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

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Message-ID: <494C8D57.7040808@goop.org>
Date:	Fri, 19 Dec 2008 22:14:47 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Evgeniy Polyakov <zbr@...emap.net>,
	Vladislav Bolkhovitin <vst@...b.net>,
	linux-scsi@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Jeff Garzik <jeff@...zik.org>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, scst-devel@...ts.sourceforge.net,
	Bart Van Assche <bart.vanassche@...il.com>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	netdev@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user
 space data

Herbert Xu wrote:
> 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.

Ah, so the lifetime of the shared_info structure doesn't match the 
lifetime of the underlying pages, and this mechanism would be 
insufficient for my purposes?  If so, how can it be solved?

Thanks,
    J
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