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Increasing memory density using KSM
Andrea Arcangeli (email@example.com)
With virtualization usage growing the amount of RAM duplication in the same host across different virtual machines possibly running the same software or handling the same data is growing at a fast pace too. KSM is a Linux Kernel module that allows to share equal anonymous memory across different processes and in turn also across different KVM virtual machines. Thanks to the KVM design and the mmu notifier feature, the KVM virtual machines aren't any different from any other process from the Linux Virtual Memory subsystem POV. And incidentally all Guest physical memory is allocated as regular Linux anonymous memory mappings. But KSM isn't just for virtual machines.
The KSM main task is to find equal pages in the system. To do that it uses two trees, one is the stable tree the other is the unstable tree. The stable tree contains only already shared and not changing KSM generated pages. The unstable tree contains only pages that aren't shared yet but that are tracked by KSM.
The content of the pages inserted into the two trees is the index of the tree, but we don't want to write-protect all the pagetables that points to the pages in the unstable tree. So we allow the content of the pages (so the tree index) to change under KSM and without knowledge of the tree balancing code. Thanks to the property of the red black trees that can keep a tree balanced without checking the node index value, even if the tree becomes unusable, the tree still remains balanced and the worst case insertion/deletion remains O(log(N)), to guarantee the ksm-tree algorithm not to degenerate in corner cases.
To reduce the number of false negative from the unstable tree lookups, a checksum is used to insert into the unstable tree only pages whose checksum didn't change recently, but in the future the checksum can be replaced by checking the dirty bit of the pagetables and shadow pagetables (not with current EPT though). After a full scan of all pages tracked by KSM, the unstable tree is rebuilt from scratch to reset all lookup errors introduced by the pages changing content during the scan.
Whenever KSM finds a match in the stable or unstable tree, it proceeds to write-protecting the pagetables that mapped to the old not shared anonymous page, and it makes them map the new shared KSM page as readonly. If any KVM shadow pagetable was mapping the page, it is updated and write-protected through the mmu notifier mechanism with a newly introduced change_pte method.