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

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Message-ID: <4712A254.4090604@goop.org>
Date:	Sun, 14 Oct 2007 16:12:20 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	David Chinner <dgc@....com>
CC:	xfs@....sgi.com, Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mark Williamson <mark.williamson@...cam.ac.uk>,
	Morten Bøgeskov 
	<xen-users@...ten.bogeskov.dk>, xfs-masters@....sgi.com,
	nickpiggin@...oo.com.au
Subject: Re: Interaction between Xen and XFS: stray RW mappings

David Chinner wrote:
> You mean xfs_buf.c.
>   

Yes, sorry.

> And yes, we delay unmapping pages until we have a batch of them
> to unmap. vmap and vunmap do not scale, so this is batching helps
> alleviate some of the worst of the problems.
>   

How much performance does it cost?  What kind of workloads would it show
up under?

> Realistically, if this delayed release of vmaps is a problem for
> Xen, then I think that some generic VM solution is needed to this
> problem as vmap() is likely to become more common in future (think
> large blocks in filesystems). Nick - any comments?
>   

Well, the only real problem is that the pages are returned to the free
pool and reallocated while still being part of a mapping.  If the pages
are still owned by the filesystem/pagecache, then there's no problem.

What's the lifetime of things being vmapped/unmapped in xfs?  Are they
necessarily being freed when they're unmapped, or could unmapping of
freed memory be more immediate than other memory?

Maybe it just needs a notifier chain or something.

    J
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