CRIU

Google Summer of Code Ideas

Revision as of 12:00, 31 January 2019 by Radostin (talk | contribs) (→‎Optimize the pre-dump algorithm: Update link to RFC for splice_process_vm syscall)

Google Summer of Code (GSoC) is a global program that offers post-secondary students an opportunity to be paid for contributing to an open source project over a three month period.

This page contains project ideas for upcoming Google Summer of Code.

Suggested ideas

Post-copy for shared memory and hugetlbfs

Summary: extend post-copy memory restore and migration to support shared memory and hugetlbfs.

CRIU relies on Userfaultfd mechanism in the Linux kernel to implement the demand paging in userspace and allow post-copy memory (or lazy) migration. However, currently this support is limited to anonymous private memory mappings, although kernel also supports shared memory areas and hugetlbfs backed memory.

The shared memory support for lazy migration can be added to CRIU without kernel modifications, while proper handling of hugetlbfs would require userfaultfd callbacks for fallocate(PUNCH_HOLE) and madvise(MADV_REMOVE) system calls.

Links:

Details:

  • Skill level: most probably advanced?
  • Language: C
  • Mentor: Mike Rapoport <rppt@linux.ibm.com> / Andrey Vagin <avagin@gmail.com>
  • Suggested by: Mike Rapoport <rppt@linux.ibm.com>

Optimize logging engine

Summary: CRIU puts a lots of logs when doing its job. Logging is done with simple fprintf function. They are typically useless, but if some operation fails -- the logs are the only way to find what was the reason for failure.

At the same time the printf family of functions is known to take some time to work -- they need to scan the format string for %-s and then convert the arguments into strings. If comparing criu dump with and without logs the time difference is notable (15%-20%), so speeding the logs up will help improve criu performance.

One of the solutions to the problem might be binary logging. The problem with binary logs is the amount of efforts to convert existing logs to binary form. Preferably, the switch to binary logging either keeps existing log() calls intact, either has some automatics to convert them.

The option to keep log() calls intact might be in pre-compilation pass of the sources. In this pass each log(fmt, ...) call gets translated into a call to a binary log function that saves fmt identifier copies all the args as is into the log file. The binary log decode utility, required in this case, should then find the fmt string by its ID in the log file and print the resulting message.


Links:

Details:

  • Skill level: intermediate
  • Language: C, though decoder/preprocessor can be in any language
  • Mentor: Andrei Vagin <avagin@gmail.com> / Pavel Emelyanov <xemul@openvz.org>
  • Suggested by: Andrei Vagin <avagin@gmail.com>

Add support for checkpoint/restore of CORK-ed UDP socket

Summary: Support C/R of corked UDP socket

There's UDP_CORK option for sockets. As man page says: 

    If this option is enabled, then all data output on this socket
    is accumulated into a single datagram that is transmitted when
    the option is disabled.  This option should not be used in
    code intended to be portable.

Currently criu refuses to dump this case, so it's effectively a bug. Supporting this will need extending the kernel API to allow criu read back the write queue of the socket (see how it's done for TCP sockets, for example). Then the queue is written into the image and is restored into the socket (with the CORK bit set too).

Links:

Details:

  • Skill level: intermediate (+linux kernel)
  • Language: C
  • Mentor: Pavel Emelianov <xemul@virtuozzo.com>
  • Suggested by: Pavel Emelianov <xemul@virtuozzo.com>

Optimize the pre-dump algorithm

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

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:

Details:

  • Skill level: advanced
  • Language: C
  • Mentor: Pavel Emelianov <xemul@virtuozzo.com>
  • Suggested by: Pavel Emelianov <xemul@virtuozzo.com>

Anonymise image files

Summary: Teach CRIT to remove sensitive information from images

When reporting a BUG it may be not acceptable for the reporter to send us raw images, as they may contain sensitive data. Need to teach CRIT to "anonymise" images for publication.

List of data to shred:

  • Memory contents. For the sake of investigation, all the memory contents can be just removed. Only the sizes of pages*.img files are enough.
  • Paths to files. Here we should keep the paths relations to each other. The simplest way seem to be replacing file names with "random" (or sequential) strings, BUT (!) keeping an eye on making this mapping be 1:1. Note, that file paths may also sit in sk-unix.img.
  • Registers.
  • Process names. (But relations should be kept).
  • Contents of streams, i.e. pipe/fifo data, sk-queue, tcp-stream, tty data.
  • Ghost files.
  • Tarballs with tmpfs-s.
  • IP addresses in sk-inet-s, ip tool dumps and net*.img.

Links:

Details:

  • Skill level: beginner
  • Language: Python
  • Mentor: Pavel Emelianov <xemul@virtuozzo.com>
  • Suggested by: Pavel Emelianov <xemul@virtuozzo.com>

Porting crit functionalities in GO

Summary: Implement image view and manipulation in Go

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:

Details:

  • Skill level: beginner
  • Language: Go
  • Mentor: Adrian Reber <areber@redhat.com>
  • Suggested by: Adrian Reber <areber@redhat.com>

Memory changes tracking with userfaultfd-WP

Summary: add ability to track memory changes of the snapshotted processes using userfaultfd-WP

Currently CRIU uses Soft-dirty mechanism in Linux kernel to track memory changes. This mechanism can be complemented (or even completely replaced) with recently proposed write protection support for userfaultfd (userfaultfd-WP).

Userfault allows implementation of paging in userspace. It allows an application to receive notifications about page faults and provide the desired memory contents for the faulting pages. In the current upstream kernels only missing page faults are supported, but there is an ongoing work to allow notifications for write faults as well. Using such notifications it would be possible to precisely track memory changes during pre-dump iterations. This approach may prove to be more efficient than soft-dirty.

Links:

Details:

  • Skill level: most probably advanced?
  • Language: C
  • Mentor: Mike Rapoport <rppt@linux.ibm.com>
  • Suggested by: Mike Rapoport <rppt@linux.ibm.com>
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