Re: [PATCH net-next v3 4/9] net: devmem: add a per-device netmem TX frag size limit

[Stanislav Fomichev <sdf.kernel@gmail.com>]

> > @@ -734,6 +734,7 @@ zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,
> >  
> >  		size = min_t(size_t, size, length);
> >  		size = min_t(size_t, size, iter_iov_len(from));
> > +		size = min_t(size_t, size, binding->tx_max_frag_size);
> 
> [Severity: Medium]
> Is this the only place the new cap is enforced? The netmem.rst text added
> by this patch says "Devmem TX frags are capped at
> netdev->netmem_tx_max_frag_size". TCP MTU probing looks like it can merge
> the capped frags back together later.

[..]

> Could tcp_clone_payload() compare skb_frag_netmem() and stop merging once
> the binding cap is reached? Another option is to skip MTU probe
> coalescing for unreadable skbs.

Hmm, let me comment on Mina's patches, might be easier not to merge
netmems in tcp_clone_payload at all?

> [Severity: High]
> This isn't a bug introduced by this patch, but the same merge check in
> tcp_clone_payload() looks like it can also combine frags from different
> net_iovs. skb_frag_page() returns NULL for both frags, so the page
> comparison always passes and only the offsets are compared.

[..]

> Should the merge test compare skb_frag_netmem(), the way
> skb_can_coalesce_netmem() does? Nothing later in the series changes
> tcp_output.c.

Mina is gonna take care of this:
https://lore.kernel.org/netdev/20261108013838.1454450-3-almasrymina@google.com/