Re: [PATCH net-next v4 0/3] net: devmem: allow rx-buf-size > PAGE_SIZE per binding

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Tue, Jul 07, 2026 at 12:24:21PM -0700, Mina Almasry wrote:
> (I'm kinda reviewing this very late here. Some suggestions/comments
> but feel free to ignore if not useful).
> 
> On Wed, Jul 1, 2026 at 12:22 PM Bobby Eshleman <bobbyeshleman@gmail.com> wrote:
> >
> > Every devmem dmabuf binding hands the page_pool PAGE_SIZE niovs today.
> > On NICs that consume one descriptor per netmem, this caps a single RX
> > descriptor at PAGE_SIZE and burns CPU on buffer churn.
> >
> > In this series, we add a bind-time netlink attribute,
> > NETDEV_A_DMABUF_RX_BUF_SIZE, that lets userspace request a larger niov size
> > (power of two >= PAGE_SIZE).
> 
> FWIW we may be able to support arbitrary sizes with devmem. Because
> the genpool supports byte-aligned allocations AFAIR. Also the
> dma-mapping happens with the dma-buf size, so the actual niov size
> doesn't matter. The only thing I can think off which may not be
> flexible to arbitrary sizes is the driver itself. IDK what happens if
> you ask the driver to dma into a buffer that is frag size 5023 or
> something like that.
> 
> But that is something that can be relaxed in the future.


I think at least for mlx5 there would be some issues, as it splits the
memory region into fixed-size strides (256B), so I'd expect it needs to
at least be divisible by the stride length. The mlx5 driver seems to
guard against this by checking for sz > PAGE_SIZE && is_power_of_2.

> 
> > Drivers must opt in via
> > queue_mgmt_ops.QCFG_RX_PAGE_SIZE.
> >
> 
> nit that probably doesn't matter: ...QCFG_RX_NETMEM_SIZE, or
> (...NIOV_SIZE). This doesn't actually work with pages, right?

I probably could have worded this in the message more clearly, but this
name is not introduced by this series, so we probably can't get away
with changing it.

> 
> If you decide to extend to arbrary sizes, I would add to the
> queue_mgmt ops supports_netmem_size(size_t size) function, and let the
> driver enforce "it has to be power of 2" if it needs to. AFAICT core
> doesn't need to.
> 
> > Selftests use udmabuf, but udmabuf sgtables were previously hardcoded to
> > PAGE_SIZE. This series modifies udmabuf to respect folio sizes in its exported
> > sgtable. The result is that when backing udmabuf with MFD_HUGETLB 2MB pages,
> > the sgtable is populated with 2MB entries, allowing devmem's gen_pool to carve
> > out large (eg. 64K) niovs.
> >
> > Measurements
> > ------------
> >
> > Setup: kperf devmem RX/TX cuda, 4 flows, 64 MB messages, 60s, dctcp,
> > num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov size,
> > mlx5.
> >
> >    niov       RX dev Gbps   RX flow avg Gbps         app sys %
> >   -----  ----------------  -----------------  ----------------
> >      4K  300.63 +/- 53.21    75.16 +/- 13.30   54.15 +/- 10.23
> >     16K  321.35 +/- 28.20    80.34 +/-  7.05   41.05 +/-  8.87
> >     32K  347.63 +/-  2.20    86.91 +/-  0.55   44.54 +/-  3.51
> >     64K  332.11 +/- 14.26    83.03 +/-  3.56   35.47 +/-  3.11
> >
> > RX app sys % drops ~19% from 4K to 64K.
> >
> 
> Hard to read the columns for me but seems like good perf data. Did
> performance become worse from 32K to 64K? I wonder why.

The drop off struck my eye too, but didn't investigate further. Given
the wide stdev, it appears to me like the trend is positive but probably
not huge. The cpu util deltas, on the other hand, look stronger to me.

> 
> I have some devmem performance fixes that are very critical for our
> production that I haven't gotten around to upstreaming yet. I wonder
> if I can send them to you for upstream submission. Are you potentially
> interested?

Definitely interested!

> 
> -- 
> Thanks,
> Mina


Thanks Mina.

Best,
Bobby