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?
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.
> 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)
+------------
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?
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
