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Message-ID: <20260129012534-mutt-send-email-mst@kernel.org>
Date: Thu, 29 Jan 2026 01:30:07 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Vishwanath Seshagiri <vishs@...a.com>
Cc: Jason Wang <jasowang@...hat.com>,
Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
Eugenio Pérez <eperezma@...hat.com>,
Andrew Lunn <andrew+netdev@...n.ch>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
David Wei <dw@...idwei.uk>, netdev@...r.kernel.org,
virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next 1/2] virtio_net: add page_pool support for
buffer allocation
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@...a.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
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