Re: [PATCH net-next 2/7] net: devmem: charge the frag length to TX skb truesize

[Mina Almasry]

On Mon, Nov 2, 2026 at 11:14 AM Stanislav Fomichev <sdf.kernel@xxxxxxxxx> wrote:
>
> truesize isn't well defined for devmem TX frags: they point into a
> dmabuf which stays mapped for the lifetime of the binding, and freeing
> the skb releases no memory. The charge only feeds sk_sndbuf and TSQ,
> which limit queued bytes.
>
> Charge the frag length instead of PAGE_SIZE:
> - TCP uncharges in bytes (tcp_trim_head(), tcp_fragment(),
>   tso_fragment()), so the charge must not be smaller than the frag.
>   PAGE_SIZE only guarantees that while net_iovs are PAGE_SIZE.
> - Frags shorter than a page now count their length, not a page.
>

I don't know about this patch. The justification reads a bit like.
"net-iov charging is weird and broken anyway, so we can do whatever we
want. Charge size instead of PAGE_SIZE for no reason."

I agree that charging for net-iov is extremely weird. Neither of
today's net-iov types actually allocates any memory. Of course when I
was developing the original infra, I took every shortcut I could and
just charged net-iov as regular memory, but that doesn't mean we can
do whatever we want.

I think either:

1. say net-iovs are not really occupied memory, so do a global cleanup
that charges 0 for all net-iovs (optionally making it conditional on
the net-iov type, so in the future someone can easily add a net-iov
that actually can be charged)", or
2. change the net-iov size to be actually size instead of PAGE_SIZE,
and keep the current 'we charge net-iovs even though they're not
really memory', and then this patch makes sense.

But I welcome disagreements.

-- 
Thanks,
Mina