mirror of https://github.com/checkpoint-restore/criu.git synced 2026-07-17 16:47:50 +00:00

History

Andrei Vagin f3b293fd85 docs: Clarify memory consistency and performance in pre-dump documentation Correct the documentation regarding `vmsplice` and `SPLICE_F_GIFT` behavior. `SPLICE_F_GIFT` does not enforce Copy-on-Write (COW) when the user-space process modifies the gifted memory. Instead, it is a zero-copy mechanism where post-resume modifications can lead to inconsistent intermediate dumps. Explain how CRIU handles this inconsistency via its iterative design, relying on the soft-dirty tracker to catch post-resume modifications and re-dump them in subsequent iterations, ensuring final consistency. Also emphasize that the `splice` scheme relies heavily on `vmsplice` performance to minimize freeze time, making the migration almost invisible to the process. Signed-off-by: Andrei Vagin <avagin@google.com>		2026-06-25 13:16:05 -07:00
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32bit-tasks-cr.md	docs: update 32-bit tasks C/R documentation	2026-06-04 11:29:31 -07:00
aio.md	docs: update AIO documentation	2026-06-04 11:29:31 -07:00
apparmor.md	docs: update AppArmor documentation	2026-06-04 11:29:31 -07:00
arm64-gcs.md	docs: update ARM64 GCS documentation	2026-06-04 11:29:31 -07:00
bpf-maps.md	docs: update BPF maps documentation	2026-06-04 11:29:31 -07:00
cgroups.md	docs: update CGroups documentation	2026-06-04 11:29:31 -07:00
change-ip-address.md	docs: update IP address change documentation	2026-06-04 11:29:31 -07:00
checkpointrestore.md	docs: update Checkpoint/Restore architecture documentation	2026-06-04 11:29:31 -07:00
code-blobs.md	docs: update Code Blobs documentation	2026-06-04 11:29:31 -07:00
comparison-to-other-cr-projects.md	docs: update Comparison to other CR projects documentation	2026-06-04 11:29:31 -07:00
copy-on-write-memory.md	docs: update COW memory documentation	2026-06-04 11:29:31 -07:00
dmtcp.md	docs: update DMTCP comparison documentation	2026-06-04 11:29:31 -07:00
dumping-files.md	docs: update Dumping Files documentation	2026-06-04 11:29:31 -07:00
faq.md	docs: update FAQ documentation	2026-06-04 11:29:31 -07:00
fdinfo-engine.md	docs: update File Restoration Engine (fdinfo) documentation	2026-06-04 11:29:31 -07:00
filesystems-pecularities.md	docs: update Filesystem Peculiarities documentation	2026-06-04 11:29:31 -07:00
final-states.md	docs: update Process Tree Final States documentation	2026-06-04 11:29:31 -07:00
freezing-the-tree.md	docs: update Freezing the Process Tree documentation	2026-06-04 11:29:31 -07:00
fsnotify.md	docs: update FSNotify documentation	2026-06-04 11:29:31 -07:00
how-hard-is-it-to-open-a-file.md	docs: update Re-opening Files documentation	2026-06-04 11:29:31 -07:00
how-to-assign-needed-file-descriptor-to-a-file.md	docs: update Descriptor Assignment documentation	2026-06-04 11:29:31 -07:00
how-to-open-a-file-without-open-system-call.md	docs: update Re-opening Nameless Files documentation	2026-06-04 11:29:31 -07:00
index.md	doc: Create index.md for Under the Hood documentation	2026-06-04 11:29:31 -07:00
invisible-files.md	docs: update Invisible Files documentation	2026-06-04 11:29:31 -07:00
irmap.md	docs: update Irmap documentation	2026-06-04 11:29:31 -07:00
kcmp-trees.md	docs: update Kcmp Trees documentation	2026-06-04 11:29:31 -07:00
kerndat.md	docs: update Kerndat documentation	2026-06-04 11:29:31 -07:00
mac-vlan.md	docs: update Mac-VLAN documentation	2026-06-04 11:29:31 -07:00
memory-changes-tracking.md	docs: update Memory Changes Tracking documentation	2026-06-04 11:29:31 -07:00
memory-dumping-and-restoring.md	docs: update Memory Dumping and Restoring documentation	2026-06-04 11:29:31 -07:00
memory-images-deduplication.md	docs: update Memory Images Deduplication documentation	2026-06-04 11:29:31 -07:00
mount-points.md	docs: update Mount Points documentation	2026-06-04 11:29:31 -07:00
mount-points20.md	docs: mark Mount Points 2.0 as legacy and redirect to Mount V2	2026-06-04 11:29:31 -07:00
mount-v2.md	docs: update Mount V2 documentation	2026-06-04 11:29:31 -07:00
mounts-v2-virtuozzo.md	docs: update Mount V2 Detailed Algorithm documentation	2026-06-04 11:29:31 -07:00
nfs-mount-points.md	doc: export under-the-hood documentations from criu.org	2026-06-04 11:29:31 -07:00
optimizing-pre-dump-algorithm.md	docs: Clarify memory consistency and performance in pre-dump documentation	2026-06-25 13:16:05 -07:00
pagemap-cache.md	docs: update Pagemap Cache documentation	2026-06-04 11:29:31 -07:00
parasite-code.md	docs: update Parasite Code documentation	2026-06-04 11:29:31 -07:00
pending-signals.md	docs: update Pending Signals documentation	2026-06-04 11:29:31 -07:00
pid-restore.md	docs: update PID Restoration documentation	2026-06-04 11:29:31 -07:00
pidfd-store.md	docs: update Pidfd Store documentation	2026-06-04 11:29:31 -07:00
pidfd.md	docs: update Pidfd documentation	2026-06-04 11:29:31 -07:00
README.md	docs: add README.md for Under the Hood documentation	2026-06-04 18:58:00 +00:00
restartable-sequences.md	docs: update Restartable Sequences documentation	2026-06-04 11:29:31 -07:00
restorer-context.md	docs: update Restorer Context documentation	2026-06-04 11:29:31 -07:00
service-descriptors.md	docs: update Service Descriptors documentation	2026-06-04 11:29:31 -07:00
shared-memory.md	docs: update Shared Memory documentation	2026-06-04 11:29:31 -07:00
sockets.md	docs: update Network Sockets documentation	2026-06-04 11:29:31 -07:00
stages-of-restoring.md	docs: update Restoration Stages documentation	2026-06-04 11:29:31 -07:00
tcp-connection.md	docs: update TCP Connection documentation	2026-06-04 11:29:31 -07:00
technologies.md	docs: update Technologies documentation	2026-06-04 11:29:31 -07:00
ttys.md	docs: update TTY documentation	2026-06-04 11:29:31 -07:00
tun-tap.md	docs: update TUN/TAP documentation	2026-06-04 11:29:31 -07:00
unix-sockets.md	docs: update Unix Sockets documentation	2026-06-04 11:29:31 -07:00
userfaultfd.md	docs: update Userfaultfd documentation	2026-06-04 11:29:31 -07:00
validate-files-on-restore.md	docs: update File Validation documentation	2026-06-04 11:29:31 -07:00
vdso.md	docs: update vDSO and VVAR documentation	2026-06-04 11:29:31 -07:00
zombies.md	docs: update Zombie Processes documentation	2026-06-04 11:29:31 -07:00

README.md

CRIU: Under the Hood

This directory contains technical documentation detailing the internal implementation of CRIU, the kernel APIs it leverages, and the complex algorithms used to achieve checkpoint and restore of various Linux resources.

Core Architecture & Lifecycle

Checkpoint and Restore Overview: High-level view of the C/R process.
Freezing the Process Tree: How CRIU stops the application using the freezer cgroup or signals.
Parasite Code: Injection and execution of code within the victim's address space.
Restorer Context: The environment in which the restorer blob executes.
Stages of Restore: Detailed breakdown of the multi-stage restoration process.
Final States: The state of processes after restore.
Technologies Used: Overview of kernel technologies CRIU depends on.
Kerndat: How CRIU probes and caches kernel feature support.

Memory Management

Memory Dumping and Restoring: The primary algorithms for memory C/R.
Memory Changes Tracking: Using dirty bits (soft-dirty) for iterative migration.
Pagemap Cache: Optimizing access to /proc/pid/pagemap.
Copy-on-Write Memory: Handling shared and private COW mappings.
Shared Memory: Restoration of SysV IPC and POSIX shared memory.
Memory Images Deduplication: Saving space in image files.
Optimizing Pre-dump Algorithm: Strategies for minimizing downtime.
Userfaultfd: Lazy migration and post-copy restoration.

Files, Mounts & I/O

Dumping Files: General overview of file descriptor C/R.
How hard is it to open a file?: The complexities of reconstructing file states.
How to open a file without open() syscall: Using linkat and other tricks for inaccessible files.
How to assign needed FD to a file: Re-mapping FDs to match original values.
Invisible Files: Handling unlinked but open files.
FD Info Engine: Parsing /proc/pid/fdinfo.
Service Descriptors: Managing CRIU's internal FDs to avoid collisions.
Mount Points: Basic mount restoration.
Mount V2: Modern mount restoration using open_tree and move_mount.
Mounts V2 Virtuozzo: Extensions for Virtuozzo-specific mount features.
Filesystem Peculiarities: Handling /dev, /proc, sysfs, etc.
IRM: Inode-to-path mapping (irmap).
KCMP Trees: Using kcmp to deduplicate shared resources.
Validate Files on Restore: Ensuring file consistency.
FSNotify: Checkpointing inotify and fanotify marks.
AIO: Checkpointing asynchronous I/O contexts.

Networking

TCP Connections: Using TCP Repair mode for zero-loss socket migration.
Unix Sockets: Reconnecting stream and dgram unix sockets.
Sockets: General socket restoration (Netlink, Raw, etc.).
Change IP Address: Handling network configuration changes during migration.
MAC-VLAN: Support for MAC-VLAN interfaces.
TUN/TAP: Virtual network device restoration.

Process & Resource Management

PID Restore: Algorithms for restoring tasks with specific PIDs.
PIDFD: Checkpointing and restoring pidfds.
PIDFD Store: Internal management of pidfds.
Zombies: Handling processes in the EXIT_ZOMBIE state.
32-bit Tasks C/R: Specifics for IA32/compat mode tasks.
Pending Signals: Capturing and re-queuing signals.
Restartable Sequences (rseq): Handling the rseq kernel feature.
vDSO: Handling the virtual dynamic shared object across kernel versions.
TTYs: The complex state of terminal devices and PTY pairs.
CGroups: Restoring cgroup hierarchy and membership.

Security & Kernel Features

AppArmor: Handling AppArmor profiles during dump and restore.
ARM64 GCS: Guarded Control Stack support on AArch64.
BPF Maps: Experimental support for checkpointing BPF map data.
Code Blobs: Management of PIE blobs (parasite, restorer).

Comparison & External Tools

Comparison to other C/R projects: How CRIU differs from DMTCP, BLCR, etc.
DMTCP: Specific notes on DMTCP integration or comparison.
FAQ: Frequently Asked Questions about CRIU internals.

Generated by Gemini CLI as part of the Documentation Audit (March 2026).