Kcmp trees

Overview

Usually we dump not just a single process but a set of them, where every process may be sharing some resources with other processes. Thus we need somehow to distinguish which resources are shared and which are not.

For this sake kcmp system call has been introduced to Linux kernel. It takes two processes and compare a resource asked, returning result similar to well known strcmp call. This allows CRIU to track resources with a sorting algorithm.

API

CRIU gather files, filesystems, vitrual memory descriptors, signal handlers and file descriptors associated with a process each into Kcmp-tree. Thus at moment we are carrying five Kcmp-trees. Each declared with DECLARE_KCMP_TREE helper. For example

DECLARE_KCMP_TREE(vm_tree, KCMP_VM);

Each tree internally represented as red-black tree.

When CRIU gathers process resources it check if a resource is already sitting inside of a tree calling kid_generate_gen() helper. If a resource is not in a tree - it pushed into a tree and a caller obtains new abstract ID which may be used inside CRIU images, otherwise the helper returns zero notifying that this kind of resource already known to CRIU and has been handled earlier.

This feature is quite important to eliminate duplication of entries inside CRIU dump images, because two processes might share a lot of resources and dumping them multiple times would cause very serious performance issue.

Two trees

In order to minimize the number of kcmp calls we use two IDs for an object -- so called gen_id and the ID itself.

The gen_id is and ID that is created based on some visible attributes of an object. E.g. for a file it's generated out of the inode number, device and position. Having two gen_id-s different we can say that the objects differ to. E.g. file with different inodes are different. But two equal gen_id-s may refer to different files too. So to check this we call kcmp.

For faster lookup we store objects in two trees. First RB-tree is the sorted by gen_id-s tree. When we fail to find an element in a tree we assume that the object we check is the new one. When we find an element in the tree we need to go on and call kcmp. But since one gen_id leaf may refer to several elements and kernel reports equals/greater/less from kcmp we create the 2nd tree under the gen_id leaf -- the sorted by ID tree where the comparison function is the kcmp.