Message-ID: <4951387D.2030003@vlnb.net>
Date: Tue, 23 Dec 2008 22:14:05 +0300
From: Vladislav Bolkhovitin <vst@vlnb.net>
User-Agent: Thunderbird 2.0.0.9 (X11/20071115)
MIME-Version: 1.0
To: Rusty Russell <rusty@rustcorp.com.au>
CC: Jeremy Fitzhardinge <jeremy@goop.org>, Evgeniy Polyakov <zbr@ioremap.net>,
       Herbert Xu <herbert@gondor.apana.org.au>, linux-scsi@vger.kernel.org,
       James Bottomley <James.Bottomley@hansenpartnership.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
       Mike Christie <michaelc@cs.wisc.edu>, Jeff Garzik <jeff@garzik.org>,
       Boaz Harrosh <bharrosh@panasas.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org, scst-devel@lists.sourceforge.net,
       Bart Van Assche <bart.vanassche@gmail.com>,
       "Nicholas A. Bellinger" <nab@linux-iscsi.org>, netdev@vger.kernel.org,
       David Miller <davem@davemloft.net>
Subject: Re: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user
 space data
References: <494C0255.8010208@goop.org> <20081220103209.GA23632@ioremap.net> <494D49E6.9000703@goop.org> <200812221113.24044.rusty@rustcorp.com.au>
In-Reply-To: <200812221113.24044.rusty@rustcorp.com.au>
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2491
Lines: 58

Rusty Russell, on 12/22/2008 03:43 AM wrote:
> On Sunday 21 December 2008 06:09:18 Jeremy Fitzhardinge wrote:
>> Evgeniy Polyakov wrote:
>>> 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).
>>>   
>> OK.
>>
>>> __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).
>>>   
>> Yes, that doesn't sound too bad.
> 
> That would be one approach.  Actually, my patch solved this by keeping a
> parent ref in various cases if the parent had a destructor: we only destroy
> the parent when all the clones are gone.
> 
> Here's the patch for reference:
> 
> net: add destructor for skb data.
> 
> If we want to notify something when an skb is truly finished (such as
> for tun vringfd support), we need a destructor on the data.
> 
> This turns out to be slightly non-trivial as fragments from one skb
> get copied to another skb: if the first skb has a destructor (or its
> parent does) we need to keep a reference to it and destroy it only
> when (all the) children are destroyed.  We add an 'orig' pointer to
> the skb_shared_info to do this.
> 
> But there's currently no way to get from the shinfo to the head (to
> kfree it), so we add a 'len' field.  A better alternative to this
> might be to move the skb_shared_info to before the head of the skb data.
> 
> Note that the destructor is responsible for calling kfree: for the tun
> device, this is critical since the destructor can be called from any
> context and it has to do a copy_to_user, so it queues the skb.

Rusty,

Can you describe how one should use your patch, please? Maybe, there is 
some code you use to test it?

Thanks,
Vlad
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/