Re: [PATCH net-next v5 2/3] selftests/net: ncdevmem: add -b option to set rx-buf-size on bind

[Mina Almasry]

On Wed, Jul 8, 2026 at 3:55 PM Bobby Eshleman <bobbyeshleman@xxxxxxxxx> wrote:
>
> From: Bobby Eshleman <bobbyeshleman@xxxxxxxx>
>
> Add -b <bytes> to request a non-default niov size via
> NETDEV_A_DMABUF_RX_BUF_SIZE. When the value exceeds PAGE_SIZE,
> udmabuf_alloc() switches to an MFD_HUGETLB-backed memfd so each 2 MB
> hugepage produces one naturally-aligned sg entry.
>
> Signed-off-by: Bobby Eshleman <bobbyeshleman@xxxxxxxx>
> Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>

Reviewed-by: Mina Almasry <almasrymina@xxxxxxxxxx>


> ---
>  tools/testing/selftests/drivers/net/hw/ncdevmem.c | 36 +++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/drivers/net/hw/ncdevmem.c b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> index d96e8a3b5a65..a16e55af51ee 100644
> --- a/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> +++ b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> @@ -40,6 +40,7 @@
>
>  #include <linux/uio.h>
>  #include <stdarg.h>
> +#include <stdint.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <unistd.h>
> @@ -61,6 +62,7 @@
>  #include <sys/time.h>
>
>  #include <linux/memfd.h>
> +#include <sys/param.h>
>  #include <linux/dma-buf.h>
>  #include <linux/errqueue.h>
>  #include <linux/udmabuf.h>
> @@ -79,6 +81,7 @@
>  #define PAGE_SHIFT 12
>  #define TEST_PREFIX "ncdevmem"
>  #define NUM_PAGES 16000
> +#define MB(x) ((x) << 20)
>
>  #ifndef MSG_SOCK_DEVMEM
>  #define MSG_SOCK_DEVMEM 0x2000000
> @@ -100,6 +103,7 @@ static unsigned int dmabuf_id;
>  static uint32_t tx_dmabuf_id;
>  static int waittime_ms = 500;
>  static bool fail_on_linear;
> +static uint32_t rx_buf_size;
>
>  /* System state loaded by current_config_load() */
>  #define MAX_FLOWS      8
> @@ -142,6 +146,7 @@ static struct memory_buffer *udmabuf_alloc(size_t size)
>  {
>         struct udmabuf_create create;
>         struct memory_buffer *ctx;
> +       unsigned int memfd_flags;
>         int ret;
>
>         ctx = malloc(sizeof(*ctx));
> @@ -156,9 +161,14 @@ static struct memory_buffer *udmabuf_alloc(size_t size)
>                 goto err_free_ctx;
>         }
>
> -       ctx->memfd = memfd_create("udmabuf-test", MFD_ALLOW_SEALING);
> +       memfd_flags = MFD_ALLOW_SEALING;
> +       if (rx_buf_size > getpagesize())
> +               memfd_flags |= MFD_HUGETLB | MFD_HUGE_2MB;
> +

The fact that you are using HUGETLM and 2MB mappings here made me
realize there is a pathological edge case in the code where the netmem
size you're requesting is greater than the mapping size, so you
actually get no netmems. So like if you ask for a 64KB netmem size but
actually you did a normal udambuf mapping and all the maps (sg len
entries) are 4K or something. IDK if the code already handles this
well with an error or what not. Worth checking.

--
Thanks,
Mina