From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH net-next 04/11] tuntap: simplify error handling in tun_build_skb() Date: Fri, 7 Sep 2018 11:22:00 +0800 Message-ID: References: <20180906040526.22518-1-jasowang@redhat.com> <20180906040526.22518-5-jasowang@redhat.com> <20180906130425-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20180906130425-mutt-send-email-mst@kernel.org> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 2018年09月07日 01:14, Michael S. Tsirkin wrote: > On Thu, Sep 06, 2018 at 12:05:19PM +0800, Jason Wang wrote: >> There's no need to duplicate page get logic in each action. So this >> patch tries to get page and calculate the offset before processing XDP >> actions, and undo them when meet errors (we don't care the performance >> on errors). This will be used for factoring out XDP logic. >> >> Signed-off-by: Jason Wang > I see some issues with this one. > >> --- >> drivers/net/tun.c | 37 ++++++++++++++++--------------------- >> 1 file changed, 16 insertions(+), 21 deletions(-) >> >> diff --git a/drivers/net/tun.c b/drivers/net/tun.c >> index 372caf7d67d9..f8cdcfa392c3 100644 >> --- a/drivers/net/tun.c >> +++ b/drivers/net/tun.c >> @@ -1642,7 +1642,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, >> int len, int *skb_xdp) >> { >> struct page_frag *alloc_frag = ¤t->task_frag; >> - struct sk_buff *skb; >> + struct sk_buff *skb = NULL; >> struct bpf_prog *xdp_prog; >> int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info)); >> unsigned int delta = 0; >> @@ -1668,6 +1668,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, >> if (copied != len) >> return ERR_PTR(-EFAULT); >> >> + get_page(alloc_frag->page); >> + alloc_frag->offset += buflen; >> + > This adds an atomic op on XDP_DROP which is a data path > operation for some workloads. Yes, I have patch on top to amortize this, the idea is to have a very big refcount once after the frag was allocated and maintain a bias and decrease them all when allocating new frags.' > >> /* There's a small window that XDP may be set after the check >> * of xdp_prog above, this should be rare and for simplicity >> * we do XDP on skb in case the headroom is not enough. >> @@ -1695,23 +1698,15 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, >> >> switch (act) { >> case XDP_REDIRECT: >> - get_page(alloc_frag->page); >> - alloc_frag->offset += buflen; >> err = xdp_do_redirect(tun->dev, &xdp, xdp_prog); >> xdp_do_flush_map(); >> if (err) >> - goto err_redirect; >> - rcu_read_unlock(); >> - local_bh_enable(); >> - return NULL; >> + goto err_xdp; >> + goto out; >> case XDP_TX: >> - get_page(alloc_frag->page); >> - alloc_frag->offset += buflen; >> if (tun_xdp_tx(tun->dev, &xdp) < 0) >> - goto err_redirect; >> - rcu_read_unlock(); >> - local_bh_enable(); >> - return NULL; >> + goto err_xdp; >> + goto out; >> case XDP_PASS: >> delta = orig_data - xdp.data; >> len = xdp.data_end - xdp.data; >> @@ -1730,23 +1725,23 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, >> local_bh_enable(); >> >> skb = build_skb(buf, buflen); >> - if (!skb) >> - return ERR_PTR(-ENOMEM); >> + if (!skb) { >> + skb = ERR_PTR(-ENOMEM); >> + goto out; > So goto out will skip put_page, and we did > do get_page above. Seems wrong. You should > goto err_skb or something like this. Yes, looks like err_xdp. > > >> + } >> >> skb_reserve(skb, pad - delta); >> skb_put(skb, len); >> - get_page(alloc_frag->page); >> - alloc_frag->offset += buflen; >> >> return skb; >> >> -err_redirect: >> - put_page(alloc_frag->page); >> err_xdp: >> + alloc_frag->offset -= buflen; >> + put_page(alloc_frag->page); >> +out: > Out here isn't an error at all, is it? You should not mix return and > error handling IMHO. If you mean the name, I can rename the label to "drop". > > > >> rcu_read_unlock(); >> local_bh_enable(); >> - this_cpu_inc(tun->pcpu_stats->rx_dropped); > Doesn't this break rx_dropped accounting? Let me fix this. Thanks >> - return NULL; >> + return skb; >> } >> >> /* Get packet from user space buffer */ >> -- >> 2.17.1