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

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Message-ID: <20081220103209.GA23632@ioremap.net>
Date:	Sat, 20 Dec 2008 13:32:09 +0300
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

On Sat, Dec 20, 2008 at 07:10:45PM +1100, Herbert Xu (herbert@...dor.apana.org.au) wrote:
> > Hm.  So if I get a destructor call from the shared_info, can I go an  
> > inspect the page refcounts to see if its really the last use?
> 
> The pages that were originally in the shared_info at creation
> time may no longer be there by the time it's freed because of
> pskb_pull_tail.

Things should work fine, since pskb_expand_head() copies whole shared
info structure (and thus will copy destructor), get all pages and then
copy all pointers into the new skb, and then release old skb's data.

So destructor for the pages should not rely on which skb it is called on
and check if pages are about to be really freed (i.e. check theirs
reference counter).

__pskb_pull_tail() is tricky, it just puts some pages it does not want
to be present in the skb, but it could be possible to add there
destructor callback from the original skb with partial flag (or just
having destructor with two parameters: skb and page, and if page is not
NULL, then actually only given page is freed, otherwise the whole skb).

-- 
	Evgeniy Polyakov
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