Re: [PATCH net] net: bcmgenet: allocate RX buffers as page fragments

[Simon Horman <horms@kernel.org>]

On Thu, Sep 24, 2026 at 12:19:21PM +0200, Nicolai Buchwitz wrote:
> Since the page_pool conversion every RX buffer is a whole page and the
> skb truesize is the page, although the hardware writes at most 2 KiB of
> it. On 64 KiB pages a packet therefore counts 65792 bytes against the
> socket buffer where it used to count 2752. As a result a UDP socket
> with the default buffer starts to drop after three packets, and the RX
> rings pin 16 MiB for 512 KiB of buffers.
> 
> Fix this and allocate the buffers as page fragments, so the truesize is
> what a packet occupies. 4 KiB pages stay one buffer per page.
> 
> Sync each buffer in the refill path, as page_pool can only sync a whole
> page on recycle. On 4 KiB pages that is twice what the hardware wrote.
> 
> Fixes: 7bc054c2d4ed ("net: bcmgenet: convert RX path to page_pool")
> Reported-by: Karl Mehltretter <kmehltretter@gmail.com>
> Closes: https://lore.kernel.org/all/20260924065839.56793-1-kmehltretter@gmail.com/
> Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>

...

> @@ -2254,11 +2257,12 @@ static int bcmgenet_rx_refill(struct bcmgenet_rx_ring *ring,
>  			      struct enet_cb *cb)
>  {
>  	struct bcmgenet_priv *priv = ring->priv;
> +	unsigned int size = GENET_RX_BUF_SIZE;
> +	unsigned int offset;
>  	dma_addr_t mapping;
>  	struct page *page;
>  
> -	page = page_pool_alloc_pages(ring->page_pool,
> -				     GFP_ATOMIC);
> +	page = page_pool_dev_alloc(ring->page_pool, &offset, &size);

I think it is not so important either way, but I'm wondering
if you considered using page_pool_alloc_frag() here.

>  	if (!page) {
>  		priv->mib.alloc_rx_buff_failed++;
>  		netif_err(priv, rx_err, priv->dev,

...