Re: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user space data

Herbert Xu
2008-12-20T02:05:51Z

On Fri, Dec 19, 2008 at 05:56:11PM -0800, Jeremy Fitzhardinge wrote:
> Evgeniy Polyakov wrote:
>> Shared info is freed when there are no skbs referring to the shared info
>> in question. Skb holds all pages in shared info in the fraglist array,
>> so when it is about to be freed, it means that network stack does not
>> use it (particulary it will putpage every page in fraglist). Usually
>> there are two skbs in the network stack per packet in TCP (allocated at
>> once though via fastclone mechanims): one is provided to the device
>> (and will be freed there) and another one is placed into retransmit
>> queue, where it will be located and freed when ack has been received.
>>
>> There may be another layers which may clone skb, but its shared info
>> structure (shared between the clones) will only be freed when all users
>> freed appropriate cloned skbs.

This is all correct. However, please note that that if any clone
does a pskb_expand_head then it will get its own private copy of
of the shared info. So you can't use the shared info to ref count
the pages in it.

Cheers,
--
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