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

From: Gregory Haskins <[email-masked]>
Date: Sat, 14 Nov 2009 00:27:46 -0500

Stephen Hemminger wrote:
> On Fri, 13 Nov 2009 21:27:57 -0500
> Gregory Haskins <[email-masked]> wrote:
> 
>> Herbert Xu wrote:
>>> On Fri, Nov 13, 2009 at 08:33:35PM -0500, Gregory Haskins wrote:
>>>> Well, not with respect to the overall protocol, of course not.  But with
>>>> respect to the buffer in question, it _has_ to be.  Or am I missing
>>>> something?
>>> sendfile() has never guaranteed that the kernel is finished with
>>> the underlying pages when it returns.
>>>
>>> Cheers,
>> Clearly there must be _some_ mechanism to synchronize (e.g.
>> flush/barrier) though, right?  Otherwise, that interface would seem to
>> be quite prone to races and would likely be unusable.   So what does
>> said flush use to know when the buffer is free?
> 
> No all the interfaces require a copy.

I'm sorry, but I do not think that is correct.  As others have pointed
out, that would not appear to be true for at least sendfile.

> Actually, sendfile makes no guarantee about synchronization
> because the receiver of said file could be arbitrarily slow, and any attempt at locking down
> current contents of file is a denial of service exposure.

I think you are inverting the problem space.  It is fully expected that
changing the "file", or the pages that represent the file before the
packet is queued would constitute the modification of the stream on the
wire.

I am more thinking about the applications of mmap+sendfile to implement
a zero-copy interface.  As David mentions in another mail, it seems that
there is no sync mechanism available, so this would not appear to be a
viable use case today, unfortunately.

> 
> People have tried doing copy-less send by page flipping, but the overhead of the IPI to
> invalidate the TLB exceeded the overhead of the copy. There was an Intel paper on this in
> at Linux Symposium (Ottawa) several years ago.

I think you are confusing copy-less tx with copy-less rx.  You can try
to do copy-less rx with page flipping, which has the IPI/TLB thrashing
properties you mention, and I agree is problematic.  We are talking
about copy-less tx here, however, and therefore no page-flipping is
involved.  Rather, we are just posting SG lists of pages directly to the
NIC (assuming the nic supports HIGH_DMA, etc).  You do not need to flip
the page, or invalidate the TLB (and thus IPI the other cores) to do
this to my knowledge.

Kind Regards,
-Greg