Subject: Re: [PATCH v2 bpf 2/2] veth: update mem type in xdp_buff

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 17 Oct 2025 19:51:58 +0200

Maciej Fijalkowski <[email-masked]> writes:

> On Fri, Oct 17, 2025 at 06:33:54PM +0200, Toke Høiland-Jørgensen wrote:
>> Maciej Fijalkowski <[email-masked]> writes:
>> 
>> > Veth calls skb_pp_cow_data() which makes the underlying memory to
>> > originate from system page_pool. For CONFIG_DEBUG_VM=y and XDP program
>> > that uses bpf_xdp_adjust_tail(), following splat was observed:
>> >
>> > [   32.204881] BUG: Bad page state in process test_progs  pfn:11c98b
>> > [   32.207167] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x11c98b
>> > [   32.210084] flags: 0x1fffe0000000000(node=0|zone=1|lastcpupid=0x7fff)
>> > [   32.212493] raw: 01fffe0000000000 dead000000000040 ff11000123c9b000 0000000000000000
>> > [   32.218056] raw: 0000000000000000 0000000000000001 00000000ffffffff 0000000000000000
>> > [   32.220900] page dumped because: page_pool leak
>> > [   32.222636] Modules linked in: bpf_testmod(O) bpf_preload
>> > [   32.224632] CPU: 6 UID: 0 PID: 3612 Comm: test_progs Tainted: G O        6.17.0-rc5-gfec474d29325 #6969 PREEMPT
>> > [   32.224638] Tainted: [O]=OOT_MODULE
>> > [   32.224639] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
>> > [   32.224641] Call Trace:
>> > [   32.224644]  <IRQ>
>> > [   32.224646]  dump_stack_lvl+0x4b/0x70
>> > [   32.224653]  bad_page.cold+0xbd/0xe0
>> > [   32.224657]  __free_frozen_pages+0x838/0x10b0
>> > [   32.224660]  ? skb_pp_cow_data+0x782/0xc30
>> > [   32.224665]  bpf_xdp_shrink_data+0x221/0x530
>> > [   32.224668]  ? skb_pp_cow_data+0x6d1/0xc30
>> > [   32.224671]  bpf_xdp_adjust_tail+0x598/0x810
>> > [   32.224673]  ? xsk_destruct_skb+0x321/0x800
>> > [   32.224678]  bpf_prog_004ac6bb21de57a7_xsk_xdp_adjust_tail+0x52/0xd6
>> > [   32.224681]  veth_xdp_rcv_skb+0x45d/0x15a0
>> > [   32.224684]  ? get_stack_info_noinstr+0x16/0xe0
>> > [   32.224688]  ? veth_set_channels+0x920/0x920
>> > [   32.224691]  ? get_stack_info+0x2f/0x80
>> > [   32.224693]  ? unwind_next_frame+0x3af/0x1df0
>> > [   32.224697]  veth_xdp_rcv.constprop.0+0x38a/0xbe0
>> > [   32.224700]  ? common_startup_64+0x13e/0x148
>> > [   32.224703]  ? veth_xdp_rcv_one+0xcd0/0xcd0
>> > [   32.224706]  ? stack_trace_save+0x84/0xa0
>> > [   32.224709]  ? stack_depot_save_flags+0x28/0x820
>> > [   32.224713]  ? __resched_curr.constprop.0+0x332/0x3b0
>> > [   32.224716]  ? timerqueue_add+0x217/0x320
>> > [   32.224719]  veth_poll+0x115/0x5e0
>> > [   32.224722]  ? veth_xdp_rcv.constprop.0+0xbe0/0xbe0
>> > [   32.224726]  ? update_load_avg+0x1cb/0x12d0
>> > [   32.224730]  ? update_cfs_group+0x121/0x2c0
>> > [   32.224733]  __napi_poll+0xa0/0x420
>> > [   32.224736]  net_rx_action+0x901/0xe90
>> > [   32.224740]  ? run_backlog_napi+0x50/0x50
>> > [   32.224743]  ? clockevents_program_event+0x1cc/0x280
>> > [   32.224746]  ? hrtimer_interrupt+0x31e/0x7c0
>> > [   32.224749]  handle_softirqs+0x151/0x430
>> > [   32.224752]  do_softirq+0x3f/0x60
>> > [   32.224755]  </IRQ>
>> >
>> > It's because xdp_rxq with mem model set to MEM_TYPE_PAGE_SHARED was used
>> > when initializing xdp_buff.
>> >
>> > Fix this by using new helper xdp_convert_skb_to_buff() that, besides
>> > init/prepare xdp_buff, will check if page used for linear part of
>> > xdp_buff comes from page_pool. We assume that linear data and frags will
>> > have same memory provider as currently XDP API does not provide us a way
>> > to distinguish it (the mem model is registered for *whole* Rx queue and
>> > here we speak about single buffer granularity).
>> >
>> > In order to meet expected skb layout by new helper, pull the mac header
>> > before conversion from skb to xdp_buff.
>> >
>> > However, that is not enough as before releasing xdp_buff out of veth via
>> > XDP_{TX,REDIRECT}, mem type on xdp_rxq associated with xdp_buff is
>> > restored to its original model. We need to respect previous setting at
>> > least until buff is converted to frame, as frame carries the mem_type.
>> > Add a page_pool variant of veth_xdp_get() so that we avoid refcount
>> > underflow when draining page frag.
>> >
>> > Fixes: 0ebab78cbcbf ("net: veth: add page_pool for page recycling")
>> > Reported-by: Alexei Starovoitov <[email-masked]>
>> > Closes: https://lore.kernel.org/bpf/[email-masked]/
>> > Signed-off-by: Maciej Fijalkowski <[email-masked]>
>> > ---
>> >  drivers/net/veth.c | 43 +++++++++++++++++++++++++++----------------
>> >  1 file changed, 27 insertions(+), 16 deletions(-)
>> >
>> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
>> > index a3046142cb8e..eeeee7bba685 100644
>> > --- a/drivers/net/veth.c
>> > +++ b/drivers/net/veth.c
>> > @@ -733,7 +733,7 @@ static void veth_xdp_rcv_bulk_skb(struct veth_rq *rq, void **frames,
>> >  	}
>> >  }
>> >  
>> > -static void veth_xdp_get(struct xdp_buff *xdp)
>> > +static void veth_xdp_get_shared(struct xdp_buff *xdp)
>> >  {
>> >  	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>> >  	int i;
>> > @@ -746,12 +746,33 @@ static void veth_xdp_get(struct xdp_buff *xdp)
>> >  		__skb_frag_ref(&sinfo->frags[i]);
>> >  }
>> >  
>> > +static void veth_xdp_get_pp(struct xdp_buff *xdp)
>> > +{
>> > +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>> > +	int i;
>> > +
>> > +	page_pool_ref_page(virt_to_page(xdp->data));
>> > +	if (likely(!xdp_buff_has_frags(xdp)))
>> > +		return;
>> > +
>> > +	for (i = 0; i < sinfo->nr_frags; i++) {
>> > +		skb_frag_t *frag = &sinfo->frags[i];
>> > +
>> > +		page_pool_ref_page(netmem_to_page(frag->netmem));
>> > +	}
>> > +}
>> > +
>> > +static void veth_xdp_get(struct xdp_buff *xdp)
>> > +{
>> > +	xdp->rxq->mem.type == MEM_TYPE_PAGE_POOL ?
>> > +		veth_xdp_get_pp(xdp) : veth_xdp_get_shared(xdp);
>> > +}
>> > +
>> >  static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>> >  					struct xdp_buff *xdp,
>> >  					struct sk_buff **pskb)
>> >  {
>> >  	struct sk_buff *skb = *pskb;
>> > -	u32 frame_sz;
>> >  
>> >  	if (skb_shared(skb) || skb_head_is_locked(skb) ||
>> >  	    skb_shinfo(skb)->nr_frags ||
>> > @@ -762,19 +783,9 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>> >  		skb = *pskb;
>> >  	}
>> >  
>> > -	/* SKB "head" area always have tailroom for skb_shared_info */
>> > -	frame_sz = skb_end_pointer(skb) - skb->head;
>> > -	frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>> > -	xdp_init_buff(xdp, frame_sz, &rq->xdp_rxq);
>> > -	xdp_prepare_buff(xdp, skb->head, skb_headroom(skb),
>> > -			 skb_headlen(skb), true);
>> > +	__skb_pull(*pskb, skb->data - skb_mac_header(skb));
>> 
>> veth_xdp_rcv_skb() does:
>> 
>> 	__skb_push(skb, skb->data - skb_mac_header(skb));
>> 	if (veth_convert_skb_to_xdp_buff(rq, xdp, &skb))
>> 
>> so how about just getting rid of that push instead of doing the opposite
>> pull straight after? :)
>
> Hi Toke,
>
> I believe this is done so we get a proper headroom representation which is
> needed for XDP_PACKET_HEADROOM comparison. Maybe we could be smarter here
> and for example subtract mac header length? However I wanted to preserve
> old behavior.

Yeah, basically what we want is to check if the mac_header offset is
larger than the headroom. So the check could just be:

    skb->mac_header < XDP_PACKET_HEADROOM

however, it may be better to use the helper? Since that makes sure we
keep hitting the DEBUG_NET_WARN_ON_ONCE inside the helper... So:

    skb_mac_header(skb) - skb->head < XDP_PACKET_HEADROOM

or, equivalently:

    skb_headroom(skb) - skb_mac_offset(skb) < XDP_PACKET_HEADROOM

I think the first one is probably more readable, since skb_mac_offset()
is negative here, so the calculation looks off...

-Toke