Re: [PATCH bpf-next v3 3/4] xdp: recycle Page Pool backed skbs built from XDP frames

[Alexander Lobakin]

From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx>
Date: Wed, 15 Mar 2023 15:55:44 +0100

> 
> On 13/03/2023 22.55, Alexander Lobakin wrote:
>> __xdp_build_skb_from_frame() state(d):
>>
>> /* Until page_pool get SKB return path, release DMA here */
>>
>> Page Pool got skb pages recycling in April 2021, but missed this
>> function.
>>
>> xdp_release_frame() is relevant only for Page Pool backed frames and it
>> detaches the page from the corresponding page_pool in order to make it
>> freeable via page_frag_free(). It can instead just mark the output skb
>> as eligible for recycling if the frame is backed by a pp. No change for
>> other memory model types (the same condition check as before).
>> cpumap redirect and veth on Page Pool drivers now become zero-alloc (or
>> almost).
>>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>> ---
>>   net/core/xdp.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>> index 8c92fc553317..a2237cfca8e9 100644
>> --- a/net/core/xdp.c
>> +++ b/net/core/xdp.c
>> @@ -658,8 +658,8 @@ struct sk_buff *__xdp_build_skb_from_frame(struct
>> xdp_frame *xdpf,
>>        * - RX ring dev queue index    (skb_record_rx_queue)
>>        */
>>   -    /* Until page_pool get SKB return path, release DMA here */
>> -    xdp_release_frame(xdpf);
>> +    if (xdpf->mem.type == MEM_TYPE_PAGE_POOL)
>> +        skb_mark_for_recycle(skb);
> 
> I hope this is safe ;-) ... Meaning hopefully drivers does the correct
> thing when XDP_REDIRECT'ing page_pool pages.

Safe when it's done by the schoolbook. For now I'm observing only one
syzbot issue with test_run due to that it assumes yet another bunch
o'things I wouldn't rely on :D (separate subthread)

> 
> Looking for drivers doing weird refcnt tricks and XDP_REDIRECT'ing, I
> noticed the driver aquantia/atlantic (in aq_get_rxpages_xdp), but I now
> see this is not using page_pool, so it should be affected by this (but I
> worry if atlantic driver have a potential race condition for its refcnt
> scheme).

If we encounter some driver using Page Pool, but mangling refcounts on
redirect, we'll fix it ;)

> 
>>         /* Allow SKB to reuse area used by xdp_frame */
>>       xdp_scrub_frame(xdpf);
> 

Thanks,
Olek