Messages in this thread | | | Date | Tue, 10 Nov 2009 15:17:22 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: [RFC PATCH] net: add dataref destructor to sk_buff |
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On Tue, Nov 10, 2009 at 05:40:50AM -0700, Gregory Haskins wrote: > >>> On 11/10/2009 at 6:53 AM, in message <20091110115335.GC6989@redhat.com>, > "Michael S. Tsirkin" <mst@redhat.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@novell.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@novell.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
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 > >
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