Messages in this thread |  | | | Date | Mon, 15 Oct 2007 08:56:18 +1000 | | From | David Chinner <> | | Subject | Re: Interaction between Xen and XFS: stray RW mappings |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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