netdev - Re: [RFC] [NET] [0/2] pskb_expand

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Date:	Wed, 26 Mar 2008 16:47:35 -0400
From:	Hideo AOKI <haoki@...hat.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>, haoki@...hat.com
Subject: Re: [RFC] [NET] [0/2] pskb_expand_head() bugfix

Hello Herbert,

Thank you for your quick response.

Herbert Xu wrote:
> On Tue, Mar 25, 2008 at 02:39:04PM -0400, Hideo AOKI wrote:
>>
>> Current pskb_expand_head() doesn't change truesize, while it
>> reallocates memory. Then, if argument nhead or ntail aren't 0, caller
>> must update truesize.
>>
>> We had this bug at audit_expand() in January and fixed it as commit
>> 406a1d868001423c85a3165288e566e65f424fe6. However, some drivers and
>> subsystems still use pskb_expand_head() without updating truesize.
> 
> Drivers usually aren't supposed to change truesize so doing
> this would actually create bugs.

I understood your point. 

Since keeping correct truesize is important to network memory
accounting, I want to fix network subsystem part at least.

I think that it is inconvenient for caller functions to need
updateing truesize by themselves. How about this change to
avoid the inconvenience?

 - Current implementation is renamed to __pskb_expand_head().
 - Drivers call __pskb_expand_head() instead of pskb_expand_head().
 - New pskb_expand_head() updates truesize after calling 
  __pskb_expand_head(). 

Or, should I simply add truesize calculation after
pskb_expand_head() calls which change truesize?
 
>> In addition, there is another problem to update truesise. Since
>> pskb_expand_head() aligns memory size before reallocation, caller
>> functions may not update turesize correctly if they just add nhaad
>> and ntail to turesize.
> 
> That should be fixable by making sure that nhead + ntail is
> aligned.

I see.

Regards,
Hideo

-- 
Hitachi Computer Products (America) Inc.

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