netdev - Re: [PATCH net-next V2 1/3] tap: use build

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Message-ID: <208fa880-c0e5-36e5-3604-eb13fe1de3b9@redhat.com>
Date:   Wed, 16 Aug 2017 21:16:56 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     davem@...emloft.net, mst@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kubakici@...pl
Subject: Re: [PATCH net-next V2 1/3] tap: use build_skb() for small packet



On 2017年08月16日 18:24, Eric Dumazet wrote:
> On Wed, 2017-08-16 at 11:55 +0800, Jason Wang wrote:
>> On 2017年08月16日 11:45, Eric Dumazet wrote:
>>> You do realize that tun_build_skb() is not thread safe ?
>> Ok, I think the issue if skb_page_frag_refill(), need a spinlock
>> probably. Will prepare a patch.
> But since tun is used from process context, why don't you use the
> per-thread generator (no lock involved)

Haven't noticed this before.

>
> tcp_sendmsg() uses this for GFP_KERNEL allocations.
>
> Untested patch :
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 5892284eb8d05b0678d820bad3d0d2c61a879aeb..c38cd840cc0b7fecf182b23976e36f709cacca1f 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -175,7 +175,6 @@ struct tun_file {
>   	struct list_head next;
>   	struct tun_struct *detached;
>   	struct skb_array tx_array;
> -	struct page_frag alloc_frag;
>   };
>   
>   struct tun_flow_entry {
> @@ -578,8 +577,6 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>   		}
>   		if (tun)
>   			skb_array_cleanup(&tfile->tx_array);
> -		if (tfile->alloc_frag.page)
> -			put_page(tfile->alloc_frag.page);
>   		sock_put(&tfile->sk);
>   	}
>   }
> @@ -1272,7 +1269,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>   				     struct virtio_net_hdr *hdr,
>   				     int len, int *generic_xdp)
>   {
> -	struct page_frag *alloc_frag = &tfile->alloc_frag;
> +	struct page_frag *alloc_frag = &current->task_frag;
>   	struct sk_buff *skb;
>   	struct bpf_prog *xdp_prog;
>   	int buflen = SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> @@ -2580,8 +2577,6 @@ static int tun_chr_open(struct inode *inode, struct file * file)
>   	tfile->sk.sk_write_space = tun_sock_write_space;
>   	tfile->sk.sk_sndbuf = INT_MAX;
>   
> -	tfile->alloc_frag.page = NULL;
> -
>   	file->private_data = tfile;
>   	INIT_LIST_HEAD(&tfile->next);
>   
>
>
>
>

Tested-by: Jason Wang <jasowang@...hat.com>
Acked-by: Jason Wang <jasowang@...hat.com>