From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: Interaction between Xen and XFS: stray RW mappings
Date: Sun, 21 Oct 2007 20:34:57 -0700
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To: dean gaudet <dean@arctic.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>, Xen-devel <xen-devel@lists.xensource.com>, =?ISO-8859-1?Q?Morten_?=, David Chinner <dgc@sgi.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, =?ISO-8859-1?Q?B=F8geskov?= <xen-users@morten.bogeskov.dk>, xfs@oss.sgi.com, xfs-masters@oss.sgi.com, Mark Williamson <mark.williamson@cl.cam.ac.uk>
List-Id: xen-devel@lists.xenproject.org

dean gaudet wrote:
> On Mon, 15 Oct 2007, Nick Piggin wrote:
>
>   
>> Yes, as Dave said, vmap (more specifically: vunmap) is very expensive
>> because it generally has to invalidate TLBs on all CPUs.
>>     
>
> why is that?  ignoring 32-bit archs we have heaps of address space 
> available... couldn't the kernel just burn address space and delay global 
> TLB invalidate by some relatively long time (say 1 second)?
>   

Yes, that's precisely the problem.  xfs does delay the unmap, leaving
stray mappings, which upsets Xen.

    J