Re: [MPTCP] [PATCH 1/2] skbuff: Add shared control buffer

[cpaasch at apple.com]

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On 09/11/17 - 06:48:09, Boris Pismenny wrote:
> > -----Original Message-----
> > From: cpaasch(a)apple.com [mailto:cpaasch(a)apple.com]
> > Sent: Thursday, November 09, 2017 13:48
> > To: Boris Pismenny <borisp(a)mellanox.com>
> > Cc: Mat Martineau <mathew.j.martineau(a)linux.intel.com>; Ilya Lesokhin
> > <ilyal(a)mellanox.com>; Liran Liss <liranl(a)mellanox.com>; mptcp(a)lists.01.org
> > Subject: Re: [MPTCP] [PATCH 1/2] skbuff: Add shared control buffer
> > 
> > On 09/11/17 - 04:32:54, Boris Pismenny wrote:
> > > +Ilya and Liran
> > >
> > > Hi,
> > >
> > > > -----Original Message-----
> > > > From: cpaasch(a)apple.com [mailto:cpaasch(a)apple.com]
> > > > Sent: Thursday, November 09, 2017 13:13
> > > > To: Mat Martineau <mathew.j.martineau(a)linux.intel.com>; Boris
> > Pismenny
> > > > <borisp(a)mellanox.com>
> > > > Cc: mptcp(a)lists.01.org
> > > > Subject: Re: [MPTCP] [PATCH 1/2] skbuff: Add shared control buffer
> > > >
> > > > +Boris
> > > >
> > > > On 20/10/17 - 16:02:31, Mat Martineau wrote:
> > > > > The sk_buff control buffer is of limited size, and cannot be enlarged
> > > > > without significant impact on systemwide memory use. However,
> > additional
> > > > > per-packet state is needed for some protocols, like Multipath TCP.
> > > > >
> > > > > An optional shared control buffer placed after the normal struct
> > > > > skb_shared_info can accomodate the necessary state without imposing
> > > > > extra memory usage or code changes on normal struct sk_buff
> > > > > users. __alloc_skb will now place a skb_shared_info_ext structure at
> > > > > skb->end when given the SKB_ALLOC_SHINFO_EXT flag. Most users of
> > struct
> > > > > sk_buff continue to use the skb_shinfo() macro to access shared
> > > > > info. skb_shinfo(skb)->is_ext is set if the extended structure is
> > > > > available, and cleared if it is not.
> > > > >
> > > > > pskb_expand_head will preserve the shared control buffer if it is present.
> > > > >
> > > > > Signed-off-by: Mat Martineau <mathew.j.martineau(a)linux.intel.com>
> > > > > ---
> > > > >  include/linux/skbuff.h | 24 +++++++++++++++++++++-
> > > > >  net/core/skbuff.c      | 56 ++++++++++++++++++++++++++++++++++++++--
> > -----
> > > > -----
> > > > >  2 files changed, 66 insertions(+), 14 deletions(-)
> > > >
> > > > Boris, below is the change I mentioned to you.
> > > >
> > > > It allows to allocate 48 additional bytes on-demand after skb_shared_info.
> > > > As it is on-demand, it won't increase the size of the skb for other users.
> > > >
> > > > For example, TLS could start using it when it creates the skb that it
> > > > pushes down to the TCP-stack. That way you don't need to handle the
> > > > tls_record lists.
> > > >
> > >
> > > One small problem is that TLS doesn't create SKBs. As a ULP it calls the
> > transport send
> > > Functions (do_tcp_sendpages for TLS). This function receives a page and not a
> > SKB.
> > 
> > yes, that's a good point. Mat has another patch as part of this series,
> > that adds an skb-arg to sendpages
> > (https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.
> > 01.org%2Fpipermail%2Fmptcp%2F2017-
> > October%2F000130.html&data=02%7C01%7Cborisp%40mellanox.com%7C8d0
> > 4f7b1c39649a6e80d08d5272d17fe%7Ca652971c7d2e4d9ba6a4d149256f461b
> > %7C0%7C0%7C636457996996299024&sdata=YjycapXvL2N%2FkrPj15oTy2ighC
> > 23j1lKmWetdJtdXLU%3D&reserved=0)
> > 
> > That should do the job for you.
> > 
> > > We decided not to create the SKB outside of the TCP layer to reduce the
> > number of
> > > changes we made to TCP.
> > >
> > > It would be nice if we could use something like that. Did you talk to DaveM
> > about
> > > upstreaming this?
> > 
> > No, we didn't talk yet to anyone outside of this list here about it.
> > 
> > We were looking for a user of it outside of MPTCP but couldn't find one.
> > Now, it seems like we found one that would be interested :)
> > 
> > 
> > I think this infrastructure here would simplify your code quite a bit, no?
> 
> I'm not sure. I'll need to think about that.
> 
> I'm worried that in some cases it won't work, i.e. the shared_info wouldn't reach our
> driver and then again we would be forced to use the tls_get_record function.
> One example, is the tcp_mtu_probe function, which creates a new skb. Another
> is tcp_collapse_retrans. We need to ensure all of these are covered when we pass
> any offloaded TLS skb to the driver.

Yes, that's a good point. Making sure that all of these are covered will be
tricky.
There could even be other places further down the stack before the driver
where an skb is copied/modified.


Christoph

> 
> > 
> > > We will definitely find it useful for the receive side, there we allocate the SKB
> > in the driver.
> > 
> > Interesting! So even there you could use it. We were under the impression
> > that it would be of less interest for the receive-side.
> > 
> > 
> > Christoph
> > 
> > >
> > > > See below for the rest of the patch.
> > > >
> > > >
> > > > Christoph
> > > >
>