GSoC completed projects

Restrict checks for open/mmaped files

Summary: Make sure the file opened (for fd or mapping) at restore is "the same" as it was on dump

Merged: https://github.com/checkpoint-restore/criu/pull/1123

CRIU doesn't carry files contents (except for ghost ones) into images. Thus on dump it saves some "meta" for file to validate it's "the same" on restore. Currently this meta includes only the file size. The task is to add some cookie value that's somehow affected by file's contents. This is primarily needed to reduce the possibility to restore with wrong libraries.

Links:

• https://www.criu.org/Category:Files
• https://en.wikipedia.org/wiki/File_verification

Optimize the pre-dump algorithm

Summary: Optimize the pre-dump algorithm to avoid pinning to many memory in RAM

Merged: https://github.com/checkpoint-restore/criu/commit/98608b90de0f853b1c8a6e15b312320e1441c359

Current pre-dump mode is used to write task memory contents into image files w/o stopping the task for too long. It does this by stopping the task, infecting it and draining all the memory into a set of pipes. Then the task is cured, resumed and the pipes' contents is written into images (maybe a page server). Unfortunately, this approach creates a big stress on the memory subsystem, as keeping all memory in pipes creates a lot of unreclaimable memory (pages in pipes are not swappable), as well as the number of pipes themselves can be huge, as one pipe doesn't store more than a fixed amount of data (see pipe(7) man page).

A solution for this problem is to use a sys_read_process_vm() syscall, which will mitigate all of the above. To do this we need to allocate a temporary buffer in criu, then walk the target process vm by copying the memory piece-by-piece into it, then flush the data into image (or page server), and repeat.

Ideally there should be sys_splice_process_vm() syscall in the kernel, that does the same as the read_process_vm does, but vmsplices the data

Links:

• Memory pre dump
• https://github.com/checkpoint-restore/criu/issues/351
• Memory dumping and restoring, Memory changes tracking
• process_vm_readv(2) vmsplice(2) RFC for splice_process_vm syscall

Porting crit functionalities in GO

Summary: Implement image view and manipulation in Go

Merged: https://github.com/checkpoint-restore/go-criu/pull/66

CRIU's checkpoint images are stored on disk using protobuf. For easier analysis of checkpoint files CRIU has a tool called CRiu Image Tool (CRIT). It can display/decode CRIU image files from binary protobuf to JSON as well as encode JSON files back to the binary format. With closer integration of CRIU in container runtimes it becomes important to be able to view the CRIU output files. Either for manipulation before restoring or for reading checkpoint statistics (memory pages written to disk, memory pages skipped, process downtime).

Currently CRIT is implemented in Python, for easier integration in other Go projects it is important to have image manipulation and analysis available from GO. This means we need a Go based library to read/modify/write/encode/decode CRIU's image files. Based on this library a Go based implementation of CRIT would be useful.

Links:

• CRIT (Go library)
• https://github.com/snprajwal/gsoc-2022

Support sparse ghosts

Merged: https://github.com/checkpoint-restore/criu/pull/1944 https://github.com/checkpoint-restore/criu/pull/1963

When criu dumps processes it also dumps files that are opened by them. It does this by saving file names by which the files are accessible. But sometimes files can have no names. It may happen if a task opened a file and then removed it. To dump this file criu cannot save its name (because the name doesn't exist). Instead criu saves the whole file. This is called "ghost file". Since saving the whole file is very expensive (copying lots of data on disk) criu limits the maximum size of a ghost file. The latter is also not good, because there are "sparse" files, that are large in size, but may be small from the real disk usage perspective. The goal of the task is to support sparse ghost files, i.e. limit the size of the ghost not by its length but by disk usage and when copying the data detect the used blocks and save only those.

Links:

• Sparse files
• Dumping files
• Invisible files
• Fiemap ioctl

Details:

• Skill level: intermediate
• Language: C
• Expected size: 350 hours
• Mentor: Pavel Emelyanov <ovzxemul@gmail.com>
• Mentor: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
• Suggested by: Pavel Emelyanov <ovzxemul@gmail.com>