netdev - Re: [PATCH net-next 7/7] net: move skb->dropcount to skb->cb[]

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Date:	Thu, 26 Feb 2015 12:17:57 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	Shmulik Ladkani <shmulik.ladkani@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Willem de Bruijn <willemb@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>, marcel@...tmann.org,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 7/7] net: move skb->dropcount to skb->cb[]

On Thu, Feb 26, 2015 at 10:49 AM, Shmulik Ladkani
<shmulik.ladkani@...il.com> wrote:
> On Thu, 26 Feb 2015 04:10:12 +0200 Eyal Birger <eyal.birger@...il.com> wrote:
>> diff --git a/include/net/sock.h b/include/net/sock.h
>> index d462f5e..8807586 100644
>> --- a/include/net/sock.h
>> +++ b/include/net/sock.h
>> @@ -2078,12 +2078,26 @@ static inline int sock_intr_errno(long timeo)
>>       return timeo == MAX_SCHEDULE_TIMEOUT ? -ERESTARTSYS : -EINTR;
>>  }
>>
>> +struct sock_skb_cb {
>> +     u32 dropcount;
>> +};
>> +
>> +/* Store sock_skb_cb at the end of skb->cb[] so protocol families
>> + * using skb->cb[] would keep using it directly and utilize its
>> + * alignement guarantee.
>> + */
>> +#define SOCK_SKB_CB_OFFSET ((FIELD_SIZEOF(struct sk_buff, cb) - \
>> +                         sizeof(struct sock_skb_cb)))
>> +
>> +#define SOCK_SKB_CB(__skb) ((struct sock_skb_cb *)((__skb)->cb + \
>> +                         SOCK_SKB_CB_OFFSET))
>> +
>>  #define sock_skb_cb_check_size(size) \
>> -     BUILD_BUG_ON((size) > FIELD_SIZEOF(struct sk_buff, cb))
>> +     BUILD_BUG_ON((size) > SOCK_SKB_CB_OFFSET)
>>
>
> You didn't take care of aligning the 'sock_skb_cb'.
> (Althogh in practive, dropcount is 4 and cb[] is 48 so you're golden).
>
> How about:
>
> struct sock_skb_cb {
>         /* protocol families specifc CBs */
>         u8 reserved[__SOCK_SKB_CB_OFFSET];
>         struct __sock_skb_cb x;                 // name me better!
> };
>
> Where '__SOCK_SKB_CB_OFFSET' and '__sock_skb_cb' are as follows:
>
> struct __sock_skb_cb {  // name me better!
>         u32 dropcount;
> };
>
> #define __SOCK_SKB_CB_OFFSET_UNALIGNED \
>         (FIELD_SIZEOF(struct sk_buff, cb) - sizeof(struct __sock_skb_cb))
>
> #define __SOCK_SKB_CB_OFFSET \
>         (__SOCK_SKB_CB_OFFSET_UNALIGNED - \
>          (__SOCK_SKB_CB_OFFSET_UNALIGNED % __alignof__(struct __sock_skb_cb)))
>
> This also takes care for alignement of '__sock_skb_cb x' within the CB.
>
> Then,
>
> #define sock_skb_cb_check_size(size) \
>         BUILD_BUG_ON((size) > FIELD_SIZEOF(struct sock_skb_cb, reserved))
>

Agreed. Will add to v2.
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