[PATCH 3.16 122/134] virtio_net: fix support for small rings

[Ben Hutchings]

3.16.47-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Michael S. Tsirkin" <m...@redhat.com>

commit d85b758f72b05a774045545f24d70980e3e9aac4 upstream.

When ring size is small (<32 entries) making buffers smaller means a
full ring might not be able to hold enough buffers to fit a single large
packet.

Make sure a ring full of buffers is large enough to allow at least one
packet of max size.

Fixes: 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag 
allocators")
Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
[bwh: Backported to 3.16:
 - There's no net_device::max_mtu, so always set packet_len = IP_MAX_MTU
 - Adjust context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 drivers/net/virtio_net.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -27,6 +27,7 @@
 #include <linux/slab.h>
 #include <linux/cpu.h>
 #include <linux/average.h>
+#include <net/route.h>
 
 static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
@@ -92,6 +93,9 @@ struct receive_queue {
        /* RX: fragments + linear part + virtio header */
        struct scatterlist sg[MAX_SKB_FRAGS + 2];
 
+       /* Min single buffer size for mergeable buffers case. */
+       unsigned int min_buf_len;
+
        /* Name of this receive queue: input.$index */
        char name[40];
 };
@@ -600,13 +604,14 @@ static int add_recvbuf_big(struct receiv
        return err;
 }
 
-static unsigned int get_mergeable_buf_len(struct ewma *avg_pkt_len)
+static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
+                                         struct ewma *avg_pkt_len)
 {
        const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
        unsigned int len;
 
        len = hdr_len + clamp_t(unsigned int, ewma_read(avg_pkt_len),
-                       GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
+                               rq->min_buf_len - hdr_len, PAGE_SIZE - hdr_len);
        return ALIGN(len, MERGEABLE_BUFFER_ALIGN);
 }
 
@@ -618,7 +623,7 @@ static int add_recvbuf_mergeable(struct
        int err;
        unsigned int len, hole;
 
-       len = get_mergeable_buf_len(&rq->mrg_avg_pkt_len);
+       len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len);
        if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp)))
                return -ENOMEM;
 
@@ -1462,6 +1467,21 @@ static void virtnet_del_vqs(struct virtn
        virtnet_free_queues(vi);
 }
 
+/* How large should a single buffer be so a queue full of these can fit at
+ * least one full packet?
+ * Logic below assumes the mergeable buffer header is used.
+ */
+static unsigned int mergeable_min_buf_len(struct virtnet_info *vi, struct 
virtqueue *vq)
+{
+       const unsigned int hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
+       unsigned int rq_size = virtqueue_get_vring_size(vq);
+       unsigned int packet_len = IP_MAX_MTU;
+       unsigned int buf_len = hdr_len + ETH_HLEN + VLAN_HLEN + packet_len;
+       unsigned int min_buf_len = DIV_ROUND_UP(buf_len, rq_size);
+
+       return max(min_buf_len, hdr_len);
+}
+
 static int virtnet_find_vqs(struct virtnet_info *vi)
 {
        vq_callback_t **callbacks;
@@ -1517,6 +1537,7 @@ static int virtnet_find_vqs(struct virtn
 
        for (i = 0; i < vi->max_queue_pairs; i++) {
                vi->rq[i].vq = vqs[rxq2vq(i)];
+               vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
                vi->sq[i].vq = vqs[txq2vq(i)];
        }
 
@@ -1601,7 +1622,8 @@ static ssize_t mergeable_rx_buffer_size_
 
        BUG_ON(queue_index >= vi->max_queue_pairs);
        avg = &vi->rq[queue_index].mrg_avg_pkt_len;
-       return sprintf(buf, "%u\n", get_mergeable_buf_len(avg));
+       return sprintf(buf, "%u\n",
+                      get_mergeable_buf_len(&vi->rq[queue_index], avg));
 }
 
 static struct rx_queue_attribute mergeable_rx_buffer_size_attribute =