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From: Andrew Gallatin <gallatin@cs.duke.edu>
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Date: Mon,  4 Dec 2000 12:43:12 -0500 (EST)
To: Matt Dillon <dillon@earth.backplane.com>
Cc: Peter Wemm <peter@netplex.com.au>,
	"Kenneth D. Merry" <ken@kdm.org>, arch@FreeBSD.ORG
Subject: Re: zero copy code review 
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Matt Dillon writes:
 > 
 > :freed).  We do not make anything COW for the kernel.
 > :
 > : > either, but I could have missed it..  What about drivers that DMA to pages
 > : > mapped into KVM without checking writability (and hence COW)?  
 > :
 > :This is a good point.  But I cannot think of any circumstance where a
 > :driver would be DMA'ing directly to a user owned page (with the
 > :exception of a vm fault, but this is impossible because the pages are
 > :resident prior to setting up the send and are wired for the duration
 > :of the send).
 > :
 > :Thanks for the input.  I'm glad to see you and Matt looking at this!
 > :
 > :Drew
 > 
 >     Careful.  If you read() from a raw device most disk drivers WILL dma
 >     directly to a user-owned page.
 > 
 > 						-Matt
 > 

That's a good point that I hadn't thought about.  All the more reason
to make the send-side code a socket option so the process has to take
careful aim before blowing off its foot.

Drew


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