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Thu, 14 May 2026 17:29:54 -0700 (PDT) Date: Thu, 14 May 2026 17:29:54 -0700 From: Stanislav Fomichev To: Jason Xing Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, horms@kernel.org, andrew+netdev@lunn.ch, bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: Re: [PATCH net 3/4] xsk: drain continuation descs after overflow in xsk_build_skb() Message-ID: References: <20260510012310.88570-1-kerneljasonxing@gmail.com> <20260510012310.88570-4-kerneljasonxing@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 05/14, Jason Xing wrote: > On Thu, May 14, 2026 at 12:27 AM Stanislav Fomichev > wrote: > > > > On 05/10, Jason Xing wrote: > > > From: Jason Xing > > > > > > When a multi-buffer packet exceeds MAX_SKB_FRAGS and triggers -EOVERFLOW, > > > only the current descriptor is released from the TX ring. The remaining > > > continuation descriptors of the same packet stay in the ring. Since > > > xs->skb is set to NULL after the drop, the TX loop picks up these > > > leftover frags and misinterprets each one as the beginning of a new > > > packet, corrupting the packet stream. > > > > > > Fix this by adding a drain_cont flag to xdp_sock. When overflow occurs > > > and the dropped descriptor has XDP_PKT_CONTD set, the flag is raised. > > > The main TX loop in __xsk_generic_xmit() then handles continuation > > > descriptors one at a time: each gets a normal CQ reservation (with > > > backpressure), its address is submitted to the completion queue, and > > > the descriptor is released from the TX ring. When the last fragment > > > (without XDP_PKT_CONTD) is processed, the flag is cleared and the > > > function returns -EOVERFLOW so the next call starts with a fresh > > > budget for normal packets. > > > > > > This reuses the existing CQ backpressure and budget mechanisms, so if > > > the CQ is full the function returns -EAGAIN and userspace drains the > > > CQ before retrying. Zero buffer leakage, zero packet stream corruption. > > > > > > Closes: https://lore.kernel.org/all/20260425041726.85FB3C2BCB2@smtp.kernel.org/ > > > Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path") > > > Signed-off-by: Jason Xing > > > --- > > > include/net/xdp_sock.h | 1 + > > > net/xdp/xsk.c | 22 ++++++++++++++++++++++ > > > 2 files changed, 23 insertions(+) > > > > > > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h > > > index 23e8861e8b25..1958d19d9925 100644 > > > --- a/include/net/xdp_sock.h > > > +++ b/include/net/xdp_sock.h > > > @@ -80,6 +80,7 @@ struct xdp_sock { > > > * call of __xsk_generic_xmit(). > > > */ > > > struct sk_buff *skb; > > > + bool drain_cont; > > > > > > struct list_head map_list; > > > /* Protects map_list */ > > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c > > > index 3f1e590c855d..232dd7126905 100644 > > > --- a/net/xdp/xsk.c > > > +++ b/net/xdp/xsk.c > > > @@ -936,6 +936,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs, > > > xs->tx->invalid_descs++; > > > } > > > xskq_cons_release(xs->tx); > > > + if (xp_mb_desc(desc)) > > > + xs->drain_cont = true; > > > } else { > > > /* Let application retry */ > > > xsk_cq_cancel_locked(xs->pool, 1); > > > @@ -982,6 +984,26 @@ static int __xsk_generic_xmit(struct sock *sk) > > > goto out; > > > } > > > > > > + if (unlikely(xs->drain_cont)) { > > > + unsigned long flags; > > > + u32 idx; > > > + > > > > [..] > > > > > + spin_lock_irqsave(&xs->pool->cq_prod_lock, flags); > > > + idx = xskq_get_prod(xs->pool->cq); > > > + xskq_prod_write_addr(xs->pool->cq, idx, desc.addr); > > > + xskq_prod_submit_n(xs->pool->cq, 1); > > > + spin_unlock_irqrestore(&xs->pool->cq_prod_lock, flags); > > > > Not sure I understand why you want this if you're still marking the desc > > invalid. > > The key point is that as long as we read the desc from txq, the desc > should be either put back to txq again or publish it in the cq (for > application to keep track of this) at this point. Or else, the > application will lose track of this desc, which breaks the whole > logic. Yes, makes sense! > > > + > > > + xs->tx->invalid_descs++; > > > + xskq_cons_release(xs->tx); > > > + if (!xp_mb_desc(&desc)) { > > > + xs->drain_cont = false; > > > + err = -EOVERFLOW; > > > + goto out; > > > + } > > > > I also don't understand why you want to return -EOVERFLOW again? Why not > > (quietly) swallow these invalid xp_mb_desc from the previous packet and move > > on? > > This particular desc is really one of overflow cases, right? We should > warn users to handle this with this error code. Right, but didn't we already return -EOVERFLOW to the user once? The part where you set drain_count=true will -EOVERFLOW. Then this remainder will also return -EOVERFLOW? But let's maybe step back, what's the expectation on the user when we return -EOVERFLOW? In theory, the user can re-submit new shorter packet (at the current prod index), right? And then your !xp_mb_desc logic will break/swallow/-EOVERFLOW it? > > > > Also, have you considered doing this drain in xsk_build_skb error path? > > Is it more messy? Carrying this drain_cont across the calls seems a bit > > too complicated (but idk, maybe the alternative drain in xsk_build_skb > > is even messier). > > Sure, it was my first version actually. One of the primary reasons is > how to handle it gracefully when CQ is full? Then those descriptors > can not be processed in this loop at this time and will be delayed > until CQ has available slots. Another important reason is that I want > to put the whole read process under the previous clear logic in > __xsk_generic_xmit(). Some protections like budget/batch still take > effect in this case. In this way, there is no strange/exceptional read > process in copy mode :) Agreed, given the fact that we need to handle prod backpressure, your approach is good (but see above comment, we should probably clarify what's the expectation on the user side).