On Thursday 03 February 2005 05:47, Shane Hathaway wrote:
The attached patch enhances the kernel's DHCP client support (in net/ipv4/ipconfig.c) to set the interface MTU if provided by the DHCP server. Without this patch, it's difficult to netboot on a network that uses jumbo frames. The patch is based on 2.6.10, but I'll update it to the latest testing kernel if that would expedite its inclusion in the kernel.
Well, I've been there before, and asked for exact the same back in June 2003, but had much less luck, nobody of kernel fame even responded:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105624464918574&w=4
I wish I had found your patch before I went to the trouble of writing my own! Yours is just as good as mine.
For what is worth it, I ported my patch to current 2.6, which raised some comments compared to yours:
- Is it really necessary to protect the dev_set_mtu call, since it is just setting up the device?
Without rtnl_shlock(), something complains about RTNL not being locked. I don't know much beyond that.
- I prefer to call dev_set_mtu only, if a change mtu request is
sent..
Yes, I can see that. Either way is fine by me.
- Are you sure, you got the endianess right?
On the MTU parameter? Yes, it's network byte order, big-endian.
Here's the "cost": ipconfig.o without my patch on x86:
3 .init.data 0000005a 00000000 00000000 00000220 2**2 CONTENTS, ALLOC, LOAD, RELOC, DATA 4 .rodata.str1.1 000001a2 00000000 00000000 0000027a 2**0 CONTENTS, ALLOC, LOAD, READONLY, DATA 5 .rodata.str1.4 000003ad 00000000 00000000 0000041c 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA 6 .init.text 00001a45 00000000 00000000 000007d0 2**4 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
With patch:
3 .init.data 0000005e 00000000 00000000 00000220 2**2 CONTENTS, ALLOC, LOAD, RELOC, DATA 4 .rodata.str1.1 000001ab 00000000 00000000 0000027e 2**0 CONTENTS, ALLOC, LOAD, READONLY, DATA 5 .rodata.str1.4 000003e5 00000000 00000000 0000042c 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA 6 .init.text 00001ab5 00000000 00000000 00000820 2**4 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
Difference: 181 Bytes (padding ignored)
The whole module takes about 9K, compared to dhcp in initrd, which takes a few hundred K! Hmm.
It's probably better to compare your patch with its apparent successor, however. The tiny DHCP client in the klibc package already supports setting the MTU.
May the linux gods indulge on this topic one day or remove the ipconfig module completely.
A friend of mine just had the misfortune of running into the exact same problem, but then he had the fortune of finding your patch. So at least the curse has a temporary remedy. :-) The long-term solution is klibc, I hope. klibc in initramfs could ease a lot of pain.
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