netdev - AW: AW: AW: PROBLEM: Kernel Oops in UDP stack

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Message-ID: <936fbc1f39424adea318bcd95f50d63d@ZCOM03.mut-group.com>
Date:   Fri, 3 Aug 2018 08:24:16 +0000
From:   Marcel Hellwig <mhellwig@...-group.com>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        David Laight <David.Laight@...LAB.COM>,
        "'davem@...emloft.net'" <davem@...emloft.net>,
        "'kuznet@....inr.ac.ru'" <kuznet@....inr.ac.ru>,
        "'yoshfuji@...ux-ipv6.org'" <yoshfuji@...ux-ipv6.org>,
        "'andrew@...n.ch'" <andrew@...n.ch>
CC:     "'netdev@...r.kernel.org'" <netdev@...r.kernel.org>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "Matthias Wystrik" <mwystrik@...-group.com>
Subject: AW: AW: AW: PROBLEM: Kernel Oops in UDP stack

>
>Well, this driver does not use NET_IP_ALIGN reservation, meaning IP header is not 4-byte aligned.
>
>No idea why mis-alignments are okay in IP layer, but not in UDP
>
>You could try to patch it to use netdev_alloc_skb_ip_align() instead of dev_alloc_skb()

Looks very promising! CPU runs for 3 hours and no panic so far. Let's hope it survives the weekend! *fingers crossed*
The strange thing I don't understand is, why does the 3.5.7 kernel does not crash (or 4.17.12, yes I managed to run a recent kernel on our machine! \o/ )? It does not use netdev_alloc_skb_ip_align either[0][1], but one thing, that I noticed it, that there is a difference in /proc/iomem

3.4.113
# cat /proc/iomem
08000000-0801ffff : lpc-eth.0
31060000-31060fff : lpc-eth.0
40088000-4008801f : serial
[...]

3.5.7
# cat /proc/iomem
20084000-20084fff : /ahb/apb/ssp@...84000
2008c000-2008cfff : /ahb/apb/ssp@...8c000
31000000-31000fff : /ahb/dma@...00000
40088000-4008801f : serial
[...]

As you notice lpc-eth.0 (or 31060000.ethernet as it's called nowadays) is not listed, so my guess it is served by DMA?

May that the reason why it does not crash (Notice: 3.5.7 runs via device tree file)?

So just my thought: If I would disable dma (somehow) would the error still occur in today's kernel?

[0]: https://elixir.bootlin.com/linux/v3.5.7/source/drivers/net/ethernet/nxp/lpc_eth.c#L1003
[1]: https://elixir.bootlin.com/linux/v4.17.12/source/drivers/net/ethernet/nxp/lpc_eth.c#L957

Mit freundlichen Grüßen / With kind regards 

Marcel Hellwig
B. Sc. Informatik
Entwickler

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