Re: [PATCH net v2] net: axienet: bound TX completion cleanup by the NAPI budget

[Sagi Maimon]

On Mon, Sep 21, 2026 at 2:57 PM <netdev-bot+sashiko@xxxxxxxxxx> wrote:
>
> Thank you for your contribution! Sashiko AI review found 1 potential
> issue(s) to consider.
>
> Critical: 0 · High: 1 · Medium: 0 (1) · Low: 0
>
> If you believe none of the issues are legitimate, please reply to each
> issue of Medium severity or higher and explain why the AI is wrong.
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>
> - [High] Incomplete fix: the new budget guard in axienet_free_tx_chain()
>   is disabled when @budget == 0 (`if (!force && budget && packets >=…
>
> Pre-existing issues:
> - [Medium] Pre-existing (not introduced or claimed to be fixed by this
>   patch): axienet_dma_bd_release()…
>
> --
>
> net: axienet: bound TX completion cleanup by the NAPI budget
>
> The patch adds a budget check to axienet_free_tx_chain() so the TX
> completion loop stops on a packet boundary once the NAPI budget is spent,
> and treats a budget of zero as "no limit" so the netpoll TX reclaim and the
> @force callers still drain the ring.  The kernel-doc for @budget is updated
> to match.
>
> > diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > index 1722b7038f340..039885814a5d2 100644
> > --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > @@ -772,7 +772,9 @@ static int axienet_device_reset(struct net_device *ndev)
> >   * @force:   Whether to clean descriptors even if not complete
> >   * @sizep:   Pointer to a u32 accumulating the total byte count of
> >   *           completed packets (using skb->len). Ignored if NULL.
> > - * @budget:  NAPI budget (use 0 when not called from NAPI poll)
> > + * @budget:  NAPI budget. When @force is false, cleanup stops after this
> > + *           many completed packets. Zero means no limit, as used by the
> > + *           netpoll TX reclaim and by callers outside NAPI poll.
> >   *
> >   * Would either be called after a successful transmit operation, or after
> >   * there was an error when setting up the chain.
> > @@ -788,6 +790,15 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
> >       dma_addr_t phys;
> >
> >       for (i = 0; i < nr_bds; i++) {
> > +             /* A NAPI poll must not return more than its budget.  Stop on a
> > +              * packet boundary once it is spent - cur_p->skb is only set on
> > +              * a packet's last descriptor, so no packet is left half-freed.
> > +              * A zero budget means no limit: netpoll polls with a budget of
> > +              * 0 to reclaim the TX path only, and must still clean the ring.
> > +              */
> > +             if (!force && budget && packets >= budget)
> > +                     break;
> > +
>
> [Severity: High]
> With the "budget &&" term, is the case the commit message is about still
> reachable on the netpoll path?

Yes.  For a budget of 0 the loop drains the ring and axienet_tx_poll()
then returns the reclaimed count, which can trip the WARN_ONCE() in
poll_one_napi().  That was already reachable before this patch, while the
loop was unbounded, but since the patch is about honouring the budget it
should cover this case as well.  v3 returns "budget ? packets : 0" from
axienet_tx_poll() as you suggest, keeping the accounting and the queue
wake.

>
> For a budget of 0 the guard never fires, so axienet_free_tx_chain() walks up
> to lp->tx_bd_num descriptors and increments packets for every completed
> descriptor with a non-NULL cur_p->skb.  axienet_tx_poll() then returns that
> count verbatim:
>
> drivers/net/ethernet/xilinx/xilinx_axienet_main.c:axienet_tx_poll() {
>         packets = axienet_free_tx_chain(lp, lp->tx_bd_ci, lp->tx_bd_num, false,
>                                         &size, budget);
>         ...
>         if (packets < budget && napi_complete_done(napi, packets)) {
>         ...
>         return packets;
> }
>
> netpoll is the caller that enforces the invariant:
>
> net/core/netpoll.c:poll_one_napi() {
>         /* We explicitly pass the polling call a budget of 0 to
>          * indicate that we are clearing the Tx path only.
>          */
>         work = napi->poll(napi, 0);
>         WARN_ONCE(work, "%pS exceeded budget in poll\n", napi->poll);
>         ...
> }
>
> lp->napi_tx is on dev->napi_list (netif_napi_add(ndev, &lp->napi_tx,
> axienet_tx_poll) in axienet_probe()), so netpoll_poll_dev()->poll_napi()->
> poll_one_napi() reaches axienet_tx_poll() with budget 0.  If netconsole or
> kgdboe is active on the interface and at least one TX descriptor has
> XAXIDMA_BD_STS_COMPLETE_MASK set with cur_p->skb != NULL, doesn't that poll
> return non-zero work for a budget of 0 and trip the WARN_ONCE?
>
> The "packets < budget" test is also false for budget 0, so
> napi_complete_done() is skipped on that pass, which is the secondary symptom
> the changelog describes.
>
> Would it work better to keep the unlimited reclaim for budget 0 (it is
> already bounded by lp->tx_bd_num) but have axienet_tx_poll() report
> "budget ? packets : 0", while still doing the byte/packet accounting?
>
> The new kernel-doc says zero means "no limit, as used by the netpoll TX
> reclaim".  netpoll asks for two things: drain the TX path, and report zero
> work.  Could the kernel-doc and the in-loop comment mention both?

Will do.

>
> One more nit on the changelog:
>
>   "Zero therefore means no limit, which also covers the @force callers that
>   clean up after a DMA mapping failure with a budget of 0."
>
> The guard is already gated on !force, so the @force callers are unaffected by
> the value of @budget here.

Right, that sentence is wrong.  v3 drops it.

>
> [Severity: Medium]
> This isn't a bug introduced by this patch, but while looking at the TX
> reclaim path: does axienet_dma_bd_release() leak the outstanding TX skbs and
> their streaming DMA mappings?

Yes.  It is independent of this patch, so I will send it as a separate
fix rather than fold it in.
v3 follows.  It passes traffic on hardware; the over-budget and netpoll
cases themselves were not exercised.

>
> The RX side walks the ring and cleans up:
>
> drivers/net/ethernet/xilinx/xilinx_axienet_main.c:axienet_dma_bd_release() {
>         for (i = 0; i < lp->rx_bd_num; i++) {
>                 ...
>                 dev_kfree_skb(lp->rx_bd_v[i].skb);
>                 ...
>                 dma_unmap_single(lp->dev, phys, lp->max_frm_size, DMA_FROM_DEVICE);
>         }
>         ...
>         /* If we end up here, tx_bd_v must have been DMA allocated. */
>         dma_free_coherent(lp->dev,
>                           sizeof(*lp->tx_bd_v) * lp->tx_bd_num,
>                           lp->tx_bd_v,
>                           lp->tx_bd_p);
> }
>
> The TX ring is freed with no equivalent pass over lp->tx_bd_v, so descriptors
> between lp->tx_bd_ci and lp->tx_bd_tail keep their cur_p->skb and their
> dma_map_single() mapping.
>
> axienet_stop() disables TX reclamation before the release, so nothing can run
> afterwards:
>
>         napi_disable(&lp->napi_tx);
>         ...
>         axienet_dma_stop(lp);
>         cancel_work_sync(&lp->dma_err_task);
>         free_irq(lp->tx_irq, ndev);
>         free_irq(lp->rx_irq, ndev);
>         axienet_dma_bd_release(ndev);
>
> So an "ip link set down" or a driver unbind with frames in flight would lose
> up to lp->tx_bd_num skbs and mappings.  The budget cap added here can leave
> completed-but-unreclaimed descriptors around for a later poll, which slightly
> widens the window, but the leak is independent of this change.
>
> --
> Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260917115657.20697-1-maimon.sagi%40gmail.com