Changes

== Basic C/R ==

== Basic C/R ==

* /proc/pid/smaps file and /proc/pid/map_files/ directory with links are used to determine

* /proc/pid/smaps file and /proc/pid/map_files/ directory with links are used to determine

** memory areas in use by task

** memory areas in use by task

** file is mapped (if any)

** mapped files (if any)

** shared memory "identifier" to resolve the MAP_SHARED areas

** shared memory "identifier" to resolve the MAP_SHARED areas

* /proc/pid/pagemap file that reveals important flags

* /proc/pid/pagemap file that reveals important flags

** ''anonymoys'' for the MAP_FILE | MAP_PRIVATE mapping indicate that the page in question is already COW-ed from the file's. Not-anonymous pages are not dumped as they are still in sync with the file

** ''anonymoys'' for the MAP_FILE | MAP_PRIVATE mapping indicate that the page in question is already COW-ed from the file's. Not-anonymous pages are not dumped as they are still in sync with the file

** ''soft-dirty'' bit is used by [[memory changes tracking]]

** ''soft-dirty'' bit is used by [[memory changes tracking]]

* Ptrace SEIZE that is used to grab pages from task's VM into pipe (with vmsplice)

* Ptrace SEIZE, used to grab pages from task's VM into a pipe (with vmsplice)

The latter step deserves some better explanation. So in order to drain memory from task we first generate the bitmap of pages needed to be dumped (using the smaps, map_files and pagemap from proc). Then we create a set of pipe-s to put pages into. Then we infect the process with [[parasite code]] which, in turn, gets the pipes and <code>vmsplice</code>-s the required pages into it. Then we <code>splice</code> the pages from pipes into image files.

The last step deserves a more detailed explanation. In order to drain memory from a task, we first generate the bitmap of pages needed to be dumped (using the smaps, map_files and [[pagemap cache]] filled from proc). Next, we create a set of pipes to put pages into. Then we infect the process with [[parasite code]], which, in turn, gets the pipes and <code>vmsplice</code>s the required pages into it. Finally, we <code>splice</code> the pages from pipes into [[memory dumps|image files]].

=== Restoring ===

=== Restoring ===

Restoring is pretty straightforward as during restore CRIU morphs itself into the target task. Two things worth mentioning.

Restoring is pretty straightforward. During restore, CRIU morphs itself into a target task. Two things worth mentioning before diving into explanation of steps.

;[[COW]]

;[[COW]]

;[[Shared memory]]

;[[Shared memory]]

:Those areas are implemented in the kernel by supporting a pseudo file on a hidden tmpfs mount. So on restore we just determine who will create the shared are and who will attach to it (see the [[postulates]]). Then the creator <code>mmap</code>-s the region and the others open the /proc/pid/map_files/ link. However, on the recent kernels, we use the new <code>memfd</code> system call that does similar thing but works for user namespaces. Briefly -- creator creates the memfd, all the others get one via /proc/pid/fd link which is not that strict as compared to the map_files.

:Those areas are implemented in the kernel by supporting a pseudo file on a hidden tmpfs mount. So on restore we just determine who will create the shared are and who will attach to it (see the [[postulates]]). Then the creator <code>mmap</code>-s the region and the others open the /proc/pid/map_files/ link. However, on the recent kernels, we use the new <code>memfd</code> system call that does similar thing but works for user namespaces. Briefly -- creator creates the memfd, all the others get one via /proc/pid/fd link which is not that strict as compared to the map_files.

=== Non linear mappings ===

=== Non linear mappings ===

Currently we don't support non-linear mappings (fail dump if present)

Currently we don't support non-linear mappings (so dump fails if such mappings are found).

== Advanced C/R ==

== Advanced C/R ==

For such things as remote dump, stackable images and incremental dumps CRIU supports a more sophisticated memory C/R policies rather than "dump all -- restore all" one. There are several CLI knobs that affect this question.

For things as remote dump, stackable images, and incremental dumps, CRIU supports a more sophisticated memory C/R policies rather than "dump all -- restore all" one. There are several CLI knobs that can be used.

* dump action

* dump action

;dump

;dump

:Without any options till will dump everything and kill the dumped tasks.

:Without any options, dump everything and kill the dumped tasks.

;dump --track-mem

;dump --track-mem

:Will dump everything, will turn on memory changes tracking and will kill tasks after this. As you might have noticed this is pretty useless combination of options.

:Dump everything, turn on memory changes tracking, and kill tasks after this. As you might have noticed, this is pretty useless combination of options!

;dump --leave-running

;dump --leave-running

:Will dump everything and leave the tasks running after dump.

:Dump everything, and leave the tasks running after dump.

;dump --track-mem --leave-running

;dump --track-mem --leave-running

:Same as above, but will turn on memory changes tracking.

:Same as above, but turn on memory changes tracking.

;dump --track-mem --leave-running --prev-images-dir <path>

;dump --track-mem --leave-running --prev-images-dir <path>

:Same as above, but during dump will also check whether the page in question is present in parent and would skip dumping it this time.

:Same as above, but during dump also check whether the page in question is present in parent, and skip dumping it this time.

;pre-dump

;pre-dump

:Will dump only the memory, turn on memory changes tracking and leave tasks running

:Only dump memory, turn on memory changes tracking and leave the tasks running.

;pre-dump --prev-images-dir <path>

;pre-dump --prev-images-dir <path>

:Same as above, but will check for pages present in parent and would skip them.

:Same as above, but check for pages present in parent and skip them.

;<pre->dump <options> --page-server

;<pre->dump <options> --page-server

:Given to any combination above would make CRIU send the pages to the page server (e.g. for [[disk-less migration]]).

:Send the pages to the page server (e.g. for [[disk-less migration]]). See [[page server]] for more details.

 

 

 

 

[[Category:Under the hood]]

[[Category:Under the hood]]

[[Category:Memory]]

[[Category:Memory]]