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

[Igor Royzis]

On Tue, May 20, 2014 at 5:28 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> 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.

Would it "ease" the memory cache penalty if we moved the parent
fragment pointer from skb_shared_info to skbuff itself?
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