netdev - Re: [PATCH] rtw88/pci: Rearrange the memory usage for skb in RX ISR

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Date:   Mon, 8 Jul 2019 16:07:23 +0800
From:   Jian-Hong Pan <jian-hong@...lessm.com>
To:     Tony Chuang <yhchuang@...ltek.com>
Cc:     Kalle Valo <kvalo@...eaurora.org>,
        "David S . Miller" <davem@...emloft.net>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux@...lessm.com" <linux@...lessm.com>,
        Daniel Drake <drake@...lessm.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] rtw88/pci: Rearrange the memory usage for skb in RX ISR

Tony Chuang <yhchuang@...ltek.com> 於 2019年7月8日 週一 下午3:23寫道：
>
> > Subject: [PATCH] rtw88/pci: Rearrange the memory usage for skb in RX ISR
>
> nit, "rtw88: pci:" would be better.

Ok.

> >
> > When skb allocation fails and the "rx routine starvation" is hit, the
> > function returns immediately without updating the RX ring. At this
> > point, the RX ring may continue referencing an old skb which was already
> > handed off to ieee80211_rx_irqsafe(). When it comes to be used again,
> > bad things happen.
> >
> > This patch allocates a new skb first in RX ISR. If we don't have memory
> > available, we discard the current frame, allowing the existing skb to be
> > reused in the ring. Otherwise, we simplify the code flow and just hand
> > over the RX-populated skb over to mac80211.
> >
> > In addition, to fixing the kernel crash, the RX routine should now
> > generally behave better under low memory conditions.
> >
> > Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=204053
> > Signed-off-by: Jian-Hong Pan <jian-hong@...lessm.com>
> > Reviewed-by: Daniel Drake <drake@...lessm.com>
> > Cc: <stable@...r.kernel.org>
> > ---
> >  drivers/net/wireless/realtek/rtw88/pci.c | 28 +++++++++++-------------
> >  1 file changed, 13 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/net/wireless/realtek/rtw88/pci.c
> > b/drivers/net/wireless/realtek/rtw88/pci.c
> > index cfe05ba7280d..1bfc99ae6b84 100644
> > --- a/drivers/net/wireless/realtek/rtw88/pci.c
> > +++ b/drivers/net/wireless/realtek/rtw88/pci.c
> > @@ -786,6 +786,15 @@ static void rtw_pci_rx_isr(struct rtw_dev *rtwdev,
> > struct rtw_pci *rtwpci,
> >               rx_desc = skb->data;
> >               chip->ops->query_rx_desc(rtwdev, rx_desc, &pkt_stat, &rx_status);
> >
> > +             /* discard current skb if the new skb cannot be allocated as a
> > +              * new one in rx ring later
> > +              * */
>
> nit, comment indentation.

Thanks.  I will fix this.

> > +             new = dev_alloc_skb(RTK_PCI_RX_BUF_SIZE);
> > +             if (WARN(!new, "rx routine starvation\n")) {
> > +                     new = skb;
> > +                     goto next_rp;
> > +             }
> > +
> >               /* offset from rx_desc to payload */
> >               pkt_offset = pkt_desc_sz + pkt_stat.drv_info_sz +
> >                            pkt_stat.shift;
> > @@ -803,25 +812,14 @@ static void rtw_pci_rx_isr(struct rtw_dev *rtwdev,
> > struct rtw_pci *rtwpci,
> >                       skb_put(skb, pkt_stat.pkt_len);
> >                       skb_reserve(skb, pkt_offset);
> >
> > -                     /* alloc a smaller skb to mac80211 */
> > -                     new = dev_alloc_skb(pkt_stat.pkt_len);
> > -                     if (!new) {
> > -                             new = skb;
> > -                     } else {
> > -                             skb_put_data(new, skb->data, skb->len);
> > -                             dev_kfree_skb_any(skb);
> > -                     }
>
> I am not sure if it's fine to deliver every huge SKB to mac80211.
> Because it will then be delivered to TCP/IP stack.
> Hence I think either it should be tested to know if the performance
> would be impacted or find out a more efficient way to send
> smaller SKB to mac80211 stack.

I remember network stack only processes the skb with(in) pointers
(skb->data) and the skb->len for data part.  It also checks real
buffer boundary (head and end) of the skb to prevent memory overflow.
Therefore, I think using the original skb is the most efficient way.

If I misunderstand something, please point out.



> >                       /* TODO: merge into rx.c */
> >                       rtw_rx_stats(rtwdev, pkt_stat.vif, skb);
> > -                     memcpy(new->cb, &rx_status, sizeof(rx_status));
> > -                     ieee80211_rx_irqsafe(rtwdev->hw, new);
> > +                     memcpy(skb->cb, &rx_status, sizeof(rx_status));
> > +                     ieee80211_rx_irqsafe(rtwdev->hw, skb);
> >               }
> >
> > -             /* skb delivered to mac80211, alloc a new one in rx ring */
> > -             new = dev_alloc_skb(RTK_PCI_RX_BUF_SIZE);
> > -             if (WARN(!new, "rx routine starvation\n"))
> > -                     return;
> > -
> > +next_rp:
> > +             /* skb delivered to mac80211, attach the new one into rx ring */
> >               ring->buf[cur_rp] = new;
> >               rtw_pci_reset_rx_desc(rtwdev, new, ring, cur_rp, buf_desc_sz);
> >
>
> --
>
> Yan-Hsuan