGE_ALLOC_COSTLY_ORDER)
-
 #define MLX4_EN_MAX_RX_FRAGS	4
 
 /* Maximum ring sizes */
@@ -256,7 +253,6 @@ struct mlx4_en_rx_alloc {
 	struct page	*page;
 	dma_addr_t	dma;
 	u32		page_offset;
-	u32		page_size;
 };
 
 #define MLX4_EN_CACHE_SIZE (2 * NAPI_POLL_WEIGHT)
@@ -579,7 +575,6 @@ struct mlx4_en_priv {
 	u8 num_frags;
 	u8 log_rx_info;
 	u8 dma_dir;
-	u8 rx_page_order;
 	u16 rx_headroom;
 
 	struct mlx4_en_tx_ring **tx_ring[MLX4_EN_NUM_TX_TYPES];