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

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Mon, Jul 20, 2026 at 02:15:42PM -0700, Mina Almasry wrote:
> On Wed, Jul 8, 2026 at 3:55 PM Bobby Eshleman <bobbyeshleman@gmail.com> wrote:
> >
> > From: Bobby Eshleman <bobbyeshleman@meta.com>
> >
> > 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@meta.com>
> > Acked-by: Stanislav Fomichev <sdf@fomichev.me>
> 
> Reviewed-by: Mina Almasry <almasrymina@google.com>
> 
> 
> > ---
> >  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.

If I follow you correctly, in the case of sg len == 4K, and requested
size of 64K, it will fail the !IS_ALIGNED(len=4k, niov_size=64K) check
and return -EINVAL.

> 
> --
> Thanks,
> Mina

Thanks for the review Mina!

Best,
Bobby