答复: 答复: [Intel-wired-lan] [PATCH 0/2] intel/xdp fixes for fliping rx buffer

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: Björn Töpel [mailto:bjorn.topel@intel.com]
> 发送时间: 2020年8月19日 14:45
> 收件人: Li,Rongqing <lirongqing@baidu.com>; Björn Töpel
> <bjorn.topel@gmail.com>
> 抄送: Netdev <netdev@vger.kernel.org>; intel-wired-lan
> <intel-wired-lan@lists.osuosl.org>; Karlsson, Magnus
> <magnus.karlsson@intel.com>; bpf <bpf@vger.kernel.org>; Maciej Fijalkowski
> <maciej.fijalkowski@intel.com>; Piotr <piotr.raczynski@intel.com>; Maciej
> <maciej.machnikowski@intel.com>
> 主题: Re: 答复: [Intel-wired-lan] [PATCH 0/2] intel/xdp fixes for fliping rx buffer
> 
> On 2020-08-19 03:37, Li,Rongqing wrote:
> [...]
>  > Hi:
>  >
>  > Thanks for your explanation.
>  >
>  > But we can reproduce this bug
>  >
>  > We use ebpf to redirect only-Vxlan packets to non-zerocopy AF_XDP, First we
> see panic on tcp stack, in tcp_collapse: BUG_ON(offset < 0); it is very hard to
> reproduce.
>  >
>  > Then we use the scp to do test, and has lots of vxlan packet at the same
> time, scp will be broken frequently.
>  >
> 
> Ok! Just so that I'm certain of your setup. You receive packets to an i40e netdev
> where there's an XDP program. The program does XDP_PASS or XDP_REDIRECT
> to e.g. devmap for non-vxlan packets. However, vxlan packets are redirected to
> AF_XDP socket(s) in *copy-mode*. Am I understanding that correct?
> 
Similar as your description, 

but the xdp program only redirects vxlan packets to af_xdp socket, other packets will go to Linux kernel networking stack, like scp/ssh packets


> I'm assuming this is an x86-64 with 4k page size, right? :-) The page flipping is a
> bit different if the PAGE_SIZE is not 4k.
> 

We use 4k page size, page flipping is 4k, we did not change the i40e drivers, 4.19 stable kernel

>  > With this fixes, scp has not been broken again, and kernel is not panic
> again  >
> 
> Let's dig into your scenario.
> 
> Are you saying the following:
> 
> Page A:
> +------------
> | "first skb" ----> Rx HW ring entry X 
> +------------
> | "second skb"----> Rx HW ring entry X+1 (or X+n)
> +------------
> 

Like:

First skb will be into tcp socket rx queue

Seconds skb is vxlan packet, will be copy to af_xdp socket, and released.

> This is a scenario that shouldn't be allowed, because there are now two users
> of the page. If that's the case, the refcounting is broken. Is that the case?
> 

True, it is broken for copy mode xsk

-Li

> Check out i40e_can_reuse_rx_page(). The idea with page flipping/reuse is that
> the page is only reused if there is only one user.
> 
>  > Seem your explanation is unable to solve my analysis:
>  >
>  >         1. first skb is not for xsk, and forwarded to another device
>  >            or socket queue
> 
> The data for the "first skb" resides on a page:
> A:
> +------------
> | "first skb"
> +------------
> | to be reused
> +------------
> refcount >>1
> 
>  >         2. seconds skb is for xsk, copy data to xsk memory, and page
>  >            of skb->data is released
> 
> Note that page B != page A.
> 
> B:
> +------------
> | to be reused/or used by the stack
> +------------
> | "second skb for xsk"
> +------------
> refcount >>1
> 
> data is copied to socket, page_frag_free() is called, and the page count is
> decreased. The driver will then check if the page can be reused. If not, it's freed
> to the page allocator.
> 
>  >         3. rx_buff is reusable since only first skb is in it, but
>  >            *_rx_buffer_flip will make that page_offset is set to
>  >            first skb data
> 
> I'm having trouble grasping how this is possible. More than one user implies
> that it wont be reused. If this is possible, the recounting/reuse mechanism is
> broken, and that is what should be fixed.
> 
> The AF_XDP redirect should not have semantics different from, say, devmap
> redirect. It's just that the page_frag_free() is called earlier for AF_XDP, instead
> of from i40e_clean_tx_irq() as the case for devmap/XDP_TX.
> 
>  >         4. then reuse rx buffer, first skb which still is living
>  >            will be corrupted.
>  >
>  >
>  > The root cause is difference you said upper, so I only fixes for non-zerocopy
> AF_XDP  >
> 
> I have only addressed non-zerocopy, so we're on the same page (pun
> intended) here!
> 
> 
> Björn
> 
>  > -Li