netdev - RE: [PATCH v7 bpf-next 6/6] xsk: build skb by page (aka generic zerocopy xmit)

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Date:   Wed, 17 Feb 2021 16:46:04 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexander Lobakin <alobakin@...me>,
        Daniel Borkmann <daniel@...earbox.net>,
        Magnus Karlsson <magnus.karlsson@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Dust Li <dust.li@...ux.alibaba.com>,
        Alexander Lobakin <alobakin@...me>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: RE: [PATCH v7 bpf-next 6/6] xsk: build skb by page (aka generic
 zerocopy xmit)

Alexander Lobakin wrote:
> From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> 
> This patch is used to construct skb based on page to save memory copy
> overhead.
> 
> This function is implemented based on IFF_TX_SKB_NO_LINEAR. Only the
> network card priv_flags supports IFF_TX_SKB_NO_LINEAR will use page to
> directly construct skb. If this feature is not supported, it is still
> necessary to copy data to construct skb.
> 
> ---------------- Performance Testing ------------
> 
> The test environment is Aliyun ECS server.
> Test cmd:
> ```
> xdpsock -i eth0 -t  -S -s <msg size>
> ```
> 
> Test result data:
> 
> size    64      512     1024    1500
> copy    1916747 1775988 1600203 1440054
> page    1974058 1953655 1945463 1904478
> percent 3.0%    10.0%   21.58%  32.3%
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@...ux.alibaba.com>
> [ alobakin:
>  - expand subject to make it clearer;
>  - improve skb->truesize calculation;
>  - reserve some headroom in skb for drivers;
>  - tailroom is not needed as skb is non-linear ]
> Signed-off-by: Alexander Lobakin <alobakin@...me>
> Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>
> ---

[...]

> +	buffer = xsk_buff_raw_get_data(pool, addr);
> +	offset = offset_in_page(buffer);
> +	addr = buffer - pool->addrs;
> +
> +	for (copied = 0, i = 0; copied < len; i++) {
> +		page = pool->umem->pgs[addr >> PAGE_SHIFT];

Looks like we could walk off the end of pgs[] if len is larger than
the number of pgs? Do we need to guard against a misconfigured socket
causing a panic here? AFAIU len here is read from the user space
descriptor so is under user control. Or maybe I missed a check somewhere.

Thanks,
John


> +		get_page(page);
> +
> +		copy = min_t(u32, PAGE_SIZE - offset, len - copied);
> +		skb_fill_page_desc(skb, i, page, offset, copy);
> +
> +		copied += copy;
> +		addr += copy;
> +		offset = 0;
> +	}
> +
> +	skb->len += len;
> +	skb->data_len += len;
> +	skb->truesize += ts;
> +
> +	refcount_add(ts, &xs->sk.sk_wmem_alloc);
> +
> +	return skb;
> +}
> +