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

[Stanislav Fomichev]

On 11/02, Mina Almasry wrote:
> 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.

SG, let me try to explain it better in the commit msg. I don't think
we can charge 0 because the uncharge path is non-zero. It currently
works ok because skb frag uncharges the small error that can accumulate.