Re: [PATCH net v1] net: netmem: fix skb_ensure_writable with unreadable skbs

[Mina Almasry]

On Mon, Jun 16, 2025 at 7:38 AM Stanislav Fomichev <stfomichev@xxxxxxxxx> wrote:
>
> On 06/15, Mina Almasry wrote:
> > skb_ensure_writable should succeed when it's trying to write to the
> > header of the unreadable skbs, so it doesn't need an unconditional
> > skb_frags_readable check. The preceding pskb_may_pull() call will
> > succeed if write_len is within the head and fail if we're trying to
> > write to the unreadable payload, so we don't need an additional check.
> >
> > Removing this check restores DSCP functionality with unreadable skbs as
> > it's called from dscp_tg.
>
> Can you share more info on which use-case (or which call sites) you're
> trying to fix?

Hi Stan,

It's the use case of setting a DSCP header, and the call site is
dscp_tg() -> skb_ensure_writable.

Repro steps should roughly be:

# Set DSCP header
sudo iptables -tmangle -A POSTROUTING -p tcp -m comment --comment
"foo" -j DSCP --set-dscp 0x08

# then run some unreadable netmem workload.

Before this change you should see 0 throughput, after this change the
unreadable netmem workload should work as expected.

-- 
Thanks,
Mina