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

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Message-ID: <20071023124137.GP66820511@sgi.com>
Date:	Tue, 23 Oct 2007 22:41:37 +1000
From:	David Chinner <dgc@....com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	David Chinner <dgc@....com>, Jeremy Fitzhardinge <jeremy@...p.org>,
	dean gaudet <dean@...tic.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Xen-devel <xen-devel@...ts.xensource.com>, Morten@...e.de,
	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: [patch] Re: Interaction between Xen and XFS: stray RW mappings

On Tue, Oct 23, 2007 at 11:30:35AM +0200, Andi Kleen wrote:
> On Tue, Oct 23, 2007 at 05:04:14PM +1000, David Chinner wrote:
> > > You mean like vmap() could record the pages passed to it in the area->pages
> > > array, and we walk and release than in __vunmap() like it already does
> > > for vfree()?
> > > 
> > > If we did this, it would probably be best to pass a page release function
> > > into the vmap or vunmap call - we'd need page_cache_release() called on
> > > the page rather than __free_page()....
> > > 
> > > The solution belongs behind the vmap/vunmap interface, not in XFS....
> > 
> > Lightly tested(*) patch that does this with lazy unmapping
> > below for comment.
> 
> Thanks
> 
> > 
> > (*) a) kernel boots, b) made an XFS filesystem with 64k directory
> > blocks, created ~100,000 files in a directory to get a wide btree
> > (~1700 blocks, still only a single level) and run repeated finds
> > across it dropping caches in between.  Each traversal maps and
> > unmaps every btree block.
> 
> Hmm, the __free_page -> page_cache_release() change in vfree() would
> have been simpler wouldn't it? 

Yes, it would, but I tried to leave vmalloc doing the same thing as
it was before. I think that it would be safe simply to call put_page()
directly in the __vunmap() code and drop all the release function
passing, but I don't know enough about that code to say for certain.
I'll spin up another patch tomorrow that does this and see how it goes.

> But if it works it is fine.

Seems to - it's passed XFSQA with 64k directories and a bunch of
dirstress workouts as well.

Nick, Jeremy, (others?) any objections to this approach to solve
the problem?

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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