Re: [PATCH] Fixed zero copy GSO without orphaning the fragments

[Eric Dumazet]

On Tue, 2014-05-20 at 14:24 +0300, Igor Royzis wrote:
> Fix accessing GSO fragments memory (and a possible corruption therefore) after
> reporting completion in a zero copy callback. The previous fix in the commit 1fd819ec
> orphaned frags which eliminates zero copy advantages. The fix makes the completion
> called after all the fragments were processed avoiding unnecessary orphaning/copying
> from userspace.
> 
> The GSO fragments corruption issue was observed in a typical QEMU/KVM VM setup that
> hosts a Windows guest (since QEMU virtio-net Windows driver doesn't support GRO).
> The fix has been verified by running the HCK OffloadLSO test.
> 
> Signed-off-by: Igor Royzis <igorr@xxxxxxxxxxx>
> Signed-off-by: Anton Nayshtut <anton@xxxxxxxxxxx>
> ---
>  include/linux/skbuff.h |    1 +
>  net/core/skbuff.c      |   18 +++++++++++++-----
>  2 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 08074a8..8c49edc 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -287,6 +287,7 @@ struct skb_shared_info {
>  	struct sk_buff	*frag_list;
>  	struct skb_shared_hwtstamps hwtstamps;
>  	__be32          ip6_frag_id;
> +	struct sk_buff  *zcopy_src;
>  

Before your patch :

sizeof(struct skb_shared_info)=0x140
offsetof(struct skb_shared_info, frags[1])=0x40

SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) -> 0x140

After your patch :

sizeof(struct skb_shared_info)=0x148
offsetof(struct skb_shared_info, frags[1])=0x48

SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) -> 0x180

Thats a serious bump, because it increases all skb truesizes, and
typical skb with one fragment will use 2 cache lines instead of one in
struct skb_shared_info, so this adds memory pressure in fast path.

So while this patch might increase performance for some workloads,
it generally decreases performance on many others.



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