Re: [PATCH net] net: add net_iov_init() and use it to initialize ->page_type

Vlastimil Babka (SUSE)
2026-04-28T07:57:13Z

+Cc Byungchul

On 4/28/26 04:53, Jakub Kicinski wrote:
> Commit db359fccf212 ("mm: introduce a new page type for page pool in
> page type") added a page_type field to struct net_iov at the same
> offset as struct page::page_type, so that page_pool_set_pp_info() can
> call __SetPageNetpp() uniformly on both pages and net_iovs.
>
> The page-type API requires the field to hold the UINT_MAX "no type"
> sentinel before a type can be set; for real struct page that invariant
> is established by the page allocator on free. struct net_iov is not
> allocated through the page allocator, so the field is left as zero
> (io_uring zcrx, which uses __GFP_ZERO) or as slab garbage (devmem,
> which uses kvmalloc_objs() without zeroing). When the page pool then
> calls page_pool_set_pp_info() on a freshly-bound niov,
> __SetPageNetpp()'s VM_BUG_ON_PAGE(page->page_type != UINT_MAX) fires
> and the kernel BUGs. Triggered in selftests by io_uring zcrx setup
> through the fbnic queue restart path:
>
> kernel BUG at ./include/linux/page-flags.h:1062!
> RIP: 0010:page_pool_set_pp_info (./include/linux/page-flags.h:1062
> net/core/page_pool.c:716)
> Call Trace:
> <TASK>
> net_mp_niov_set_page_pool (net/core/page_pool.c:1360)
> io_pp_zc_alloc_netmems (io_uring/zcrx.c:1089 io_uring/zcrx.c:1110)
> fbnic_fill_bdq (./include/net/page_pool/helpers.h:160
> drivers/net/ethernet/meta/fbnic/fbnic_txrx.c:906)
> __fbnic_nv_restart (drivers/net/ethernet/meta/fbnic/fbnic_txrx.c:2470
> drivers/net/ethernet/meta/fbnic/fbnic_txrx.c:2874)
> fbnic_queue_start (drivers/net/ethernet/meta/fbnic/fbnic_txrx.c:2903)
> netdev_rx_queue_reconfig (net/core/netdev_rx_queue.c:137)
> __netif_mp_open_rxq (net/core/netdev_rx_queue.c:234)
> io_register_zcrx (io_uring/zcrx.c:818 io_uring/zcrx.c:903)
> __io_uring_register (io_uring/register.c:931)
> __do_sys_io_uring_register (io_uring/register.c:1029)
> do_syscall_64 (arch/x86/entry/syscall_64.c:63
> arch/x86/entry/syscall_64.c:94)
> </TASK>
>
> The same path is reachable through devmem dmabuf binding via
> netdev_nl_bind_rx_doit() -> net_devmem_bind_dmabuf_to_queue().
>
> Add a net_iov_init() helper that stamps ->owner, ->type and the
> ->page_type sentinel, and use it from both the devmem and io_uring
> zcrx niov init loops.
>
> Fixes: db359fccf212 ("mm: introduce a new page type for page pool in page type")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
> CC: asml.silence@gmail.com
> CC: axboe@kernel.dk
> CC: almasrymina@google.com
> CC: sdf@fomichev.me
> CC: hawk@kernel.org
> CC: akpm@linux-foundation.org
> CC: rppt@kernel.org
> CC: vbabka@kernel.org
> CC: io-uring@vger.kernel.org
> ---
> include/net/netmem.h | 15 +++++++++++++++
> io_uring/zcrx.c | 3 +--
> net/core/devmem.c | 3 +--
> 3 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index 507b74c9f52d..78fe51e5756b 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -127,6 +127,21 @@ static inline unsigned int net_iov_idx(const struct net_iov *niov)
> return niov - net_iov_owner(niov)->niovs;
> }
>
> +/* Initialize a niov: stamp the owning area, the memory provider type,
> + * and the page_type "no type" sentinel expected by the page-type API
> + * (see PAGE_TYPE_OPS in <linux/page-flags.h>) so that
> + * page_pool_set_pp_info() can later call __SetPageNetpp() on a niov
> + * cast to struct page.
> + */
> +static inline void net_iov_init(struct net_iov *niov,
> + struct net_iov_area *owner,
> + enum net_iov_type type)
> +{
> + niov->owner = owner;
> + niov->type = type;
> + niov->page_type = UINT_MAX;
> +}
> +
> /* netmem */
>
> /**
> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> index 7b93c87b8371..19837e0b5e91 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -495,10 +495,9 @@ static int io_zcrx_create_area(struct io_zcrx_ifq *ifq,
> for (i = 0; i < nr_iovs; i++) {
> struct net_iov *niov = &area->nia.niovs[i];
>
> - niov->owner = &area->nia;
> + net_iov_init(niov, &area->nia, NET_IOV_IOURING);
> area->freelist[i] = i;
> atomic_set(&area->user_refs[i], 0);
> - niov->type = NET_IOV_IOURING;
> }
>
> if (ifq->dev) {
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index cde4c89bc146..468344739db2 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -297,8 +297,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
>
> for (i = 0; i < owner->area.num_niovs; i++) {
> niov = &owner->area.niovs[i];
> - niov->type = NET_IOV_DMABUF;
> - niov->owner = &owner->area;
> + net_iov_init(niov, &owner->area, NET_IOV_DMABUF);
> page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov),
> net_devmem_get_dma_addr(niov));
> if (direction == DMA_TO_DEVICE)