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Tsirkin" To: Omar Elghoul Cc: vishs@meta.com, andrew+netdev@lunn.ch, davem@davemloft.net, dw@davidwei.uk, edumazet@google.com, eperezma@redhat.com, ilias.apalodimas@linaro.org, jasowang@redhat.com, kernel-team@meta.com, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, technoboy85@gmail.com, virtualization@lists.linux.dev, xuanzhuo@linux.alibaba.com Subject: Re: [PATCH net-next v11] virtio_net: add page_pool support for buffer allocation Message-ID: <20260323131033-mutt-send-email-mst@kernel.org> References: <20260310183107.2822016-1-vishs@meta.com> <20260323150136.14452-1-oelghoul@linux.ibm.com> <20260323114313-mutt-send-email-mst@kernel.org> <8e0a5562-4511-41df-9993-a77e51025e95@linux.ibm.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <8e0a5562-4511-41df-9993-a77e51025e95@linux.ibm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: _u6TUEWtSM4LXOqU7MOVX0H5vbftOEP8LgNua8Y2KjM_1774285842 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Mon, Mar 23, 2026 at 12:54:03PM -0400, Omar Elghoul wrote: > On 3/23/26 11:52 AM, Michael S. Tsirkin wrote: > > On Mon, Mar 23, 2026 at 11:01:31AM -0400, Omar Elghoul wrote: > > > [...] > > Well... I am not sure how I missed it. Obvious in hindsight: > > > > static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq, > > void *buf, unsigned int len, void **ctx, > > unsigned int *xdp_xmit, > > struct virtnet_rq_stats *stats) > > { > > struct net_device *dev = vi->dev; > > struct sk_buff *skb; > > u8 flags; > > if (unlikely(len < vi->hdr_len + ETH_HLEN)) { > > pr_debug("%s: short packet %i\n", dev->name, len); > > DEV_STATS_INC(dev, rx_length_errors); > > virtnet_rq_free_buf(vi, rq, buf); > > return; > > } > > /* About the flags below: > > * 1. Save the flags early, as the XDP program might overwrite them. > > * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID > > * stay valid after XDP processing. > > * 2. XDP doesn't work with partially checksummed packets (refer to > > * virtnet_xdp_set()), so packets marked as > > * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing. > > */ > > if (vi->mergeable_rx_bufs) { > > flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags; > > skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit, > > stats); > > } else if (vi->big_packets) { > > void *p = page_address((struct page *)buf); > > flags = ((struct virtio_net_common_hdr *)p)->hdr.flags; > > skb = receive_big(dev, vi, rq, buf, len, stats); > > } else { > > flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags; > > skb = receive_small(dev, vi, rq, buf, ctx, len, xdp_xmit, stats); > > } > > > > > > So we are reading the header, before dma sync, which is within > > receive_mergeable and friends: > Thank you for your analysis and explanation. > > > > static struct sk_buff *receive_mergeable(struct net_device *dev, > > struct virtnet_info *vi, > > struct receive_queue *rq, > > void *buf, > > void *ctx, > > unsigned int len, > > unsigned int *xdp_xmit, > > struct virtnet_rq_stats *stats) > > { > > struct virtio_net_hdr_mrg_rxbuf *hdr = buf; > > int num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers); > > struct page *page = virt_to_head_page(buf); > > int offset = buf - page_address(page); > > struct sk_buff *head_skb, *curr_skb; > > unsigned int truesize = mergeable_ctx_to_truesize(ctx); > > unsigned int headroom = mergeable_ctx_to_headroom(ctx); > > head_skb = NULL; > > if (rq->use_page_pool_dma) > > page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, len); > > > > > > Just as a test, the below should fix it (compiled only), but the real > > fix is more complex since we need to be careful to avoid expensive syncing > > twice. > > I applied your patch and tested it on my system. With this change, I could > not reproduce the same error anymore. I would be happy to test a proper fix > once you have one. > > > > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > > index 97035b49bae7..57b4f5954bed 100644 > > --- a/drivers/net/virtio_net.c > > +++ b/drivers/net/virtio_net.c > > @@ -931,9 +931,19 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi, > > static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 *len, void **ctx) > > { > > + void *buf; > > + > > BUG_ON(!rq->page_pool); > > - return virtqueue_get_buf_ctx(rq->vq, len, ctx); > > + buf = virtqueue_get_buf_ctx(rq->vq, len, ctx); > > + if (buf && rq->use_page_pool_dma && *len) { > > + struct page *page = virt_to_head_page(buf); > > + int offset = buf - page_address(page); > > + > > + page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, *len); > > + } > > + > > + return buf; > > } > > static void virtnet_rq_unmap_free_buf(struct virtqueue *vq, void *buf) > > > > > > > > just sent