Re: Skbuff Trimming

Re: Skbuff Trimming

[Andi Kleen]

Bendi Vinaya Kumar <vbendi@cs.clemson.edu> writes:

> But, it does not do the same on
> "frag_list". Why?

frag_list is not a general purpose skbuff facility and is not used by
most protocols and not directly supported by most of skbuff.c It is just 
supported by some specific paths to enable lazy defragmenting. It is not
an attempt to turn skbuffs into mbufs.

-Andi

Re: Skbuff Trimming

[Bendi Vinaya Kumar]

On 28 Jun 2002, Andi Kleen wrote:

> Bendi Vinaya Kumar <vbendi@cs.clemson.edu> writes:
>
> > But, it does not do the same on
> > "frag_list". Why?
>
> frag_list is not a general purpose skbuff facility and is not used by
> most protocols and not directly supported by most of skbuff.c It is just
> supported by some specific paths to enable lazy defragmenting. It is not
> an attempt to turn skbuffs into mbufs.
>
> -Andi

I agree with you. I couldn't find a
case when it (frag_list) is used.

Function

ip_rcv
(http://lxr.linux.no/source/net/ipv4/ip_input.c#L383)

uses two functions, namely,

1) pskb_may_pull, which might
   result in a call to __pskb_pull_tail

and 2) __pskb_trim, which might
       result in a call to ___pskb_trim.

Function __pskb_pull_tail
operates on data in both frags
array and frag_list. As mentioned
earlier, ___pskb_trim doesn't operate
on frag_list. Since these two functions
are called from ip_rcv, they must be
operating on the same sk buff. One function
handles frag_list, the other doesn't.

Isn't there an inconsistency in treatment here,
even though frag_list is not commonly used
in practice?

Thank you.

Regards,
Vinaya Kumar Bendi
P.S.: Kindly CC any answers/comments
      to vbendi@cs.clemson.edu.

Re: Skbuff Trimming

[Bendi Vinaya Kumar]

June 29, 2002

Hi,

Guess, I understand now.

__pskb_pull_tail operates on
frag_list as well, since it is
required to pull a header (completely)
into kmalloc'd area pointed to by
skb->data.

It is, however, not required of
___pskb_trim. So, frag_list, in this
case, may be avoided.

Thanks.

Regards,
Vinaya Kumar Bendi
P.S.: Kindly CC any answers/comments
      to vbendi@cs.clemson.edu.

Skbuff Trimming

[Bendi Vinaya Kumar]

June 27, 2002

Hi,

I have a question for anyone
who has been through the following
function,

___pskb_trim
(http://lxr.linux.no/source/net/core/skbuff.c#L739)

This function tries to trim an
skbuff off any trailing pad bytes.
It goes about doing its job, going
through "frags" array.

But, it does not do the same on
"frag_list". Why?

Thank you for your time.

Regards,
Vinaya Kumar Bendi
P.S.: Kindly CC any answers/comments
	to vbendi@cs.clemson.edu.