Subject: Re: [Intel-wired-lan] [PATCH net-next v11 12/12] libeth: xdp: access ->pp through netmem_desc instead of page

From: Byungchul Park <[email-masked]>
Date: Fri, 18 Jul 2025 10:14:07 +0900

On Fri, Jul 18, 2025 at 09:43:46AM +0900, Byungchul Park wrote:
> On Fri, Jul 18, 2025 at 01:42:38AM +0800, kernel test robot wrote:
> > Hi Byungchul,
> > 
> > kernel test robot noticed the following build warnings:
> > 
> > [auto build test WARNING on c65d34296b2252897e37835d6007bbd01b255742]
> > 
> > url:    https://github.com/intel-lab-lkp/linux/commits/Byungchul-Park/netmem-introduce-struct-netmem_desc-mirroring-struct-page/20250717-150253
> > base:   c65d34296b2252897e37835d6007bbd01b255742
> > patch link:    https://lore.kernel.org/r/20250717070052.6358-13-byungchul%40sk.com
> > patch subject: [Intel-wired-lan] [PATCH net-next v11 12/12] libeth: xdp: access ->pp through netmem_desc instead of page
> > config: arm-randconfig-r072-20250717 (https://download.01.org/0day-ci/archive/20250718/[email-masked]/config)
> > compiler: arm-linux-gnueabi-gcc (GCC) 8.5.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250718/[email-masked]/reproduce)
> > 
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <[email-masked]>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/[email-masked]/
> > 
> > All warnings (new ones prefixed by >>):
> > 
> >    In file included from include/linux/container_of.h:5,
> >                     from include/linux/list.h:5,
> >                     from include/linux/timer.h:5,
> >                     from include/linux/netdevice.h:24,
> >                     from include/trace/events/xdp.h:8,
> >                     from include/linux/bpf_trace.h:5,
> >                     from include/net/libeth/xdp.h:7,
> >                     from drivers/net/ethernet/intel/libeth/tx.c:6:
> >    include/net/libeth/xdp.h: In function 'libeth_xdp_prepare_buff':
> > >> include/net/libeth/xdp.h:1295:23: warning: passing argument 1 of 'page_pool_page_is_pp' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
> >         pp_page_to_nmdesc(page)->pp->p.offset, len, true);
> >                           ^~~~
> >    include/linux/build_bug.h:30:63: note: in definition of macro 'BUILD_BUG_ON_INVALID'
> >     #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
> >                                                                   ^
> >    include/net/netmem.h:301:2: note: in expansion of macro 'DEBUG_NET_WARN_ON_ONCE'
> >      DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(p));  \
> >      ^~~~~~~~~~~~~~~~~~~~~~
> >    include/net/libeth/xdp.h:1295:5: note: in expansion of macro 'pp_page_to_nmdesc'
> >         pp_page_to_nmdesc(page)->pp->p.offset, len, true);
> >         ^~~~~~~~~~~~~~~~~
> >    In file included from arch/arm/include/asm/cacheflush.h:10,
> >                     from include/linux/cacheflush.h:5,
> >                     from include/linux/highmem.h:8,
> >                     from include/linux/bvec.h:10,
> >                     from include/linux/skbuff.h:17,
> >                     from include/net/net_namespace.h:43,
> >                     from include/linux/netdevice.h:38,
> >                     from include/trace/events/xdp.h:8,
> >                     from include/linux/bpf_trace.h:5,
> >                     from include/net/libeth/xdp.h:7,
> >                     from drivers/net/ethernet/intel/libeth/tx.c:6:
> >    include/linux/mm.h:4176:54: note: expected 'struct page *' but argument is of type 'const struct page *'
> >     static inline bool page_pool_page_is_pp(struct page *page)
> >                                             ~~~~~~~~~~~~~^~~~
> 
> Oh.  page_pool_page_is_pp() in the mainline code already has this issue
> that the helper cannot take const struct page * as argument.
> 
> How should we resolve the issue?  Changing page_pool_page_is_pp() to
> macro and using _Generic again looks too much.  Or should we?  Any idea?

option 1. Remove 'const' on declaration of struct page in
	  libeth_xdp_prepare_buff() but maybe bad.

option 2. Use __pp_page_to_nmdesc() instead of pp_page_to_nmdesc() in
	  libeth_xdp_prepare_buff() to skip checking if it's a pp page.

option 3. Change page_pool_page_is_pp() to macro and use _Generic to
	  cover const casting.

More?

	Byungchul

> 	Byungchul
> 
> > vim +1295 include/net/libeth/xdp.h
> > 
> >   1263
> >   1264  bool libeth_xdp_buff_add_frag(struct libeth_xdp_buff *xdp,
> >   1265                                const struct libeth_fqe *fqe,
> >   1266                                u32 len);
> >   1267
> >   1268  /**
> >   1269   * libeth_xdp_prepare_buff - fill &libeth_xdp_buff with head FQE data
> >   1270   * @xdp: XDP buffer to attach the head to
> >   1271   * @fqe: FQE containing the head buffer
> >   1272   * @len: buffer len passed from HW
> >   1273   *
> >   1274   * Internal, use libeth_xdp_process_buff() instead. Initializes XDP buffer
> >   1275   * head with the Rx buffer data: data pointer, length, headroom, and
> >   1276   * truesize/tailroom. Zeroes the flags.
> >   1277   * Uses faster single u64 write instead of per-field access.
> >   1278   */
> >   1279  static inline void libeth_xdp_prepare_buff(struct libeth_xdp_buff *xdp,
> >   1280                                             const struct libeth_fqe *fqe,
> >   1281                                             u32 len)
> >   1282  {
> >   1283          const struct page *page = __netmem_to_page(fqe->netmem);
> >   1284
> >   1285  #ifdef __LIBETH_WORD_ACCESS
> >   1286          static_assert(offsetofend(typeof(xdp->base), flags) -
> >   1287                        offsetof(typeof(xdp->base), frame_sz) ==
> >   1288                        sizeof(u64));
> >   1289
> >   1290          *(u64 *)&xdp->base.frame_sz = fqe->truesize;
> >   1291  #else
> >   1292          xdp_init_buff(&xdp->base, fqe->truesize, xdp->base.rxq);
> >   1293  #endif
> >   1294          xdp_prepare_buff(&xdp->base, page_address(page) + fqe->offset,
> > > 1295                           pp_page_to_nmdesc(page)->pp->p.offset, len, true);
> >   1296  }
> >   1297
> > 
> > --
> > 0-DAY CI Kernel Test Service
> > https://github.com/intel/lkp-tests/wiki