From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C4B18C2D0E4 for ; Mon, 23 Nov 2020 13:54:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7954C206F1 for ; Mon, 23 Nov 2020 13:54:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388798AbgKWNxL (ORCPT ); Mon, 23 Nov 2020 08:53:11 -0500 Received: from www62.your-server.de ([213.133.104.62]:60006 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388791AbgKWNxJ (ORCPT ); Mon, 23 Nov 2020 08:53:09 -0500 Received: from sslproxy06.your-server.de ([78.46.172.3]) by www62.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92.3) (envelope-from ) id 1khCHB-0007wF-Ho; Mon, 23 Nov 2020 14:53:05 +0100 Received: from [85.7.101.30] (helo=pc-9.home) by sslproxy06.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1khCHB-0007fL-A7; Mon, 23 Nov 2020 14:53:05 +0100 Subject: Re: [PATCH bpf] net, xsk: Avoid taking multiple skbuff references To: =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , ast@kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org Cc: =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , jonathan.lemon@gmail.com, yhs@fb.com, weqaar.janjua@gmail.com, magnus.karlsson@intel.com, weqaar.a.janjua@intel.com References: <20201123131215.136131-1-bjorn.topel@gmail.com> From: Daniel Borkmann Message-ID: <12b970c5-6b44-5288-0c79-2df5178d1165@iogearbox.net> Date: Mon, 23 Nov 2020 14:53:04 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20201123131215.136131-1-bjorn.topel@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.102.4/25996/Sun Nov 22 14:25:48 2020) Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 11/23/20 2:12 PM, Björn Töpel wrote: > From: Björn Töpel > > Commit 642e450b6b59 ("xsk: Do not discard packet when NETDEV_TX_BUSY") > addressed the problem that packets were discarded from the Tx AF_XDP > ring, when the driver returned NETDEV_TX_BUSY. Part of the fix was > bumping the skbuff reference count, so that the buffer would not be > freed by dev_direct_xmit(). A reference count larger than one means > that the skbuff is "shared", which is not the case. > > If the "shared" skbuff is sent to the generic XDP receive path, > netif_receive_generic_xdp(), and pskb_expand_head() is entered the > BUG_ON(skb_shared(skb)) will trigger. > > This patch adds a variant to dev_direct_xmit(), __dev_direct_xmit(), > where a user can select the skbuff free policy. This allows AF_XDP to > avoid bumping the reference count, but still keep the NETDEV_TX_BUSY > behavior. > > Reported-by: Yonghong Song > Fixes: 642e450b6b59 ("xsk: Do not discard packet when NETDEV_TX_BUSY") > Signed-off-by: Björn Töpel > --- > include/linux/netdevice.h | 1 + > net/core/dev.c | 9 +++++++-- > net/xdp/xsk.c | 8 +------- > 3 files changed, 9 insertions(+), 9 deletions(-) > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h > index 964b494b0e8d..e7402fca7752 100644 > --- a/include/linux/netdevice.h > +++ b/include/linux/netdevice.h > @@ -2815,6 +2815,7 @@ u16 dev_pick_tx_cpu_id(struct net_device *dev, struct sk_buff *skb, > struct net_device *sb_dev); > int dev_queue_xmit(struct sk_buff *skb); > int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev); > +int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id, bool free_on_busy); > int dev_direct_xmit(struct sk_buff *skb, u16 queue_id); > int register_netdevice(struct net_device *dev); > void unregister_netdevice_queue(struct net_device *dev, struct list_head *head); > diff --git a/net/core/dev.c b/net/core/dev.c > index 82dc6b48e45f..2af79a4253bb 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -4180,7 +4180,7 @@ int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev) > } > EXPORT_SYMBOL(dev_queue_xmit_accel); > > -int dev_direct_xmit(struct sk_buff *skb, u16 queue_id) > +int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id, bool free_on_busy) > { > struct net_device *dev = skb->dev; > struct sk_buff *orig_skb = skb; > @@ -4211,7 +4211,7 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id) > > local_bh_enable(); > > - if (!dev_xmit_complete(ret)) > + if (free_on_busy && !dev_xmit_complete(ret)) > kfree_skb(skb); > > return ret; Hm, but this way free_on_busy, even though constant, cannot be optimized away? Can't you just move the dev_xmit_complete() check out into dev_direct_xmit() instead? That way you can just drop the bool, and the below dev_direct_xmit() should probably just become an __always_line function in netdevice.h so you avoid the double call. > @@ -4220,6 +4220,11 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id) > kfree_skb_list(skb); > return NET_XMIT_DROP; > } > + > +int dev_direct_xmit(struct sk_buff *skb, u16 queue_id) > +{ > + return __dev_direct_xmit(skb, queue_id, true); > +} > EXPORT_SYMBOL(dev_direct_xmit); > > /************************************************************************* > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c > index 5a6cdf7b320d..c6ad31b374b7 100644 > --- a/net/xdp/xsk.c > +++ b/net/xdp/xsk.c > @@ -411,11 +411,7 @@ static int xsk_generic_xmit(struct sock *sk) > skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr; > skb->destructor = xsk_destruct_skb; > > - /* Hinder dev_direct_xmit from freeing the packet and > - * therefore completing it in the destructor > - */ > - refcount_inc(&skb->users); > - err = dev_direct_xmit(skb, xs->queue_id); > + err = __dev_direct_xmit(skb, xs->queue_id, false); > if (err == NETDEV_TX_BUSY) { > /* Tell user-space to retry the send */ > skb->destructor = sock_wfree; > @@ -429,12 +425,10 @@ static int xsk_generic_xmit(struct sock *sk) > /* Ignore NET_XMIT_CN as packet might have been sent */ > if (err == NET_XMIT_DROP) { > /* SKB completed but not sent */ > - kfree_skb(skb); > err = -EBUSY; > goto out; > } > > - consume_skb(skb); > sent_frame = true; > } > > > base-commit: 178648916e73e00de83150eb0c90c0d3a977a46a >