Re: [PATCH net v1] net: stmmac: TSO: Fix unbalanced DMA map/unmap for non-paged SKB data

Thierry Reding
2024-12-04T18:18:50Z

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On Wed, Dec 04, 2024 at 05:45:43PM +0000, Russell King (Oracle) wrote:
> On Wed, Dec 04, 2024 at 05:02:19PM +0000, Jon Hunter wrote:
> > Hi Russell,
> >
> > On 04/12/2024 16:39, Russell King (Oracle) wrote:
> > > On Wed, Dec 04, 2024 at 04:58:34PM +0100, Thierry Reding wrote:
> > > > This doesn't match the location from earlier, but at least there's
> > > > something afoot here that needs fixing. I suppose this could simply be
> > > > hiding any subsequent errors, so once this is fixed we might see other
> > > > similar issues.
> > >
> > > Well, having a quick look at this, the first thing which stands out is:
> > >
> > > In stmmac_tx_clean(), we have:
> > >
> > > if (likely(tx_q->tx_skbuff_dma[entry].buf &&
> > > tx_q->tx_skbuff_dma[entry].buf_type != STMMAC_TXBUF_T
> > > _XDP_TX)) {
> > > if (tx_q->tx_skbuff_dma[entry].map_as_page)
> > > dma_unmap_page(priv->device,
> > > tx_q->tx_skbuff_dma[entry].buf,
> > > tx_q->tx_skbuff_dma[entry].len,
> > > DMA_TO_DEVICE);
> > > else
> > > dma_unmap_single(priv->device,
> > > tx_q->tx_skbuff_dma[entry].buf,
> > > tx_q->tx_skbuff_dma[entry].len,
> > > DMA_TO_DEVICE);
> > > tx_q->tx_skbuff_dma[entry].buf = 0;
> > > tx_q->tx_skbuff_dma[entry].len = 0;
> > > tx_q->tx_skbuff_dma[entry].map_as_page = false;
> > > }
> > >
> > > So, tx_skbuff_dma[entry].buf is expected to point appropriately to the
> > > DMA region.
> > >
> > > Now if we look at stmmac_tso_xmit():
> > >
> > > des = dma_map_single(priv->device, skb->data, skb_headlen(skb),
> > > DMA_TO_DEVICE);
> > > if (dma_mapping_error(priv->device, des))
> > > goto dma_map_err;
> > >
> > > if (priv->dma_cap.addr64 <= 32) {
> > > ...
> > > } else {
> > > ...
> > > des += proto_hdr_len;
> > > ...
> > > }
> > >
> > > tx_q->tx_skbuff_dma[tx_q->cur_tx].buf = des;
> > > tx_q->tx_skbuff_dma[tx_q->cur_tx].len = skb_headlen(skb);
> > > tx_q->tx_skbuff_dma[tx_q->cur_tx].map_as_page = false;
> > > tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
> > >
> > > This will result in stmmac_tx_clean() calling dma_unmap_single() using
> > > "des" and "skb_headlen(skb)" as the buffer start and length.
> > >
> > > One of the requirements of the DMA mapping API is that the DMA handle
> > > returned by the map operation will be passed into the unmap function.
> > > Not something that was offset. The length will also be the same.
> > >
> > > We can clearly see above that there is a case where the DMA handle has
> > > been offset by proto_hdr_len, and when this is so, the value that is
> > > passed into the unmap operation no longer matches this requirement.
> > >
> > > So, a question to the reporter - what is the value of
> > > priv->dma_cap.addr64 in your failing case? You should see the value
> > > in the "Using %d/%d bits DMA host/device width" kernel message.
> >
> > It is ...
> >
> > dwc-eth-dwmac 2490000.ethernet: Using 40/40 bits DMA host/device width
>
> So yes, "des" is being offset, which will upset the unmap operation.
> Please try the following patch, thanks:
>
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 9b262cdad60b..c81ea8cdfe6e 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -4192,8 +4192,8 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
> struct stmmac_txq_stats *txq_stats;
> struct stmmac_tx_queue *tx_q;
> u32 pay_len, mss, queue;
> + dma_addr_t tso_des, des;
> u8 proto_hdr_len, hdr;
> - dma_addr_t des;
> bool set_ic;
> int i;
>
> @@ -4289,14 +4289,15 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
>
> /* If needed take extra descriptors to fill the remaining payload */
> tmp_pay_len = pay_len - TSO_MAX_BUFF_SIZE;
> + tso_des = des;
> } else {
> stmmac_set_desc_addr(priv, first, des);
> tmp_pay_len = pay_len;
> - des += proto_hdr_len;
> + tso_des = des + proto_hdr_len;
> pay_len = 0;
> }
>
> - stmmac_tso_allocator(priv, des, tmp_pay_len, (nfrags == 0), queue);
> + stmmac_tso_allocator(priv, tso_des, tmp_pay_len, (nfrags == 0), queue);
>
> /* In case two or more DMA transmit descriptors are allocated for this
> * non-paged SKB data, the DMA buffer address should be saved to

I see, that makes sense. Looks like this has been broken for a few years
(since commit 34c15202896d ("net: stmmac: Fix the problem of tso_xmit"))
and Furong's patch ended up exposing it.

Anyway, this seems to fix it for me. I can usually trigger the issue
within one or two iperf runs, with your patch I haven't seen it break
after a dozen or so runs.

It may be good to have Jon's test results as well, but looks good so
far.

Thanks!
Thierry

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