Subject: Re: [RFC PATCH] net: add dataref destructor to sk_buff

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 10 Nov 2009 15:17:22 +0200

On Tue, Nov 10, 2009 at 05:40:50AM -0700, Gregory Haskins wrote:
> >>> On 11/10/2009 at  6:53 AM, in message <[email-masked]>,
> "Michael S. Tsirkin" <[email-masked]> 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 <[email-masked]>
> >> 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 <[email-masked]>
> >> ---
> >> 
> >>  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

You are comparing with skb destructors, not skb data destructors :) skb
data destructor is Rusty's patch which he wanted to use for vringfd.  I
mean e.g. this:
http://lists.openwall.net/netdev/2008/04/18/7

> > 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.

VFS does not know about shinfo either, does it?

> > 
> > 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).

Well, I hope the reviews are helpful.  I'll be happy if we learn to
track pages involved in transmit, but need to be careful.

> > 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.

Not if someone else is referencing the pages without a reference to shinfo.

> > 
> > 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.

I don't seem to find where does skb_split reference the shinfo.
It seems to do get_page on individual pages?

> Kind Regards,
> -Greg