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

Alexander Lobakin wrote:

From: Xuan Zhuo <xuanzhuo@linux.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@linux.alibaba.com>
Reviewed-by: Dust Li <dust.li@linux.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 <redacted>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.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;
+}
+

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