netdev - Re: [PATCH v4 bpf 2/2] veth: update mem type in xdp

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Message-ID: <87ms5ce7qm.fsf@toke.dk>
Date: Mon, 27 Oct 2025 15:31:45 +0100
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>, bpf@...r.kernel.org,
 ast@...nel.org, daniel@...earbox.net, hawk@...nel.org
Cc: netdev@...r.kernel.org, magnus.karlsson@...el.com,
 aleksander.lobakin@...el.com, ilias.apalodimas@...aro.org,
 lorenzo@...nel.org, kuba@...nel.org, Maciej Fijalkowski
 <maciej.fijalkowski@...el.com>
Subject: Re: [PATCH v4 bpf 2/2] veth: update mem type in xdp_buff

Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:

> When skb's headroom is not sufficient for XDP purposes,
> skb_pp_cow_data() returns new skb with requested headroom space. This
> skb was provided by page_pool.
>
> For CONFIG_DEBUG_VM=y and XDP program that uses bpf_xdp_adjust_tail()
> against a skb with frags, and mentioned helper consumed enough amount of
> bytes that in turn released the page, 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).
>
> 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 <ast@...nel.org>
> Closes: https://lore.kernel.org/bpf/CAADnVQ+bBofJDfieyOYzSmSujSfJwDTQhiz3aJw7hE+4E2_iPA@mail.gmail.com/
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>

Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>