On Fri, 2017-01-27 at 17:24 +0000, David Laight wrote: > 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?
We try hard to pre-allocate enough headroom. Because copies are expensive. Look for MAX_HEADER, MAX_TCP_HEADER, and things like that. For the tail, we add 128 bytes of extra tail when __pskb_pull_tail() wants to expand skb->head. If you believe you found a use case where we do stupid reallocations, please fix the caller. pskb_expand_head() should never be called, really. Only for some pathological/malicious traffic we have to, eventually.
