netdev - Re: [PATCH v2 bpf 1/2] xdp: update xdp_rxq_info's mem type in XDP generic hook

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Message-ID: <025d2281-caf0-4f88-8f31-b0bfa5596aec@intel.com>
Date: Mon, 20 Oct 2025 17:36:06 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>
CC: <bpf@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
	<ilias.apalodimas@...aro.org>, <toke@...hat.com>, <lorenzo@...nel.org>,
	<kuba@...nel.org>, <netdev@...r.kernel.org>, <magnus.karlsson@...el.com>,
	<andrii@...nel.org>, <stfomichev@...il.com>,
	<syzbot+ff145014d6b0ce64a173@...kaller.appspotmail.com>, Ihor Solodrai
	<ihor.solodrai@...ux.dev>, Octavian Purdila <tavip@...gle.com>
Subject: Re: [PATCH v2 bpf 1/2] xdp: update xdp_rxq_info's mem type in XDP
 generic hook

From: Jesper Dangaard Brouer <hawk@...nel.org>
Date: Mon, 20 Oct 2025 13:20:57 +0200

> 
> 
> On 17/10/2025 16.31, Maciej Fijalkowski wrote:
>> Currently, generic XDP hook uses xdp_rxq_info from netstack Rx queues
>> which do not have its XDP memory model registered. There is a case when
>> XDP program calls bpf_xdp_adjust_tail() BPF helper, which in turn
>> releases underlying memory. This happens when it consumes enough amount
>> of bytes and when XDP buffer has fragments. For this action the memory
>> model knowledge passed to XDP program is crucial so that core can call
>> suitable function for freeing/recycling the page.
>>
>> For netstack queues it defaults to MEM_TYPE_PAGE_SHARED (0) due to lack
>> of mem model registration. The problem we're fixing here is when kernel
>> copied the skb to new buffer backed by system's page_pool and XDP buffer
>> is built around it. Then when bpf_xdp_adjust_tail() calls
>> __xdp_return(), it acts incorrectly due to mem type not being set to
>> MEM_TYPE_PAGE_POOL and causes a page leak.
>>
> 
> Does the code not set the skb->pp_recycle ?

You mean this CoW code which replaces the buffers in the skb with system
PP-backed ones?
Maybe that's the problem (I don't remember the details of the function)?

> 
>> Pull out the existing code from bpf_prog_run_generic_xdp() that
>> init/prepares xdp_buff onto new helper xdp_convert_skb_to_buff() and
>> embed there rxq's mem_type initialization that is assigned to xdp_buff.

[...]

>> +    if (skb_is_nonlinear(skb)) {
>> +        skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>> +        xdp_buff_set_frags_flag(xdp);
>> +    } else {
>> +        xdp_buff_clear_frags_flag(xdp);
>> +    }
>> +
> 
> The SKB should be marked via skb->pp_recycle, but I guess you are trying
> to catch code that doesn't set this correctly?
> (Slightly worried this will "paper-over" some other buggy code?)
> 
>> +    xdp->rxq->mem.type = page_pool_page_is_pp(virt_to_page(xdp->data)) ?

BTW this may return incorrect results if the page is not order-0.
IIRC system PPs always return order-0 pages, what about veth code etc?

>> +                MEM_TYPE_PAGE_POOL : MEM_TYPE_PAGE_SHARED;
>> +}
> 
> In the beginning PP_MAGIC / PP_SIGNATURE was primarily used as a
> debugging feature to catch faulty code that released pp pages to the
> real page allocator.  It seems to have evolved into something more
> critical.  Someone also tried to elevate this into a page flag, which
> would make this more reliable.
Thanks,
Olek