Difference between revisions of "Checkpoint/Restore"

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m (add link to "Pid restore")
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* Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').
 
* Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').
 
* Pipes parameters.
 
* Pipes parameters.
* Memory maps (via '''/proc/$pid/maps''').
+
* Memory maps (via '''/proc/$pid/maps''' and '''/proc/$pid/map_files/''').
 +
* etc.
  
 
The process dumper (lets call it a dumper further) does the following steps during checkpoint stage
 
The process dumper (lets call it a dumper further) does the following steps during checkpoint stage
  
# '''$pid''' of a process group leader is obtained from the command line.
+
==== Collect process tree and freeze it ====
# By using this '''$pid''' the dumper walks though '''/proc/$pid/task/$tid/children''' and gathers children '''$pids''' recursively. At the end we will have a process tree.
+
The '''$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.
# Then we take every '''$pid''' from a process tree, seize and them with ptrace ''PTRACE_SEIZE'' call (which put tasks into seized state, where tasks do not know that they are actually stopped and someone does nasty things with them :), and performs the following steps on each '''$pid'''.
+
 
# Collect VMA areas by parsing '''/proc/$pid/maps'''.
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==== Collect tasks' resources and dump them ====
# Collect file descriptor numbers the task has via '''/proc/$pid/fd'''.
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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
 +
# File descriptor numbers are read via '''/proc/$pid/fd'''
 
# 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.
# The dumper 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.
+
 
# 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 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.
# The procedure continues for every '''$pid'''.
+
 
 +
From parsite context CRIU does more information such as
 +
# Credentials
 +
# Contents of memory
 +
 
 +
 
 +
==== Cleanup ====
 +
 
 +
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.
  
 
=== Restore ===
 
=== Restore ===

Revision as of 11:06, 1 December 2014

Basic design

Checkpoint

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

  • Files descriptors information (via /proc/$pid/fd and /proc/$pid/fdinfo).
  • Pipes parameters.
  • Memory maps (via /proc/$pid/maps and /proc/$pid/map_files/).
  • etc.

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

Collect process tree and freeze it

The $pid of a process group leader is obtained from the command line (--tree 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 ptrace's PTRACE_SEISE command.

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

  1. VMAs areas are parsed from /proc/$pid/smaps and mapped files are read from /proc/$pid/map_files links
  2. File descriptor numbers are read via /proc/$pid/fd
  3. Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing /proc/$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.

From parsite context CRIU does more information such as

  1. Credentials
  2. Contents of memory


Cleanup

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.

Restore

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

  1. A process tree has been read from a file.
  2. Every process started with saved (i.e. original) $pid (see Pid restore) via clone() call.
  3. Files and pipes are restored (by restored it's meant - they are opened and positioned).
  4. A new memory map is created, filled with data the program had at checkpoint time.
  5. Finally the program is kicked to start with rt_sigreturn system call.