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Tsirkin" To: Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Xuan Zhuo , Eric Dumazet , Jason Wang , virtualization@lists.linux-foundation.org, "David S. Miller" Subject: Re: [PATCH 5.10 055/188] virtio_net: Do not pull payload in skb->head Message-ID: <20210412051010-mutt-send-email-mst@kernel.org> References: <20210412084013.643370347@linuxfoundation.org> <20210412084015.479443671@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210412084015.479443671@linuxfoundation.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 12, 2021 at 10:39:29AM +0200, Greg Kroah-Hartman wrote: > From: Eric Dumazet > > commit 0f6925b3e8da0dbbb52447ca8a8b42b371aac7db upstream. > > Xuan Zhuo reported that commit 3226b158e67c ("net: avoid 32 x truesize > under-estimation for tiny skbs") brought a ~10% performance drop. > > The reason for the performance drop was that GRO was forced > to chain sk_buff (using skb_shinfo(skb)->frag_list), which > uses more memory but also cause packet consumers to go over > a lot of overhead handling all the tiny skbs. > > It turns out that virtio_net page_to_skb() has a wrong strategy : > It allocates skbs with GOOD_COPY_LEN (128) bytes in skb->head, then > copies 128 bytes from the page, before feeding the packet to GRO stack. > > This was suboptimal before commit 3226b158e67c ("net: avoid 32 x truesize > under-estimation for tiny skbs") because GRO was using 2 frags per MSS, > meaning we were not packing MSS with 100% efficiency. > > Fix is to pull only the ethernet header in page_to_skb() > > Then, we change virtio_net_hdr_to_skb() to pull the missing > headers, instead of assuming they were already pulled by callers. > > This fixes the performance regression, but could also allow virtio_net > to accept packets with more than 128bytes of headers. > > Many thanks to Xuan Zhuo for his report, and his tests/help. > > Fixes: 3226b158e67c ("net: avoid 32 x truesize under-estimation for tiny skbs") > Reported-by: Xuan Zhuo > Link: https://www.spinics.net/lists/netdev/msg731397.html > Co-Developed-by: Xuan Zhuo > Signed-off-by: Xuan Zhuo > Signed-off-by: Eric Dumazet > Cc: "Michael S. Tsirkin" > Cc: Jason Wang > Cc: virtualization@lists.linux-foundation.org > Acked-by: Jason Wang > Signed-off-by: David S. Miller > Signed-off-by: Greg Kroah-Hartman Note that an issue related to this patch was recently reported. It's quite possible that the root cause is a bug elsewhere in the kernel, but it probably makes sense to defer the backport until we know more ... > --- > drivers/net/virtio_net.c | 10 +++++++--- > include/linux/virtio_net.h | 14 +++++++++----- > 2 files changed, 16 insertions(+), 8 deletions(-) > > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -406,9 +406,13 @@ static struct sk_buff *page_to_skb(struc > offset += hdr_padded_len; > p += hdr_padded_len; > > - copy = len; > - if (copy > skb_tailroom(skb)) > - copy = skb_tailroom(skb); > + /* Copy all frame if it fits skb->head, otherwise > + * we let virtio_net_hdr_to_skb() and GRO pull headers as needed. > + */ > + if (len <= skb_tailroom(skb)) > + copy = len; > + else > + copy = ETH_HLEN + metasize; > skb_put_data(skb, p, copy); > > if (metasize) { > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -65,14 +65,18 @@ static inline int virtio_net_hdr_to_skb( > skb_reset_mac_header(skb); > > if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) { > - u16 start = __virtio16_to_cpu(little_endian, hdr->csum_start); > - u16 off = __virtio16_to_cpu(little_endian, hdr->csum_offset); > + u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start); > + u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset); > + u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16)); > + > + if (!pskb_may_pull(skb, needed)) > + return -EINVAL; > > if (!skb_partial_csum_set(skb, start, off)) > return -EINVAL; > > p_off = skb_transport_offset(skb) + thlen; > - if (p_off > skb_headlen(skb)) > + if (!pskb_may_pull(skb, p_off)) > return -EINVAL; > } else { > /* gso packets without NEEDS_CSUM do not set transport_offset. > @@ -102,14 +106,14 @@ retry: > } > > p_off = keys.control.thoff + thlen; > - if (p_off > skb_headlen(skb) || > + if (!pskb_may_pull(skb, p_off) || > keys.basic.ip_proto != ip_proto) > return -EINVAL; > > skb_set_transport_header(skb, keys.control.thoff); > } else if (gso_type) { > p_off = thlen; > - if (p_off > skb_headlen(skb)) > + if (!pskb_may_pull(skb, p_off)) > return -EINVAL; > } > } >