Re: [PATCH] net: allow configuration of the size of page in __netdev_alloc_frag

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2012-10-24 at 12:42 +0100, Ian Campbell wrote:
> The commit 69b08f62e174 "net: use bigger pages in __netdev_alloc_frag"
> lead to 70%+ packet loss under Xen when transmitting from physical (as
> opposed to virtual) network devices.
> 
> This is because under Xen pages which are contiguous in the physical
> address space may not be contiguous in the DMA space, in fact it is
> very likely that they are not. I think there are other architectures
> where this is true, although perhaps non quite so aggressive as to
> have this property at a per-order-0-page granularity.
> 
> The real underlying bug here most likely lies in the swiotlb not
> correctly handling compound pages, and Konrad is investigating this.
> However even with the swiotlb issue fixed the current arrangement
> seems likely to result in a lot of bounce buffering which seems likely
> to more than offset any benefit from the use of larger pages.
> 
> Therefore make NETDEV_FRAG_PAGE_MAX_ORDER configurable at runtime and
> use this to request order-0 frags under Xen. Also expose this setting
> via sysctl.
> 
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: netdev@vger.kernel.org
> Cc: xen-devel@lists.xen.org
> ---

I understand your concern, but this seems a quick/dirty hack at this
moment. After setting the sysctl to 0, some tasks may still have some
order-3 pages in their cache.

Your driver must already cope with skb->head being split on several
pages.

So what fundamental difference exists with frags ?