Changes

== Basic design ==

This page describes the overall design of how Checkpoint and Restore work in CRIU.

=== Checkpoint ===

== Checkpoint ==

The checkpoint procedure relies heavily on '''/proc''' file system (it's a general place where criu takes all the information it needs).

The checkpoint procedure relies heavily on '''/proc''' file system (it's a general place where criu takes all the information it needs).

Which includes

The information gathered from /proc includes:

* Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').

* Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').

* etc.

* etc.

The process dumper (lets call it a dumper further) does the following steps during checkpoint stage

The process dumper (called a dumper below) does the following steps during the checkpoint stage.

==== Collect process tree and freeze it ====

=== Collect process tree and freeze it ===

The '''$pid''' of a process group leader is obtained from the command line (<code>--tree</code> option). By using this '''$pid''' the dumper walks though '''/proc/$pid/task/''' directory collecting threads and through the '''/proc/$pid/task/$tid/children''' to gathers children recursively. While walking tasks are stopped using the <code>ptrace</code>'s <code>PTRACE_SEISE</code> command.

The '''$pid''' of a process group leader is obtained from the command line (<code>--tree</code> option). By using this '''$pid''' the dumper walks though '''/proc/$pid/task/''' directory collecting threads and through the '''/proc/$pid/task/$tid/children''' to gathers children recursively. While walking tasks are stopped using the <code>ptrace</code>'s <code>PTRACE_SEIZE</code> command.

==== Collect tasks' resources and dump them ====

 

=== Collect tasks' resources and dump them ===

At this step CRIU reads all the information (it knows) about collected tasks and writes them to dump files. The resources are obtained via

At this step CRIU reads all the information (it knows) about collected tasks and writes them to dump files. The resources are obtained via

# VMAs areas are parsed from '''/proc/$pid/smaps''' and mapped files are read from '''/proc/$pid/map_files''' links

# VMAs areas are parsed from '''/proc/$pid/smaps''' and mapped files are read from '''/proc/$pid/map_files''' links

# Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing '''/proc/$pid/stat''' entry.

# Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing '''/proc/$pid/stat''' entry.

Then CRIU injects a parasite code into a task via ptrace interface. This is done in two steps -- at first we inject only a few bytes for ''mmap'' syscall at CS:IP the task has at moment of seizing. Then ptrace allow us to run an injected syscall and we allocate enough memory for a parasite code chunk we need for dumping. After that the parasite code is copied into new place inside dumpee address space and CS:IP set respectively to point to our parasite code.

Then CRIU injects a [[parasite code]] into a task via ptrace interface. This is done in two steps -- at first we inject only a few bytes for ''mmap'' syscall at CS:IP the task has at moment of seizing. Then ptrace allow us to run an injected syscall and we allocate enough memory for a parasite code chunk we need for dumping. After that the parasite code is copied into new place inside dumpee address space and CS:IP set respectively to point to our parasite code.

From parsite context CRIU does more information such as

From parsite context CRIU does more information such as

# Contents of memory

# Contents of memory

After everything dumped (such as memory pages, which can be written out only from inside dumpee address space) we use ptrace facility again and cure dumpee by dropping out all our parasite code and restoring original code. Then CRIU detaches from tasks and they continue to operate.

 

At this step CRIU reads in image files and finds out which processes share which resources. Later shared resources are restored by some one process and all the others either inherit one on the 2nd stage (like session) or obtain in some other way. The latter is, for example, shared files which are sent with SCM_CREDS messages via unix sockets, or shared memory areas that are restoring via <code>memfd</code> file descriptor.

 

 

At this step CRIU calls fork() many times to re-created the processes needed to be restored. Note, that threads are not restored here, but on the 4th step.

 

 

 

 

The restoration of the above four types of resources are delayed till the last stage for the reasons described below. On this stage CRIU opens files, prepares [[namespaces]], maps (and fills with data) private memory areas, creates sockets, calls chdir() and chroot() and doing some more.

 

=== Switch to restorer context, restore the rest and continue ===

The restore procedure (aka restorer) proceed in the following steps

At the same place we restore timers not to make them fire too early, here we restore credentials to let criu do privileged operations (like fork-with-pid) and threads not to make them suffer from sudden memory layout change.

''See also: [[restorer context]], [[tree after restore]].''

# Every process started with saved (i.e. original) '''$pid''' (see [[Pid restore]]) via <code>clone()</code> call.

# Files and pipes are restored (by restored it's meant - they are opened and positioned).

[[Category:Under the hood]]

[[Category:Under the hood]]