I had some terrible troubles last month within two host that I have with Gentoo over HP Blade 460c G6. The problem was simple, the most IO that I generated in the host, the more memory in SUnreclaim (in /proc/meminfo).
The problem (after a log of time and testing), was solved by configuring the kernel with:
# CONFIG_IOMMU_API is not set
And, after a whole night of testing (that in prior test sessions in 2 hours the memory claimed up to 3 or 5 GB), was just in 80 MB the whole night.
The problem is quite known with the iommu module, but at first time I thought that was the cciss module (because everything was related with the disk IO). But after a further analysis, the iommu came across my path and then I saw a lot of bugs related to this module (and most of them were related to the memory leaks).