netdev - Re: [PATCH net] bnxt_en: do not map packet buffers twice

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Date: Thu, 14 Dec 2023 15:18:41 -0800
From: David Wei <dw@...idwei.uk>
To: Michael Chan <michael.chan@...adcom.com>, davem@...emloft.net
Cc: netdev@...r.kernel.org, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, bpf@...r.kernel.org, hawk@...nel.org, ast@...nel.org,
 daniel@...earbox.net, john.fastabend@...il.com,
 Andy Gospodarek <andrew.gospodarek@...adcom.com>,
 Somnath Kotur <somnath.kotur@...adcom.com>
Subject: Re: [PATCH net] bnxt_en: do not map packet buffers twice

On 2023-12-14 13:31, Michael Chan wrote:
> From: Andy Gospodarek <andrew.gospodarek@...adcom.com>
> 
> Remove double-mapping of DMA buffers as it can prevent page pool entries
> from being freed.  Mapping is managed by page pool infrastructure and
> was previously managed by the driver in __bnxt_alloc_rx_page before
> allowing the page pool infrastructure to manage it.
> 
> Fixes: 578fcfd26e2a ("bnxt_en: Let the page pool manage the DMA mapping")
> Reviewed-by: Somnath Kotur <somnath.kotur@...adcom.com>
> Signed-off-by: Andy Gospodarek <andrew.gospodarek@...adcom.com>
> Signed-off-by: Michael Chan <michael.chan@...adcom.com>
> ---
>  drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c b/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
> index 96f5ca778c67..8cb9a99154aa 100644
> --- a/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
> +++ b/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
> @@ -59,7 +59,6 @@ struct bnxt_sw_tx_bd *bnxt_xmit_bd(struct bnxt *bp,
>  	for (i = 0; i < num_frags ; i++) {
>  		skb_frag_t *frag = &sinfo->frags[i];
>  		struct bnxt_sw_tx_bd *frag_tx_buf;
> -		struct pci_dev *pdev = bp->pdev;
>  		dma_addr_t frag_mapping;
>  		int frag_len;
>  
> @@ -73,16 +72,10 @@ struct bnxt_sw_tx_bd *bnxt_xmit_bd(struct bnxt *bp,
>  		txbd = &txr->tx_desc_ring[TX_RING(prod)][TX_IDX(prod)];
>  
>  		frag_len = skb_frag_size(frag);
> -		frag_mapping = skb_frag_dma_map(&pdev->dev, frag, 0,
> -						frag_len, DMA_TO_DEVICE);
> -
> -		if (unlikely(dma_mapping_error(&pdev->dev, frag_mapping)))
> -			return NULL;
> -
> -		dma_unmap_addr_set(frag_tx_buf, mapping, frag_mapping);

If this is no longer set, what would happen to dma_unmap_single() in
bnxt_tx_int_xdp() that is then reading `mapping` via dma_unmap_addr()?

> -
>  		flags = frag_len << TX_BD_LEN_SHIFT;
>  		txbd->tx_bd_len_flags_type = cpu_to_le32(flags);
> +		frag_mapping = page_pool_get_dma_addr(skb_frag_page(frag)) +
> +			       skb_frag_off(frag);
>  		txbd->tx_bd_haddr = cpu_to_le64(frag_mapping);
>  
>  		len = frag_len;