Re: [PATCH net-next v1 03/11] xdp: Copy buffers with restricted lifetime

[Mina Almasry]

On Tue, Nov 10, 2026 at 2:21 AM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> UMEM stays mapped writable in userspace, and it has a fixed number of
> chunks. The stack must not keep its chunks in an skb or an xdp_frame.
> Mark the providers whose buffers the stack may keep with
> MP_CAP_RETAIN, and copy packets of the others on XDP_PASS, on
> conversion to a frame, and on redirects other than XSKMAP.
>

I'm not understanding why that needs special handling. Devmem stays
mapped and writeable and did not need MP_CAP_RETAIN. Also, today the
stack can keep any pp page in the socket's receive queue for arbitrary
time without _RETAIN. I'm espcially confused you're not adding _RETAIN
as a cap to existing providers, but their netmems can be retained.

> The head chunk is writable by userspace, so it cannot hold
> skb_shared_info, and virt_to_netmem() cannot find a net_iov head.
> Drivers pass kernel memory for both to xdp_init_buff_netmem().
>
> A static key keeps the check off while no such page pool exists. A
> frame copy is linear, so a multi-buffer packet of such a provider
> cannot go to a device or CPU map.
>

Did this actually improve perf? I recall that when I benchmarked the
performance improvement, it was questionable.

-- 
Thanks,
Mina