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

[Jon Hunter <jonathanh@nvidia.com>]

On 05/12/2024 10:57, Jon Hunter wrote:
> 
> On 04/12/2024 18:18, Thierry Reding wrote:
>> 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.
> 
> 
> I have been running tests on my side and so far so good too. I have not 
> seen any more mapping failure cases.
> 
> Russell, if you are planning to send a fix for this, please add my ...
> 
> Tested-by: Jon Hunter <jonathanh@nvidia.com>

Nevermind I see Furong already sent a fix.

Jon

-- 
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