Re: [PATCH v2 net-next 1/2] virtio_net: add page_pool support for buffer allocation

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 28, 2026 at 01:20:30PM -0800, Vishwanath Seshagiri wrote:
> Use page_pool for RX buffer allocation in mergeable and small buffer
> modes to enable page recycling and avoid repeated page allocator calls.
> skb_mark_for_recycle() enables page reuse in the network stack.
> 
> Big packets mode is unchanged because it uses page->private for linked
> list chaining of multiple pages per buffer, which conflicts with
> page_pool's internal use of page->private.
> 
> Implement conditional DMA premapping using virtqueue_dma_dev():
> - When non-NULL (vhost, virtio-pci): use PP_FLAG_DMA_MAP with page_pool
>   handling DMA mapping, submit via virtqueue_add_inbuf_premapped()
> - When NULL (VDUSE, direct physical): page_pool handles allocation only,
>   submit via virtqueue_add_inbuf_ctx()
> 
> This preserves the DMA premapping optimization from commit 31f3cd4e5756b
> ("virtio-net: rq submits premapped per-buffer") while adding page_pool
> support as a prerequisite for future zero-copy features (devmem TCP,
> io_uring ZCRX).
> 
> Page pools are created in probe and destroyed in remove (not open/close),
> following existing driver behavior where RX buffers remain in virtqueues
> across interface state changes.
> 
> The rx_mode_work_enabled flag prevents virtnet_rx_mode_work() from
> sending control virtqueue commands while ndo_close is tearing down
> device state, avoiding virtqueue corruption during concurrent operations.
> 
> Signed-off-by: Vishwanath Seshagiri <vishs@meta.com>
> ---
>  drivers/net/Kconfig      |   1 +
>  drivers/net/virtio_net.c | 353 ++++++++++++++++++++++-----------------
>  2 files changed, 203 insertions(+), 151 deletions(-)
> 
> diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
> index ac12eaf11755..f1e6b6b0a86f 100644
> --- a/drivers/net/Kconfig
> +++ b/drivers/net/Kconfig
> @@ -450,6 +450,7 @@ config VIRTIO_NET
>  	depends on VIRTIO
>  	select NET_FAILOVER
>  	select DIMLIB
> +	select PAGE_POOL
>  	help
>  	  This is the virtual network driver for virtio.  It can be used with
>  	  QEMU based VMMs (like KVM or Xen).  Say Y or M.
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index db88dcaefb20..df2a5fc5187e 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -26,6 +26,7 @@
>  #include <net/netdev_rx_queue.h>
>  #include <net/netdev_queues.h>
>  #include <net/xdp_sock_drv.h>
> +#include <net/page_pool/helpers.h>
>  
>  static int napi_weight = NAPI_POLL_WEIGHT;
>  module_param(napi_weight, int, 0444);
> @@ -359,6 +360,11 @@ struct receive_queue {
>  	/* Page frag for packet buffer allocation. */
>  	struct page_frag alloc_frag;
>  
> +	struct page_pool *page_pool;
> +
> +	/* True if page_pool handles DMA mapping via PP_FLAG_DMA_MAP */
> +	bool use_page_pool_dma;
> +
>  	/* RX: fragments + linear part + virtio header */
>  	struct scatterlist sg[MAX_SKB_FRAGS + 2];
>  
> @@ -521,11 +527,13 @@ static int virtnet_xdp_handler(struct bpf_prog *xdp_prog, struct xdp_buff *xdp,
>  			       struct virtnet_rq_stats *stats);
>  static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *rq,
>  				 struct sk_buff *skb, u8 flags);
> -static struct sk_buff *virtnet_skb_append_frag(struct sk_buff *head_skb,
> +static struct sk_buff *virtnet_skb_append_frag(struct receive_queue *rq,
> +					       struct sk_buff *head_skb,
>  					       struct sk_buff *curr_skb,
>  					       struct page *page, void *buf,
>  					       int len, int truesize);
>  static void virtnet_xsk_completed(struct send_queue *sq, int num);
> +static void free_unused_bufs(struct virtnet_info *vi);
>  
>  enum virtnet_xmit_type {
>  	VIRTNET_XMIT_TYPE_SKB,
> @@ -706,15 +714,21 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
>  	return p;
>  }
>  
> +static void virtnet_put_page(struct receive_queue *rq, struct page *page,
> +			     bool allow_direct)
> +{
> +	page_pool_put_page(rq->page_pool, page, -1, allow_direct);
> +}
> +
>  static void virtnet_rq_free_buf(struct virtnet_info *vi,
>  				struct receive_queue *rq, void *buf)
>  {
>  	if (vi->mergeable_rx_bufs)
> -		put_page(virt_to_head_page(buf));
> +		virtnet_put_page(rq, virt_to_head_page(buf), false);
>  	else if (vi->big_packets)
>  		give_pages(rq, buf);
>  	else
> -		put_page(virt_to_head_page(buf));
> +		virtnet_put_page(rq, virt_to_head_page(buf), false);
>  }


what I dislike here is how big_packets mode still pokes
at give_pages but other modes use the page pool.

Given all modes operate with struct page it's hard to
shake the feeling we could be trying to put a page
we did not get from the pool back into the pool,
or vice versa.



>  
>  static void enable_rx_mode_work(struct virtnet_info *vi)
> @@ -877,9 +891,6 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  		if (unlikely(!skb))
>  			return NULL;
>  
> -		page = (struct page *)page->private;
> -		if (page)
> -			give_pages(rq, page);
>  		goto ok;
>  	}
>

For example above you did not touch give_pages, here
you are ripping out give_pages. Superficially, weird.


I ask myself whether page pool is not better than the
homegrown linked list that give_pages uses, anyway.

Will need some perf testing though.

-- 
MST