Re: Interaction between Xen and XFS: stray RW mappings

[David Chinner <dgc@sgi.com>]

On Fri, Oct 12, 2007 at 09:58:43AM -0700, Jeremy Fitzhardinge wrote:
> Hi Dave & other XFS folk,
> 
> I'm tracking down a bug which appears to be a bad interaction between XFS
> and Xen.  It looks like XFS is holding RW mappings on free pages, which Xen
> is trying to get an exclusive RO mapping on so it can turn them into
> pagetables.
> 
> I'm assuming the pages are actually free, and this isn't a use after free
> problem.  From a quick poke around, the most likely pieces of XFS code is
> the stuff in xfs_map.c which creates a virtually contiguous mapping of pages
> with vmap, and seems to delay unmapping them.

You mean xfs_buf.c.

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.

> When pinning a pagetable, Xen tries to eliminate any RW aliases of the pages
> its using.  This is generally trivial because pages returned by
> get_free_page don't have any mappings aside from the normal kernel mapping.
> High pages, when using CONFIG_HIGHPTE, may have a residual kmap mapping, so
> we clear out the kmap cache if we encounter a highpage in the pagetable.
> 
> I guess we could create a special-case interface to do the same thing with
> XFS mappings, but it would be nicer to have something more generic.

*nod*

> Is my analysis correct?  Or should XFS not be holding stray mappings?  Or is
> there already some kind of generic mechanism I can use to get it to release
> its mappings?

The xfsbufd cleans out any stale mappings - it's woken by the memory
shrinker interface (i.e. calls xfsbufd_wakeup()). Otherwise every
64th buffer being vmap()d will flush out stale mappings.

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?

Cheers,

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