linux-kernel - Re: Interaction between Xen and XFS: stray RW mappings

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Date:	Mon, 22 Oct 2007 12:11:58 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	dean gaudet <dean@...tic.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Xen-devel <xen-devel@...ts.xensource.com>, Morten@...e.de,
	David Chinner <dgc@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Bøgeskov <xen-users@...ten.bogeskov.dk>,
	xfs@....sgi.com, xfs-masters@....sgi.com,
	Mark Williamson <mark.williamson@...cam.ac.uk>
Subject: Re: Interaction between Xen and XFS: stray RW mappings

Andi Kleen wrote:
> It's hidden now so it causes any obvious failures any more. Just
> subtle ones which is much worse.
>   

I think anything detected by Xen is still classed as "obscure" ;)

> But why not just disable it?  It's not critical functionality,
> just a optimization that unfortunately turned out to be incorrect.
>
> It could be made correct short term by not freeing the pages until
> vunmap for example.
>   

I think it only ends up holding 64 pages (or is it 64 mappings?), so its
not a horrible use of memory.  Particularly since it responds to memory
pressure callbacks.

>> does his grand unified vmap manager.  I guess a clean workaround would
>> be to add a CONFIG_XFS_LAZY_UNMAP, and do it at the Kconfig level... 
>> I'll cook up a patch.
>>     
>
> This option could only be safely set in architectures that don't
> care about caching attribute aliases or never remap kernel pages
> uncached. That's not x86, powerpc, ia64 at a minimum.
>
> I think the only good short term fix is to turn your ifdef CONFIG_XEN
> into a #if 0
>   

#if 1, but yes, I see your point.

    J
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