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Message-ID: <20091113184503.13f6d447@s6510> Date: Fri, 13 Nov 2009 18:45:03 -0800 From: Stephen Hemminger <shemminger@...tta.com> To: Gregory Haskins <gregory.haskins@...il.com> Cc: Herbert Xu <herbert@...dor.apana.org.au>, Gregory Haskins <ghaskins@...ell.com>, "Michael S. Tsirkin" <mst@...hat.com>, alacrityvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Re: [RFC PATCH] net: add dataref destructor to sk_buff On Fri, 13 Nov 2009 21:27:57 -0500 Gregory Haskins <gregory.haskins@...il.com> 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. 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. 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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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