From: Eric Dumazet
> Sent: 27 January 2017 14:44
...
> > I'm also guessing that extra headroom can be generated by stealing unused
> > tailroom.
>
> This is already done.
>
> Quoting
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=87fb4b7b533073eeeaed0b6bf7c
> 2328995f6c075
>
> At skb alloc phase, we put skb_shared_info struct at the exact end of
> skb head, to allow a better use of memory (lowering number of
> reallocations), since kmalloc() gives us power-of-two memory blocks.
Does that actually have the expected effect?
Allocate an skb for 512 bytes, copy in some data with 64 bytes of headroom.
This is (probably) a 1k memory block with skb_shared_info at the end.
Some code needs to add a header that doesn't fit, calls pskb_expand_head()
to get another 4 bytes.
Since the existing amount of 'tailroom' must be kept kmalloc(1024+4) is called.
This allocates a 2k memory block, again skb_shared_info is put at the end.
So the headroom has been increased by 4 bytes and the tailroom by 1020.
Another layer needs to add another header.
The memory block becomes 4k large.
What have I missed?
David
