Date:	Tue, 10 Nov 2009 05:40:50 -0700
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	<alacrityvm-devel@...ts.sourceforge.net>, <herbert.xu@...hat.com>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] net: add dataref destructor to sk_buff

>>> On 11/10/2009 at  6:53 AM, in message <20091110115335.GC6989@...hat.com>,
"Michael S. Tsirkin" <mst@...hat.com> wrote: 
> On Fri, Oct 02, 2009 at 10:20:00AM -0400, Gregory Haskins wrote:
>> (Applies to davem/net-2.6.git:4fdb78d30)
>> 
>> Hi David, netdevs,
>> 
>> The following is an RFC for an attempt at addressing a zero-copy solution.
>> 
>> To be perfectly honest, I have no idea if this is the best solution, or if
>> there is truly a problem with skb->destructor that requires an alternate
>> mechanism.  What I do know is that this patch seems to work, and I would
>> like to see some kind of solution available upstream.  So I thought I would
>> send my hack out as at least a point of discussion.  FWIW: This has been
>> tested heavily in my rig and is technically suitable for inclusion after
>> review as is, if that is decided to be the optimal path forward here.
>> 
>> Thanks for your review and consideration,
>> 
>> Kind regards,
>> -Greg
>> 
>> ----------------------------------------
>> From: Gregory Haskins <ghaskins@...ell.com>
>> Subject: [RFC PATCH] net: add dataref destructor to sk_buff
>> 
>> What: The skb->destructor field is reportedly unreliable for ensuring
>> that all shinfo users have dropped their references.  Therefore, we add
>> a distinct ->release() method for the shinfo structure which is closely
>> tied to the underlying page resources we want to protect.
>> 
>> Why: We want to add zero-copy transmit support for AlacrityVM guests.
>> In order to support this, the host kernel must map guest pages directly
>> into a paged-skb and send it as normal.  put_page() alone is not
>> sufficient lifetime management since the pages are ultimately allocated
>> from within the guest.  Therefore, we need higher-level notification
>> when the skb is finally freed on the host so we can then inject a proper
>> "tx-complete" event into the guest context.
>> 
>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>> ---
>> 
>>  include/linux/skbuff.h |    2 ++
>>  net/core/skbuff.c      |    9 +++++++++
>>  2 files changed, 11 insertions(+), 0 deletions(-)
>> 
>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>> index df7b23a..02cdab6 100644
>> --- a/include/linux/skbuff.h
>> +++ b/include/linux/skbuff.h
>> @@ -207,6 +207,8 @@ struct skb_shared_info {
>>  	/* Intermediate layers must ensure that destructor_arg
>>  	 * remains valid until skb destructor */
>>  	void *		destructor_arg;
>> +	void *          priv;
>> +	void           (*release)(struct sk_buff *skb);
>>  };
>>  
>>  /* We divide dataref into two halves.  The higher 16 bits hold references
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index 80a9616..a7e40a9 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -219,6 +219,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t 
> gfp_mask,
>>  	shinfo->tx_flags.flags = 0;
>>  	skb_frag_list_init(skb);
>>  	memset(&shinfo->hwtstamps, 0, sizeof(shinfo->hwtstamps));
>> +	shinfo->release = NULL;
>> +	shinfo->priv = NULL;
>>  
>>  	if (fclone) {
>>  		struct sk_buff *child = skb + 1;
>> @@ -350,6 +352,9 @@ static void skb_release_data(struct sk_buff *skb)
>>  		if (skb_has_frags(skb))
>>  			skb_drop_fraglist(skb);
>>  
>> +		if (skb_shinfo(skb)->release)
>> +			skb_shinfo(skb)->release(skb);
>> +
>>  		kfree(skb->head);
>>  	}
>>  }
>> @@ -514,6 +519,8 @@ int skb_recycle_check(struct sk_buff *skb, int skb_size)
>>  	shinfo->tx_flags.flags = 0;
>>  	skb_frag_list_init(skb);
>>  	memset(&shinfo->hwtstamps, 0, sizeof(shinfo->hwtstamps));
>> +	shinfo->release = NULL;
>> +	shinfo->priv = NULL;
>>  
>>  	memset(skb, 0, offsetof(struct sk_buff, tail));
>>  	skb->data = skb->head + NET_SKB_PAD;
>> @@ -856,6 +863,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int 
> ntail,
>>  	skb->hdr_len  = 0;
>>  	skb->nohdr    = 0;
>>  	atomic_set(&skb_shinfo(skb)->dataref, 1);
>> +	skb_shinfo(skb)->release = NULL;
>> +	skb_shinfo(skb)->priv = NULL;
>>  	return 0;
>>  
>>  nodata:
> 
> This is basically subset of the skb data destructors patch, isn't it?

Sort of, but the emphasis is different.  Here are the main differences:

1) skb->destructor() is on the skb level, shinfo->release() is at the shared-info/page level

2) skb->destructor is (iiuc) modified during the skb's lifetime (for instance rmem hooks here to
manage the buffer-space dynamically), whereas shinfo->release is designed to be used by
"the owner" and is thus only set at creation.

3) shinfo->release tracks the lifetime of the pages, not the skb.  This means it transcends
the skb's lifetime (such as splits for clone, etc) and focuses on the shared component: the pages

> Last time this was tried, this is the objection that was voiced:
> 
> 	The problem with this patch is that it's tracking skb's, while
> 	you want use it to track pages for zero-copy.  That just doesn't
> 	work.  Through mechanisms like splice, individual pages in the
> 	skb can be detached and metastasize to other locations, e.g.,
> 	the VFS.

Right, and I don't think this applies here because I specifically chose the shinfo level to try to properly
track the page level avoid this issue.  Multiple skb's can point to a single shinfo, iiuc.

> 
> and I think this applies here.

I don't think so, but if you think I missed something, do not be shy (not that you ever are).

> In other words, this only *seems*
> to work for you because you are not trying to do things like
> guest to host communication, with host doing smart things.

I am not following what you mean here, as I do use this for guest->host and guest->host->remote, and
it works quite nicely.  I map the guest pages in, and when the last reference to the pages are dropped,
I release the pages back to the guest.  It doesn't matter if the skb egresses out a physical adapter or is
received locally.  All that matters is the lifetime of the shinfo (and thus its pages) is handled correctly.

> 
> Cc Herbert which was involved in the original discussion.
> 
> In the specific case, it seems that things like pskb_copy,
> skb_split and others will also be broken, won't they?

Not to my knowledge.   They up the reference to the shinfo before proceeding.

Kind Regards,
-Greg



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